View Full Version : mobile install options/observations

CO Hunter
02-14-2014, 08:09 PM
So a while back I installed an FT7900 and Larsen NMO 2/70, 5/8 wave (great mobile reputation) in my 80 series with an NMO mount through the roof (centered). I have since sold my FT7900 and about a year ago installed a Kenwood TM-D710. Here are a few head scratchers.

Mount options.

Last week I bought a RTT that will now cover the roof so I need an option for when it is mounted and I don't want to drill a second hole in the roof. I was thinking of a hood mount vs. lift gate mount. Anyone have experience comparing TX/RX at these two locations? I want to minimize my performance loss so I want a 5/8 wave.

Here is another question that is a bit tougher. I have an NMO to SO239 adapter that I use for my ATAS 120 and FT857 (stationary). I was wondering if I mount the Larsen, say on a load bar (not grounded) in front of the RTT and plug the antenna into that NMO/SO239 adapter if that would work? Seems the roof would still be the ground plane, with the hood acting as a less efficient ground plane in the forward direction. yes/no?

Here is an interesting observation.

The person that sold the FT7900 to mounted it in a 100 series with a Diamond NR73BNMO 1/2 wave antenna on the rear lift gate, maybe a foot below roof line, right/passenger side. On a trail run a few months ago he was able to hear one of our club members that was just on the other side of a slight ridge and I was not. We did have a rock outcrop to our right and the TX was from left. He was right behind me. I attributed it to possible odd signal bouncing but didn't know if anyone might know off hand of RX sensitivity differences between the two radios? By antenna and mount locations I should be out performing him. Other explanations?

Any thoughts on any of this?

02-14-2014, 10:16 PM
Two somewhat pedestrian comments- I have tested a few radios and discovered that they are different, even when they are supposed to be the same. Like my vx-3r on 1watt won't hit the 145.310 repeater from my driveway, while my vx-7r On 1 watt will hit it. Both using the exact same antenna. So it's definitely possible that the Yaesu just receives better than your Kenwood. I trust you were turning your squelch down? :confused:

Second is I have resolved to mount my antennas more for antenna preservation than optimum propagation. Lots of us here with wagons all got our first radios around the same time, all mounted the antennas on the hatches, and most all have broken at least one. Some still fight that battle, others have lowered them to transmit another day. It's only in those circumstances where someone else can hear and you can't that it makes a difference, which for me has been at most about once per year. I have been ok with that compromise...

CO Hunter
02-15-2014, 07:31 AM
.. I trust you were turning your squelch down? :confused:

Dang it, didn't think of it at the time, seems like a real possibility. I hate when I overlook the obvious. I will keep that in mind for next time.

Second is I have resolved to mount my antennas more for antenna preservation than optimum propagation. Lots of us here with wagons all got our first radios around the same time, all mounted the antennas on the hatches, and most all have broken at least one. ...

My buddy has broken two antennas on his hatch mount, that gets expensive.

When I get in low overhang places I just remove my roof mount antenna, cap the NMO connector and put up a small mag mount quarter wave (MFJ). I have an antenna switch under the drivers seat so I don't have to unplug antennas. Takes 1-2 minutes tops to swap. This is mainly for rig -rig comm but the little 1/4 wave does ok reaching out a bit. This way I can't still get the max propagation most of the time. I really don't need the 1/4 all that often as I find I can pick lines well enough to avoid branches. It is generally the sweepers that make me change over.

02-15-2014, 01:54 PM
This is my solution to low hanging stuff, the STI-CO Flexi-whip. At least for 2m, the rear one. The front antenna is a Larsen for 70cm and both are 1/4λ and I run a duplexer. Anyway, the Flexi-whip is extremely durable and flexible, it's claimed it can be tied into a knot. Never tried that but it's lasted several years up there and I never remove it just about ever, including driving into the garage, parking structures, etc. It just flops over and pops right back straight.



If I find I need extra range I have 5/8λ whips that I can put on in place of the shorty whips but I have no trouble making Denver repeaters from up here even at low and mid power (5W and 25W), so I don't find that necessary much. Most of that is due to the ideal ground plane, it really makes a big different over lip mounts even with unity gain whips. I can't stress enough how well drilling an NMO mount in the middle of your roof works... Don't have to deal with large antennas and get essentially zero fade since a 1/4λ has equal gain through complete azimuth and elevation.

CO Hunter
02-16-2014, 09:35 AM
Running independant VHF/UHF is the best way, I thought I was brave for drilling one hole ;)

That antenna whip flexibility looks similar to my MFJ mini mag mount though I have not tried to tie a knot with it but for $20 maybe I should. What I really like about that one you have is that it is an NMO. I may look into getting one of those and ditch the mini mag mount except for use in vehicles with no mobile install. Thanks for posting that.

Interesting that we have similar but different approaches here. I keep the 5/8 wave on all the time and switch to the 1/4 wave when needed and you keep the 1/4 wave on and carry a 5/8 wave. I guess the take home message is you can make a NMO roof mount work for a variety of conditions if you take a second antenna and that really is not a burden, IMHO.

Any thoughts on this part of my questions/post, I modified a bit for clarity:

I was wondering if I mount the Larsen, say on a load bar (not grounded) in front of the RTT (with a second NMO mount) and then plug the antenna into that original, centered NMO with SO239 adapter if that would work? Seems the roof would still be the ground plane, with the hood acting as a less efficient ground plane in the forward direction.

02-16-2014, 11:21 AM
Let make sure I understand, you are using a plane-mounted NMO (or mag-mount, lip mount, it doesn't matter really) with a SO239 adapter to a short coax feedline to a bar-mount antenna?

Electrically it will show conductivity but you can't necessarily assume it to be a good or universal ground. Reason for this is the radiating element itself needs to couple to the return, so the distance and coax impact the characteristic of this. The physical distance and material used between the antenna and the mount and around the ground plane aren't random.

Short answer would be that it will work but there is a performance impact. How much? Probably not enough to notice normally but you are losing efficiency, so it will ultimately be non-ideal. The nice thing about the NMO mount is it's a uniform impedance across it, so the discontinuity will be primarily in the SO239 & PL259 parts.

Also, yes, the sheet metal of the truck will still act like a ground plane. The difference is that you have to visualize the extra mount and coax as an impedance between the hot and return. You will be forcing RF current on the shield, which is a restriction but no worse really than any other feedline.

Would a sketch be useful? What will actually happen is the short patch cable will act as a small antenna most likely. I'll have to mull it over, but you might want to put a choke on the outside, although if it's short I don't know that it will matter much either way.

CO Hunter
02-16-2014, 01:52 PM
Let make sure I understand, you are using a plane-mounted NMO (or mag-mount, lip mount, it doesn't matter really) with a SO239 adapter to a short coax feedline to a bar-mount antenna?

That is correct, about a 3 foot pig tail would take me to a Yakima roof bar that will only be on when I have the RTT. I agree it would be sub-optimal, just how much.

I understand what you are are saying, I think. But as you stated, for a short run, it may not make a difference. I wish I had an antenna analyzer, that could help answer some of these questions. NMO mounts are cheap so maybe I can just try it out. It think a sketch could help the discussion along, as I am not sure I understand the PL259/SO259 discontinuity concern.

02-16-2014, 02:49 PM
I have an MFJ-269. Don't mind helping at all. If it works in your installation, I wouldn't worry much about it. The truth is all the connections are more of a problem mechanically, lots of places to break and leak.

CO Hunter
02-17-2014, 07:42 AM
Thanks for the offer to help, too bad I don't get that far north too often. Based on what we discussed I think I am going to order an NMO mount and give the bar mount a shot. I hate to use so many connectors, but in the end it may not be that bad.

I looked up your Flexi-whip and I did not see a dual band version, bummer. I will need to find another one now that I have the idea of a flexible 1/4 wave NMO mount stuck in my head.

02-17-2014, 09:22 AM
why not run a fender mount?

CO Hunter
02-17-2014, 09:57 AM
Up above I said hood mount, what I meant was fender mount.

I think up on roof would be better ground plane and I would not have to route another cable and have an exposed mount all the time. This is a secondary antenna location for when I have a RTT mounted.

02-17-2014, 01:41 PM
ahh, got it. Didn't realize this was secondary.

CO Hunter
02-17-2014, 08:47 PM
So our family found ourselves in Denver for an unexpected trip today so I stopped in at HRO to pick up some antenna supplies. Got an NMO mount that I soldered up tonight. While looking at small 1/4 wave antnennas the guy said a 2m 1/4 wave would work on 70cm but not perfect. Looks like this antenna would be 3/4 wave on 70cm. For my uses I think that would be aceptable so...I think I will order up that Flexi-whip ;)

Just need to quickly fab up a mounting bracket for the new NMO and try it out. It may be a couple of days before I can get to it.

CO Hunter
02-17-2014, 10:23 PM
Dave, If you have an MFJ-269 you should be able to check the SWR across the 70 cm band on the 2m 1/4 wave Flexi-whip. Would that be too much trouble?

02-18-2014, 05:39 AM
Yes, he's right, that a 1/4 at 145MHz is just slightly longer than 3/4 at 450MHz. IIRC it's about an inch different. Sure, I'll measure what that Flexi-whip looks like.

02-18-2014, 01:36 PM
OK, it'll tune but you have to very careful. I started with a whip that was slightly long on 2m and it was about 2:1 on the upper end of 70cm. For reference we get 144-148MHz on 2m and 420-450MHz on 70cm.

Starting point:
144 - 1.3:1
145 - 1.4:1
146 - 1.5:1
147 - 1.7:1
148 - 1.9:1

This gave:
420 - 2.8:1
425 - 2:1
430 - 1.3:1
435 - 1.5:1
440 - 1.6:1
445 - 1.3:1
450 - 1.6:1

I knew this was long and not ideal but didn't realize it was quite that bad. So I trimmed this antenna to minimize at 146MHz, which got it down to 1.2:1 at the middle of 2m. But this made 70cm go completely useless, 3:1 and worse throughout our band. It's perfect for 2m now, though! I actually knew this would be the case since the 2m and 70cm bands do not line-up as perfect multiples, so I figured if/when this happened I would either have a perfect 2m antenna or could always just cut it to a 1/4λ on 70cm.

Non-the-less, when you sweep a 3/4λ antenna it will not respond with a flat SWR curve as the various characteristics dominate. It's kinda like a 1/4λ sometimes but then it'll look a little like a 1/2λ or 5/8λ mixed in sometimes.

So you can't just pick a single frequency to dip and aim for it, you have to sweep across your bandwidth and pick a happy compromise. I let the lower part of 70cm do what it would since here in Colorado and most places the lower frequencies are SSTV, EME, SSB, weak signal and QRP. Since this is on my truck which transmit on FM for simplex and repeater phone only, I tuned from 433 to 450MHz, so let it bounce around 1.6:1 across there.

Trimmed my second:
144 - 1.2:1
148 - 1.6:1

This gave (approximately):
420 - 3.2:1
430 - 2:1
440 - 1.5:1
450 - 1.6:1
460 - 2:1

CO Hunter
02-18-2014, 09:39 PM
Awesome data set, thanks!! Based on your retuning to 146 mhz , it appears that on 70cm in the band range I am interested in for phone 445 - 448 the SWR should be just over 1.5, certainly acceptable.

This seems like it may be a good option for me instead of using my 1/4 wave mini mag mount, even if it is quite a bit more expensive.

Does the antenna come with a trim guide for frequency? I think I may be placing an order tomorrow. I found a couple of places that have it for ~$65, where did you buy yours?

02-19-2014, 05:55 AM
I get them at Tessco. Should be about $35 for one without a mount, which is the STI-CO part number ROOF-FT-NITI-M.

They are $60 if you get the ones with a mag mount or that include an NMO mount. What your are looking for is the replacement whip option, no connector, no mount. Well, unless you need a mount or NMO, in which case the replacement whip is not the right option.

As I recall they just come in a plastic bag with a rubber seal and not much else. They are targeted for commercial installations and I suspect they buy them by the case, so they don't need instructions that will just clutter up the shop.



CO Hunter
02-19-2014, 08:39 PM
Unfortunately it appears no one sells the whip and base, don't need the NMO mount but I may be stuck buying all of it. Tomorrow I will call STI-CO directly.

I found the cut guide on the website.

The mast length (includes whip and what they call mast adapter (I would call it base)) for 145 Mhz should be 19.8". Do you recall what you wound up with?

I reread your results above and did so a bit slower and I became a bit confused by your saying when you trimmed/tuned for 146 Mhz the 70 cm went useless at worse than 3:1. But farther down I interpret the trimmed whip for 146 Mhz yielded a SWR of 1.2:1 and the resulting SWR for ~445 Mhz was slightly more than 1.5:1. It seems like SWR for that portion of 70cm actually improved over the original length. Maybe I am missing something :confused: Sorry, not trying to be a pest.

02-20-2014, 12:05 PM
Tessco has sold the whip on an NMO base for $34, that's the link I put in there. It does not come with any NMO, but screws onto my Larsen NMO mounts just fine. Be sure to use the thin rubber gasket they include, anything much thicker and the center contact may not reach the NMO base.

When I give a frequency, it's literally that frequency, not a shorthand for the band. IOW, 146MHz is 146MHz, not a substitute meaning the 2m band.

It takes some finesse and will have to be done carefully (I was taking 1/16" off at a time), but it looks like it can be done. It's also important to note that I have not done any testing while driving, this antenna is very flexible and my suspicion is that the SWR will swing fairly wildly as it moves around.

The take-away is you will have the SWR get beyond 2:1 somewhere in either the 2m or 70cm band and if you're not careful it will be in the FM phone portion in one of the bands. I selected the longer of the two values because I use 70cm quite a bit so wanted it to be low SWR on 440MHz through 450MHz. The ideal VHF length for my truck would be somewhere between these two cut lengths, I went too short by ~1/4" with 18-5/8". It would be fine on 2m, however it would be a very poor choice for UHF. It's 2.1:1 right in the center of my intended use on 70cm.

BTW, I will probably not need the shorter one if you're interested in it. How about $20? This would be definitely good for a 2m-only installation if someone else is following this thread.

I'm constructing a mathematical model for this, hopefully I get that working today or tomorrow. When I do I'll know more about how this antenna is operating.

Measured data with these conditions:

NMO-K on roof, approximately the middle left-to-right.
The mount is roughly 12" from the back and 36" from the windshield towards the rear of the cab
Cut to length RG-58 coax, runs under the headliner, down the B-pillar to under the passenger's seat
MFJ-269 analyzer
The value is OAL as measured from the tip of the whip to the bottom of the NMO base, the actual whip is about 1/16" shorter
Line is smoothed with a cubic spline


02-20-2014, 07:16 PM
Model started, so far using the longer of the two (19" = 482.6mm) over an ideal ground is giving a fairly reasonable (as in not completely unbelievable) result. It looks like the feed line, NMO and maybe the SO239 are impacting the real installation, particularly on 70cm but even my 2m measurements vary a bit. I'm just modeling this as a single element with 124 segments, so it's pretty basic.

In any case, the elevation plot is interesting for the 3/4 wavelength case. My experience driving around today using this antenna on 70cm matches the pattern, it tends to fade a lot more than the 1/4 did. This is probably the biggest hurdle to doing this, that huge null at 22 degrees. Being down -20dB there is going to be a problem since that is where the majority of your repeater aspects are going to sit. OTOH it probably won't be the least bit deaf up against the hogbacks along C470. ;-)





CO Hunter
02-20-2014, 08:20 PM
Tessco has sold the whip on an NMO base for $34, that's the link I put in there. It does not come with any NMO, but screws onto my Larsen NMO mounts just fine. Be sure to use the thin rubber gasket they include, anything much thicker and the center contact may not reach the NMO base.

When I called Tessco yesterday the description said whip only. He researched it a bit and said he had conflicting information as to what was included. When I called STI-CO this morning they confirmed the whip comes with the base as you stated.

In any case, the elevation plot is interesting for the 3/4 wavelength case. My experience driving around today using this antenna on 70cm matches the pattern, it tends to fade a lot more than the 1/4 did. This is probably the biggest hurdle to doing this, that huge null at 22 degrees. Being down -20dB there is going to be a problem since that is where the majority of your repeater aspects are going to sit.

Wow, that is a big fat null! I don't use 70cm that much just a local linked repeater that has a low azimuth so I should be ok. I want the flexibility of hitting a 70cm repeater if needed but if I am in an emcomm situation I could always put the 5/8 wave back on. I think I will take a shot at this and trim it to get the 19 1/16" that you used. The primary use for this antenna would be low overhang areas, otherwise the 5/8 wave will be mounted.

Can't thank you enough for this analysis, I owe you a :beer: someday. I will pass along the info for the good deal on the shorter Flexi-whip.

What modeling software did you use? I have played around (a little) with EZNEC a while back and if I recall correctly, it wasn't all that well documented or intuitive, at least the free version. I was trying to model my 80m, full wave, horizontal loop.

02-21-2014, 06:31 AM
Yeah, just remember that -20dB is a factor of 100, the equivalent is the difference between 0.5W and 50W. It's not an insignificant null. However through our band it's really more like -16dB, which is about a factor of 40. Feeding 50W the repeater see the same as if you're using about 1.25W with a unity gain. IOW, like trying to use a HT on low power. It's unfortunate it's at such an critical take-off angle. :-/

My memory is not good but I think this is exactly why I added the second 70cm. I wanted the Flexi-whip fer sure and use 70cm quite a lot. More now that I'm getting into digital, all the DMR is being done on 70cm here and in most places since the bulk of radios in the commercial world operate in UHF. There are a couple of VHF models but the UHF outnumber them probably 25:1 or more.

Anyway, a repeater I routinely talk through up on Squaw Peak (WB5YOE) couldn't hear me at all with 50W through this antenna. I usually get full quieting at 5W until I get north of Ft. Collins or happen to be in a hole. Most of the time at 50W I can hit the repeater even up into Cheyenne. On RX with the 1/4 wave the S-meter is fully pegged but it bounces around S-6/7 with 3/4 wavelength.

I'm just using NEC-2 to analyze, nothing special. All NEC-2 is free and technically NEC-4/4.2 is free, although being export controlled it's a pain to get. You also have to give Lawrence-Livermore money to obtain it, you know, to administer the software.

Yes, NEC is anything but point-n-click intuitive. But if you're familiar with SPICE or assembly & embedded code it's not tough to figure out. It's just 80-character fields with two-letter line headers. The tough part is accurately describing the system to produce what you think it should, but Dr. Johnk did a fantastic job pounding EM wave into my mush-for-brains noodle.

The actual software I used for those plots is cocoaNEC 2.0, written by Kok Chen, W7AY.

02-21-2014, 06:42 AM
BTW, I would start with at least a 19-1/2" length, if not a bit longer. My actuals ended up shorter so if you just copy them they might be much too short. This is what a 0.4953m long whip over perfect ground shows in NEC. Most of the time the cut charts for 1/4 wavelength whips says 19-3/8", so I'm not sure why I'm ending up shorter.




CO Hunter
02-21-2014, 07:34 PM
okay, will do. I ordered the antenna today from Tessco. The real world performance differences are likely due to all the little differences in theory assumptions vs. actual conditions.

Okay, so now back to the original question after a long but awesome detour.

This evening I finally had time to finish work on the new antenna mount and try out the radio. With 5 w on 70cm to the low azimuth repeater it was pretty much full quieting based on signal reports. This repeater is only a few miles away. The 80's hood was pretty much pointed at the repeater. Next check was on 2m, the Pikes Peak repeater would be more of a challenge due to the terrain between my house and Pikes Peak, no line of sight as I am up against the foothills. Only on 50w was I able to open the repeater only, no audio. I went back to my original center of the roof antenna mount and was full quieting on 5w to Pikes Peak. The 80's orientation to Pikes Peak was pretty much perpendicular to the drivers side. With the secondary antenna mount so close to the windshield I suspect I have a null off in the direction of the repeater. Not much metal for a ground plane is what I suspect as the antenna is between the windshield and the sunroof. So, being off perpendicular a little could be the difference? I will get some pictures of the setup tomorrow and I will change the orientation of the 80 to see if I get any difference in performance.

02-22-2014, 05:03 PM
Well, I know why my antennas are supposed to be a little shorter than the simple formula says, the patch cable to the duplexer is acting like a transformer. What bugs me is the extra 1/2" or so I have to trim to hit resonance. I can't figure out where it comes from. I have a decent transmission line model so I'm assuming it's in the PL259 or in the NMO on 70cm, even though they are supposed to be impedance matched to 1GHz I just don't buy it.

On your other issue, this is something I preach, lip and fender mounts distort your signal patterns, especially with 1/4λ and 5/8λ antennas. You lose about 3dB compared to the center of a roof in the plane direction and will have very little signal to the non-plane side.




CO Hunter
02-22-2014, 08:32 PM
..What bugs me is the extra 1/2" or so I have to trim to hit resonance. I can't figure out where it comes from. I have a decent transmission line model so I'm assuming it's in the PL259 or in the NMO on 70cm, even though they are supposed to be impedance matched to 1GHz I just don't buy it.

This is why it must be so nice to have an antenna analyzer.

On your other issue, this is something I preach, lip and fender mounts distort your signal patterns, especially with 1/4λ and 5/8λ antennas. You lose about 3dB compared to the center of a roof in the plane direction and will have very little signal to the non-plane side.

That is exactly why I have the center roof mount! Those are some good references, one I had not seen before.

I did some testing with a 1/4 wave mini mag mount located between the sunroof and the windshield. At 5w I was full quieting into the Pikes Peak repeater, with any vehicle orientation with rear facing having slightly more static. Here are a few pics of the various setups. Obviously the 5/8 wave struggled MUCH more with the limited metal in the immediate surroundings. A correction to the location for the 5/8 wave, it was over the sunroof opening. I was really surprised by these differences in performance, especially how poorly the 5/8 wave was in this configuration.

5/8 wave bar mounted over sunroof and fed into NMO roof mount. I am guessing this will be the location with the RTT.

NMO roof mount adapters

NMO roof mount adapters

MFJ 1/4 wave mini mag mount into fed into NMO roof mount. Located just in front of load bar, between sunroof and windshield. I am very surprised this location worked so well in the forward direction.

I wish I could see RF. Maybe I need to get a pair of those Geordi La Forge visors...Anyhow, I need to think about this a bit more.

02-22-2014, 10:21 PM
1/2 wave be any better? I'd be curious to see what its specs for those are in the different mounting locations.

I'm just now working on a fender mount 1/2 wave single band. Switching from a rooftop mag-mount 1/4 dual band. I'm not dedicated enough yet to drill a hole in the roof.

CO Hunter
02-25-2014, 08:36 PM
The other night, after thinking about my results over the weekend, I read the following webpage and had one of those moments.


I went out and moved my cross bar, with the second NMO and 5/8 wave antenna attached, to near the center of the roof. On 5w I hit the Pikes Peak repeater and got a strong return signal from the repeater. So feeling encouraged, I then moved that whole set up to the very front of the roof so the bar and antenna were now centered between the sunroof and the windshield. I hit the Pikes Peak repeater again and had a nice talk on 5w with a very good signal report! So the issue was indeed lack of ground plane when my first test was setup over the sunroof. I really thought all that metal near the antenna would be sufficient, NOT. Common mode current is not your friend. The amount of sheet metal directly under the antenna is what is important.

Really good information in the link above although I think the discussion of the electrons moving is probably to make the concept more understandable since as written it seems to contradict the ARRL antenna resource "Why an Antenna Radiates".

Here is another good discussion on antennas.


I will be reconfiguring the mount to try and get it even closer the the roof.

1/2 wave be any better? I'd be curious to see what its
specs for those are in the different mounting locations.

I'm just now working on a fender mount 1/2 wave single band. Switching from a rooftop mag-mount 1/4 dual band. I'm not dedicated enough yet to drill a hole in the roof.

Obviously I am not an expert. While the 1/2 wave does not require a ground plane weather it is better than what you are using will depend on several factors discussed in the links Dave and I posted. Specifically Dave's first link. All else being equal, a center mag mount is not a bad performer. Change antennas and it is a tougher question to answer. IMHO, the bottom line is the roof is the best place for an antenna. Obviously there are other considerations such as overhead obstructions but there are also many options for a roof mount YMMV.

02-25-2014, 10:21 PM
Be careful when mixing terms like ground and return. A lot of what he professes is highly relevant to HF and less so to VHF, just simply due to the wavelengths involved. The basic principles remain, though earth, ground and return get muddled in mobile HF while higher frequencies tend to be easier to complete circuits in a mobile environment. It's really essentially impossible to have a good return when doing HF mobile, so everything is a compromise.

Not sure what that mass statement means, mass has nothing to do with anything. Surface area and lengths are critical. A 5 lb slug of metal under an antenna isn't going to work while a very light region of foil might, practically speaking. The use of foil and braid is fine to couple surfaces. What exactly constitutes a conductors, transmission line and radiator is pretty clearly defined but in the real world sometimes you have to do what you gotta.

In any case, he presents a lot of practical information even if it some of it is dubious technically. I don't know that he's studied RF physics or engineering, although that does not make his experienced recommendations without value. The ARRL paper about why antennas radiate is sound with respect to classical EM wave principles.

As far as 1/2 wavelength end fed monopole antennas, they are said to be ground plane independent but this is only because they have very high feedpoint impedance and thus the return path losses are negligible in comparison. They work better over radials or a plane just like any other configuration, it's just that they don't require one strictly to radiate RF while a 1/4 in space without a return won't work at all. All antennas will be much more efficient with a well designed return path.

I don't think it was a lack of return but the configuration. The pattern is distorted, the energy was going somewhere, just not where you thought it was.

CO Hunter
02-26-2014, 05:40 PM
Not sure what that mass statement means, mass has nothing to do with anything. Surface area and lengths are critical. A 5 lb slug of metal under an antenna isn't going to work while a very light region of foil might, practically speaking.

You are correct. However, I interpreted his use of the word mass to be in the context of the amount of sheet metal for a mobile install. I deleted that quote in my post above and added a sheet metal comment to avoid confusion on the subject. Good point.

I got my Flexi-whip antenna today. I was looking over the provided cut chart and your previous posts about the mast length. If one assumes linearity over the short 5mhz increments (yes I know it is not linear but lacking more information...) I get 19.62 inches total mast length for 146mhz. This is a bit longer than what you have but is in line with what you said about starting at 19.5 or a bit longer. I can then try to find someone with an antenna analyzer and check it out. This antenna is very flexible but much thicker than my MFJ mini mag mount 1/4 wave.

CO Hunter
02-26-2014, 05:54 PM
Can't believe I just cut it too short, 1/2 inch too short.

Had the 1/8" over the 19.5 stuck in my head and cut it to 19 1/8", which is about 149Mhz :mad::mad:

02-26-2014, 06:48 PM
How far off are you now? You can fix it by adding length electrically and it might be pretty easy, modify the transmission line between the NMO adapter and the antenna or maybe add some inductance at the base. I was wondering how it would go tuning with the dual SO239/PL259 discontinuities at each end of the feedline.

CO Hunter
02-26-2014, 09:41 PM
I am off by 0.5 inches, too short. I understand the cut chart is for a "typical" installation such as a roof mount NMO. I understand adding the SO259/PL259 connectors will change things but I suspect not that much. Getting it tuned for my primary location is my concern at the moment as I will likely be running my 5/8 wave when secondary NMO location is in use. Maybe if I call Tessco and tell them what happened, just maybe I can get an idiot discount on another one...haha.

I was able to open the 2m Pikes Peak repeater on 5w but the return signal was noticeably weaker than normal and got a call back that my TX was 95% static and unreadable. I was able to open the nearby low azimuth 70cm repeater and got a strong return signal. I was on the run this evening so I didn't have time to get into a discussion with anyone about the audio quality on 70cm. The 2m signal seems to be a lot worse than what it should be for the amount of cutting error. Maybe I need to recheck that the antenna is seated securely, that gasket was very tight on the mount.

Maybe I should just weld it back together and recut...haha

As a side note, my current set up runs through an antenna switch.

02-26-2014, 10:13 PM
Actually the UHF connector is a terrible discontinuity. Nominal impedance of 34 ohms and about 1.3:1 VSWR at 500MHz. Putting them at each end of a length of 50 ohm coax is a textbook transmission line reflection diagram problem. So what is the SWR right now? You could just add a short length of wire, maybe 3 or 4uH of inductance, to bring it back into resonance. That's the great thing about VHF and UHF, it won't take much to move things.

CO Hunter
02-27-2014, 08:57 PM
So this is what I found. Apparently the gasket had caught a bit on the NMO mount, so I removed the gasket and reseated the antenna without it. I don't have a SWR meter for my TM D710 so I took my Ft857 and my little analog LDG meter and hooked those up in the 80. My signal report at 5w, on 2m into the Pikes Peak repeater was a little weaker than normal put clear with a SWR of 1.1 (did not move the needle). I tried the 70cm repeater at 2w and the meter showed an SWR of roughly 2.1-2.2. I did not get a signal report as the FT857 rolled back the power (did not open the repeater). I have seen this happen on 70cm in the past with SWR near this level but higher. I can reconnect my Kenwood tomorrow and try the 70cm repeater again. Just didn't feel like working on it more tonight.

Hopefully I can get this tuned back up for the area of 70cm I want. If not, no great loss but very annoying. How would I add the inductance back in? Very intrigued. I am guessing it does not involve welding ...

02-28-2014, 05:56 AM
Yeah, I know what you mean. I usually leave out the gasket when tuning the Flexiwhip. The Larsen NMO-Q is the same way, with a fixed center conductor. So I want to make sure the antenna is making solid contact with the base while tuning so I know it's good. Then I recheck after putting in the gasket and smearing some dielectric grease to make sure nothing has changed.

The difference between good and bad with that gasket is very slight. The generic rubber gaskets you find in hardware stores are usually too thick and don't compress enough, so be sure to use the one they include or one just like it. I also found that the hardware store plumbing gaskets deteriorate and turn to goo after a couple of years being exposed to UV and mag chloride...

I may have mentioned, but you have to interpret the meter readings. A 50Ω resistor has a prefect load match but radiates almost zero RF. So you may not have or even want to see a 1.0:1 SWR reading. My MFJ-269 only has I'm guessing a 5% precision, so 1.0:1~1.05:1 is what I read into a calibration load and precision gets worse at greater than 2:1 mismatches.

That 2.2:1 or so SWR does not surprise me, the bandwidth of a 3/4λ antenna is narrow, so if you've dialed it in for a 2m repeater on the high side it's quite likely you'll be on the long side of the curve for 70cm repeater inputs. See posts #21 and #24, when SWR at 147Mhz is 1.4:1 the same antenna is 2:1 at 440MHz. Those NEC models don't factor in parasitics or discontinuities inherent in a real antenna. Those are just feedpoint characteristics. We already know the UHF connector is lousy at UHF frequencies (remember 'UHF' when the PL259/SO239 was designed in WWII was what we now call VHF), it's likely the RG58 is start to get lossy at 440MHz, etc.

VK3JEG ran some tests on the UHF connector in fact: http://www.qsl.net/vk3jeg/pl259tst.html

I was suggesting you add a short length of wire, like 1/2", between the coax and antenna. That's not a practical solution, though. It's very common with mobile HF antennas to put coils at the base to change impedance because being physically short relative to wavelength yields extremely narrow bandwidths. So you'll be dead on in the phone spectrum but completely mismatched in the CW and data. So you add reactance to bring resonance down or up. You might also be able to add capacitance but it's easier to add inductance. I'll have to think about it, it might be possible to add a coil at the top end or maybe a parallel cap. Problem is mechanically how to make it work.

CO Hunter
02-28-2014, 09:29 PM
I don't care for their gasket. Today I seated the gasket carefully against the roof and tightened the antenna. I could still get a piece of paper between the gasket and antenna base along most of the circumference. I like the Larsen gasket much better but the diameter is wrong for the flex-whip.

So I tried the 70cm band again but after a time or two the ft857 started getting out on 70cm with a SWR of about 1.25:1 ! I have found this radio to be a bit finicky on 70cm. The Pike Peak repeater (146.970) was still at 1.1:1 (or no needle movement). The 70cm repeaters I was hitting were 448.450 (Pikes Peak) and the Marc 448.650 (low azimuth). For these higher frequencies on the 70cm band the graphs on page 21 show a much lower SWR than 2:1. Interestingly, the length in that model was 19" only 1/8" less than what I cut mine to. Is it possible I actually got very lucky after all? Need to find someone local with an antenna analyzer.

I have used a mix of UHF connector manufacturers to build cables over the last few years. I use those cables for HF/VHF/UHF. Now I wonder what I have done...

CO Hunter
03-02-2014, 08:48 AM
The weather has kept me from playing around this weekend. I posted the UHF SWR behavior in the FT857 yahoo group and got this response.

"Dear Sir: I repaired a radio with same similar symptoms. Get a Service Manual. Follow the RF trace from the VHF/UHF finals to two PIN Diodes leading out to the low pass filter section and from there going out to the antenna SO-239. In my case it was the first pin diode, which you can check from the ((((((circuit trace to the diode))))))) and then the diode itself. Mine was not soldered from the factory, but worked for some time before the symptoms appeared. When I touched it with the DVM probe the pin diode went flying over my head. It's a 25 watt pin diode, I replaced it with a 50 watt pin diode. Your SWR intermittent is caused by the Foward/Reflected sensor is seeing the loss of connection through the LPF, and antenna conn. Good Luck"

I have the circuit diagrams. This looks like it may be a bit above what I am able to do. I may open up the case and take a peak to see if I can identify the area of the PA circuit involved.

03-02-2014, 11:07 AM
He seems to be talking about D3021 & D3019 or maybe D3023 & D3039

03-02-2014, 11:41 AM
Harder to find D3023 and D3039.

03-02-2014, 11:47 AM
If I had to guess it would be D3021 on 2m or D3019 on 70cm that are suspect.

CO Hunter
03-03-2014, 07:39 AM
How in the world do you get this stuff so fast, wow??? Are you an EE?

I haven't had chance to open it up yet. I emailed the guy from the yahoo group back and got this. By the way it seems like you two are on the same page.

"Look on page 55 Service Manual bottom Right Hand corner. If you follow circuit trace from Q3024 (2SC3102) collector lead towards the right, notice the trace leads to input of LPF----VU TX RF trace. Follow to R3069 33ohm 1 watt resistor which leads off to right from VU TX RF trace. Throght L3031
to D3019 and D3022 which lead off right to end of Schematic These are the two PIN diodes I mentioned , D3019 is the one I found bad in the rig I worked on. It is SMD components and not for the faint hearted soldering tech.
Should be 5vdc on tX at anode and about 0.2 to 0.5vdc drop at cathode or just do the usual check from circuit trace solder pad to anode, circuit trace solder pad to cathode, then anode to cathode, and the reverse ie; cathode to anode for ohmic checks. NOTE*****SMD capacitors at each end woould block DC continuity, but couples RF through, therefore in circuit checks okay.
Most DVMs have a diode check function selection or just measure the resistance thru. Should be close to infinite reverse biased. Will be low ohms resistance forward.

Also note from my previous email to you. The section that I'm pointing out is the UHF transmit path. If, the diodes not bad-----something in that section indicated most likely your problem as that is confined to 440Mhz. There is a separate section above it for two meters or VHF. At the end of that chain trace is the sensor for your SWR/FWD metering-----any variance in SWR readings would have to be outboard of that sensor-----hint hint.

One more note of interest. I had a similar problem on this radio when I was trying to TX on 70cm using a 2m home brew copper J-pole. The radio wold occasionally work on 70cm as I originally described the problem. However, at times when it would not TX on 70cm it would not TX on 2m. I did not use the radio in this manner too often for fear of damaging something. When I switched to an Arrow dual band j-pole the TX was flawless but I have not used it more than one time so far. Seems I am in the phase of more building/configuring things than talking these days.

I am not sure where this leaves me other than Yaesu tech facility as I do not posses the SMD tools nor skills. I very much appreciate the time you have taken with this.

In reply to my j-pole update here is what he said: "sometimes it's as simple as a resonant length of coax at 144 or 440 Mhz. and RF getting back into the transmitter on the 857. Bad ground, especially with a vertical antenna can be instigator. If, you have an external meter or a antenna analyzer, by process of elimination one could discover if it's RF getting back into circuits or if a component is bad. Even water getting in the coax from a leak at the PL 259 connectors outdoors can cause crazy intermittent.

I wish you the best of luck with your situation. Here I use a two step process: What is working-what is not working. What "can" cause it to exhibit these symptoms."

A leaky coax at the j-pole could have been at least part of the cause for that issue as that antenna had been up for a few years even though I did water proof it pretty well.

Unfortunately I am not near as versed in these type of diagnosis as you two are and then there is the matter of the SMD skills to fix it!!

03-03-2014, 07:56 AM
Yup, B.S. in electrical and electronics engineering in 2002 from CU-Denver, emphasis in RF, microelectronics and power. I took a couple of graduate classes in EM/RF, but never really got anywhere towards the M.S.E.E. Also hold my professional engineer's registration in Colorado. Up until about a year ago I worked designing control & data handling and solid state recorder modules for spacecraft. Not a lot on the uplink and downlink stuff, so the EM/RF side I really last did big time in engineering school and the radio hobby is to keep somewhat familiar with it. Well, other than EMI/EMC and signal integrity on PCB designs and the dynamics of SMPS.

I can solder SMC at home. So even though I don't currently have access to a really nice qualified lab, I can deal with anything in my radios. I never got good enough to even try taking the NASA qualification tests, so I obviously could not do flight hardware, but I fixed and modified breadboards and engineering models all the time and the engineers at SEAKR sometimes would build our own module test jigs, especially at the conceptual stage. Don't mistake, though, I am /not/ good at soldering. These 0603 and 0804 sized stuff and ICs with HUGE tabs in radios is cake compared to 0402 discretes and BGAs in really dense designs, though.

03-03-2014, 08:59 AM
BTW, based on your SWR readings I somewhat doubt it's a resonant length of coax causing your issues. A short or high impedance causing an issue is possible on the J-pole issue, hard to say there. But measuring SWR in this case is a solid way to know what your load is doing. IOW, it doesn't matter if an antenna is effective or not at radiating RF but if SWR is low then you can reasonably conclude you are not getting RF reflected, at least none at the measurement point (e.g. it's possible for load reflected energy to be absorbed by the feedline in theory, although you'd see issue like melted insulation or RF interference).

CO Hunter
03-06-2014, 08:11 PM
Ha, I knew it. Good for you to have the educational background compatible with one's hobby. Must be nice to be able to get into a radio like that and fix it. I will keep those pics handy should I decide to pursue. So I guess you have a FT857 as well?

I have been out of town so I will do a bit more antenna testing in the next few days. By the way, on the FT857 group I read this below that was at the end of an unrelated question to the one I posted. Seems like this type of radio response may be associated with what I am experiencing.

"Also I was too anxious to concern myself with it's technical performance when I first got it and discovered a weird issue with 70cm that really is no problem at all, but it caused me to think mine was limited to about 6 or 7 watts on 70cm. On 70cm the radio is quite sensitive to reflected power and apparently reactance near the VHF/UHF connector. I have one particular VHF/UHF SWR/Power meter that works fine connected with a 2 or 3 foot patch cable on other radios, but when connected to the FT-857D causes the power to fold back even if the SWR is low. With that same meter connected on the end of a 10 foot patch cable everything is fine. It quickly folds back power from 22 watts to 10 watts anytime the SWR reaches 3:1. It does not fold back power so much on 2 meters at 3:1. On 2 meters it folds back from 50 watts to 40 watts and then more as the SWR increases. So if you get low power out on 70cm, don't be concerned. "

CO Hunter
03-15-2014, 09:36 AM
So I thought I would wrap this up. Using my Kenwood TM D710, some more testing shows the flexi whip to work very well on 2m and okay on 70cm. On 70 cm I can hit the nearby low azimuth repeater full quieting. The Pikes Peak 70 cm repeater report was 40% white noise but readable (inside my garage on 5 watts), recall no line of sight to the Pike Peak repeater. For my 70cm mobile needs I think this will be just fine as it is my low overhang/tight trail antenna. Looks like I got lucky on trimming the antenna ;) Normally I run the 5/8 wave Larsen in the roof mount.

Now for my FT857. I took it back inside and hooked it up to the Arrow dual band J-pole. The SWR went up to 2.5 and no TX on 70cm. On VHF it was fine. So there is definitely something wrong with the circuit as described above. I was referred to affordable radio repair in Floraence, CO (http://www.affordableradiorepair.com) (http://www.affordableradiorepair.com) by Dave at Centennial electronics here in the Springs.

Thanks again for all the help Dave. By the way, you may be interested in our NM expo trip, we plan on stopping by the VLA :D

CO Hunter
03-20-2014, 07:13 PM
So I didn't really close this out with what I did for an auxiliary mount. Here are a few pics. It bolts against the tabs for the RTT mounts. Its hard to tell but the antenna sits halfway between the sunroof and windshield.




03-20-2014, 07:34 PM
That's cool, I like that solution a lot. You could add a couple of radials pointed forward. Bet that would improve the performance.

03-24-2014, 01:21 PM
Took a few sweeps of the STI-CO antennas I have. These are not lab quality, done in the driveway with a hasty calibration and a couple of adapters. Each grid line represents a change in VSWR ratio of 0.5 and the reference lines are 1:1 (e.g. zero reflection measured at the S11 port) and 3:1.

The one cut correctly for 2m shown across the 70cm band. Looks awesome at 430MHz and pretty good generally. Sweep goes from 420 to 480MHz, center is 450MHz and each step is 25MHz. The prominent dip is ~435MHz at almost 1:1 SWR. Through the bulk of the phone part of 70cm the SWR flirts with 2:1.

Here's a correctly cut (per formula) 1/4 wave across the same sweep. It doesn't look all that great to be honest, most of the band sits around 2:1 SWR. Main dip is ~443MHz at 1.05:1 SWR (I calculated 445MHz as my intended center). I think the noise is not due to the antenna but the feedline and connectors. Also seems there is some sort of a strong local interference that I can't find, which is partially why my plots have a sine wave imposed on top of them.

The same antenna in my full configuration, patch cable to duplexer broken out to two antennas cut for each band. Difference in this one to the previous is the cable and MFJ duplexer, which clearly have influence. I suspect the length of patch cable at 9" probably is the main reason, but both seem to show a lot of effect of the connectors, coax, etc. Very ugly plots. Also shows my antenna is still not trimmed short enough on 70cm, but before I go cutting more off I want to figure out the duplexer dependency.

Same as last one just swept across 2m. No surprise, very little to see, narrow frequency range, wide bandwidth and VHF. Range is 140 to 152MHz, center is 146MHz and the SWR is 1.05:1 at 146.50MHz.

A 50 ohm resistor at the VNA output for reference. This is a crappy test load, a PL259 with a RF resistor soldered inside of it. This shows how bad the PL259/SO239 interface is at UHF. The resistor is useful out to 1GHz and is 51 ohms, e.g. should show almost a perfect 1:1 SWR (within reason) but at 70cm the connector impedance is anything but uniform. Can't blame much of this on my skills since it does show correctly at lower frequencies (you can see the SWR rising coming in from the left and continuing across the sweep).