Rising Sun Member Forums  

Go Back   Rising Sun Member Forums > Other Hobbies > Ham & CB Radio Corner

Notices

Reply
 
Thread Tools
  #21  
Old 02-20-2014, 07:16 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,552
Default

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. ;-)

Name:  2m_vswr_thumb.jpg
Views: 100
Size:  117.7 KB

Name:  2m_elevation_thumb.jpg
Views: 449
Size:  126.4 KB

Name:  70cm_vswr_thumb.jpg
Views: 99
Size:  120.7 KB

Name:  70cm_elevation_thumb.jpg
Views: 462
Size:  140.9 KB
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler
Reply With Quote
  #22  
Old 02-20-2014, 08:20 PM
CO Hunter's Avatar
CO Hunter CO Hunter is offline
Lifted
 
Join Date: May 2010
Location: Colorado Springs
Posts: 106
Default

Quote:
Originally Posted by DaveInDenver View Post
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.

Quote:
Originally Posted by DaveInDenver View Post
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 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.
__________________
1996 LX 450 OME heavy front/med rears, factory e-lockers, 285/75-16 Discoverer STT, home made t-case skid plate, ARB front bumper, XD9000i, Redline sliders, HID off road lights, Kenwood TM-D710

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -- Benjamin Franklin
Reply With Quote
  #23  
Old 02-21-2014, 06:31 AM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,552
Default

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.
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler
Reply With Quote
  #24  
Old 02-21-2014, 06:42 AM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,552
Default

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.

Name:  19_5_2m_vswr_thumb.jpg
Views: 454
Size:  73.2 KB

Name:  19_5_70cm_vswr_thumb.jpg
Views: 455
Size:  77.8 KB

Name:  19_5_70cm_el_thumb.jpg
Views: 450
Size:  92.5 KB
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler
Reply With Quote
  #25  
Old 02-21-2014, 07:34 PM
CO Hunter's Avatar
CO Hunter CO Hunter is offline
Lifted
 
Join Date: May 2010
Location: Colorado Springs
Posts: 106
Default

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.
__________________
1996 LX 450 OME heavy front/med rears, factory e-lockers, 285/75-16 Discoverer STT, home made t-case skid plate, ARB front bumper, XD9000i, Redline sliders, HID off road lights, Kenwood TM-D710

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -- Benjamin Franklin
Reply With Quote
  #26  
Old 02-22-2014, 05:03 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,552
Default

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.

http://www.repeater-builder.com/ante...e-antennas.pdf

http://kathrein.pl/down/BasicAntenna.pdf

mobile_vhf_ant.pdf
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler
Reply With Quote
  #27  
Old 02-22-2014, 08:32 PM
CO Hunter's Avatar
CO Hunter CO Hunter is offline
Lifted
 
Join Date: May 2010
Location: Colorado Springs
Posts: 106
Default

Quote:
Originally Posted by DaveInDenver View Post
..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.

Quote:
Originally Posted by DaveInDenver View Post
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.

Name:  nmo bar.jpg
Views: 73
Size:  90.4 KB
5/8 wave bar mounted over sunroof and fed into NMO roof mount. I am guessing this will be the location with the RTT.

Name:  nmo2.jpg
Views: 73
Size:  119.2 KB
NMO roof mount adapters

Name:  nmo.jpg
Views: 73
Size:  107.1 KB
NMO roof mount adapters

Name:  quarter wave.jpg
Views: 73
Size:  64.0 KB
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.
__________________
1996 LX 450 OME heavy front/med rears, factory e-lockers, 285/75-16 Discoverer STT, home made t-case skid plate, ARB front bumper, XD9000i, Redline sliders, HID off road lights, Kenwood TM-D710

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -- Benjamin Franklin
Reply With Quote
  #28  
Old 02-22-2014, 10:21 PM
gahi's Avatar
gahi gahi is offline
Locked
 
Join Date: Oct 2006
Location: Moab, UT
Posts: 262
Default

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.
__________________
Gary

1996 FZJ80
Reply With Quote
  #29  
Old 02-25-2014, 08:36 PM
CO Hunter's Avatar
CO Hunter CO Hunter is offline
Lifted
 
Join Date: May 2010
Location: Colorado Springs
Posts: 106
Default solved

The other night, after thinking about my results over the weekend, I read the following webpage and had one of those moments.

http://www.k0bg.com/ground.html

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.

http://www.k0bg.com/options.html

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

Quote:
Originally Posted by gahi View Post
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.
__________________
1996 LX 450 OME heavy front/med rears, factory e-lockers, 285/75-16 Discoverer STT, home made t-case skid plate, ARB front bumper, XD9000i, Redline sliders, HID off road lights, Kenwood TM-D710

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -- Benjamin Franklin

Last edited by CO Hunter; 02-26-2014 at 05:24 PM.
Reply With Quote
  #30  
Old 02-25-2014, 10:21 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
 
Join Date: Jun 2006
Location: Larimer County
Posts: 6,552
Default

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.
__________________
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29

"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler

Last edited by DaveInDenver; 02-26-2014 at 05:28 AM.
Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


All times are GMT -6. The time now is 04:54 PM.


Powered by vBulletin® Version 3.7.1
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.