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Old 02-14-2014, 08:09 PM
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Default mobile install options/observations

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?
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Old 02-14-2014, 10:16 PM
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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?

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...
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Old 02-15-2014, 07:31 AM
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Quote:
Originally Posted by nakman View Post
.. I trust you were turning your squelch down?
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.


Quote:
Originally Posted by nakman View Post
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.
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Old 02-15-2014, 01:54 PM
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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.
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Old 02-16-2014, 09:35 AM
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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.
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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

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Last edited by CO Hunter; 02-16-2014 at 01:43 PM.
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Old 02-16-2014, 11:21 AM
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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.
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Old 02-16-2014, 01:52 PM
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Quote:
Originally Posted by DaveInDenver View Post
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.
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Old 02-16-2014, 02:49 PM
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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.
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Old 02-17-2014, 07:42 AM
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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.
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Old 02-17-2014, 09:22 AM
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why not run a fender mount?
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