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Old 02-12-2020, 08:39 AM   #1
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Ham + GMRS antenna setup

I am new to ham radio. I have the technician license and plan on staying at that level for a while. My main interest is using the ham while on the trails in my jeep.

Gmrs is so heavily used, and has other benefits in the fact that all of my family can legally use the gmrs, since I have that license as well.

I have the mxt400 50watt gmrs radio in my jeep, and want to be able to use it in the rv while driving for convoy conversations.

Since my RV has the girard roof rails, I have to be concerned with overall height. Ideally I would mount it like I have placed my surecall or panorama cellular antennas.

The gmrs uses 450-470Mhz range of the 70cm, so I assume that a dedicated gmrs antenna would be better.

For Ham, I would be using my HT, so I would want a 2m/70cm dual band.

There are so many antennas, I am hoping that a fellow RV /ham user has already found the antenna that would work for me.

I could make a 19" work, but that would place it on the endangered list for trees.

Obviously I am going to sacrifice distance. I dont plan on doing contests.

Thanks for your input.
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Old 02-13-2020, 06:48 PM   #2
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Quote:
Originally Posted by redbaron73 View Post
I am new to ham radio. I have the technician license and plan on staying at that level for a while. My main interest is using the ham while on the trails in my jeep.

Gmrs is so heavily used, and has other benefits in the fact that all of my family can legally use the gmrs, since I have that license as well.

I have the mxt400 50watt gmrs radio in my jeep, and want to be able to use it in the rv while driving for convoy conversations.

Since my RV has the girard roof rails, I have to be concerned with overall height. Ideally I would mount it like I have placed my surecall or panorama cellular antennas.

The gmrs uses 450-470Mhz range of the 70cm, so I assume that a dedicated gmrs antenna would be better.

For Ham, I would be using my HT, so I would want a 2m/70cm dual band.

There are so many antennas, I am hoping that a fellow RV /ham user has already found the antenna that would work for me.

I could make a 19" work, but that would place it on the endangered list for trees.

Obviously I am going to sacrifice distance. I dont plan on doing contests.

Thanks for your input.
Attachment 274940Attachment 274941
What's the construction of your coach roof? Is it metal or fiberglass?
I'm asking this as the performance of the antenna is as dependent on the ground plane of the antenna as the antenna itself. The length of a true 1/4 wave CB (27mhz) antenna is about 9 feet and your average RV has no suitable ground plane for it, therefore the performance sucks. My RV has a fiberglass body and the previous owner mounted a CB antenna to the side of the body at roof level and above the drivers seat... and wondered why he could see further than he could talk. When I replace the roof on my RV I'm going to include a 3-foot-square stainless steel plate as the ground plane for the 2m antenna, and a 6-inch square one for the UHF repeater.

A 1/4 wave antenna for the 2 meter band, on the other hand, is about 18 inches, and for amateur UHF is about 6 inches.

If you have a grounded roof, look here (put take out the extra spaces I put in to make it visible):
https:// www.arcantenna.com/ laird-antenex-qw152-152-162-mhz-chrome-quarter-wave-whip-antenna-with-nmo-mount-metallic-ground-plane-required.html
The web page says 152-162 but it comes uncut at 18 inches long... and the 18 inch length perfect for the ham 2m band at 144-148 MHz. The same antenna will be resonant as a 3/4 wave at 440 (a true 1/4 wave 440 antenna is about 6 inches)... we are taking advantage of the fact that the ham UHF band is about 3 times the frequency of 2m.

The actual formula is 2808 divided by MHz equals inches.

That 18 inch stainless steel whip is CHEAP... under $10... think "disposable"... and if you whack a low hanging tree it will probably survive just fine... if you actually do bend it flat you can swap it in minutes.

GMRS repeaters are at 467 (transmit) and 462 (receive) and GMRS simplex is all at 462... that ham 440 antenna will not be optimum but will be be close enough to work...

BTW I work part time at a 2-way radio shop.... If you are in the Los Angeles area I'd be willing to help... Some of the radios we sell are 440-512 MHz in one radio (some customers are at 452, Orange County Sheriffs are at 460, Los Angeles County Sheriffs are at 482, and some police departments are at 511)
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Old 02-13-2020, 07:32 PM   #3
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Thanks for the response. I should have mentioned the roof materials.


It is a fiberglass roof, with Girard awnings on the side. These are approx 5 inches above the roof line, and have a mix of metal and other materials in it.



For my cellular antenna's, I had to get above the girard's to get a good signal. The antennas are only 3-5 inches.




Your suggestion about treating the antenna as disposable is intriguing, and makes a lot of sense. I already do this with the cellular, as I used VHB and Dicor to secure it, so that a strike would not destroy the roof.



Do you have any experience with the thru-glass VHF/UHF antennas? Are these worth considering?
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Old 02-13-2020, 08:07 PM   #4
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Your suggestion about treating the antenna as disposable is intriguing, and makes a lot of sense. I already do this with the cellular, as I used VHB and Dicor to secure it, so that a strike would not destroy the roof.
They make "no-ground-plane" antennas - they are physically shortened 1/2 wave antennas. They do not work as well as a true 1/2 wave or a 1/4 wave with a ground plane, but they are better than nothing. Might want to google for them. They are more expensive than a throwaway stainless steel whip, and are usually only one band. In your case you will probably want to use a breakaway mount.
Some dual band ham radios use one coax connector, some have a separate one for each band. What do you have?
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Do you have any experience with the thru-glass VHF/UHF antennas? Are these worth considering?
Don't waste your time. 20 years ago they sort-of worked.

The glass-mount antennas treat the windshield as the center dielectric of a capacitor. They did not work very well as the plates were pretty far apart and a lot of the RF energy didn't make it... then about 10 years ago the auto manufacturers started metalizing the glass to cut down on the "greenhouse effect" of the the windshield and back window - the coating cut down on the IR / heat absorption of the glass. Well the metalizing also acts as a grounded shield for the RF energy!
Your handheld always worked better if you stick the antenna out the window due to the shielding of the door, roof and door posts... now it is worse due to them adding more shielding in the glass!

Mike
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Old 02-13-2020, 08:40 PM   #5
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I don't have the awning, just flat fiberglass roof. I used a square piece of white steel about 18" and eternabonded it to the roof and used a cheap mag mount for a two meter antenna. If it does get into a tree branch it just knocks it over. After 4 years its still there. If you could center it on the roof the awnings may not interfere too much. I got lucky and had a dryer die about the time I was installing the radio and cut the top out of it for the ground plane. Its baked on white paint and with the eternabond holding it down its not getting any moisture under it to cause rust. So far its still looking good, no rust or discoloring. Works OK, like you said I'm not trying to do contests, just local.
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Old 02-13-2020, 09:09 PM   #6
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I have a handheld dual band HT, the Radioddity GD77. It is a single connection for both bands. If a single dual band antenna doesn't cut it, then I may have to install a Duplexer in line and dedicated antenna's per band.
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Old 02-13-2020, 10:18 PM   #7
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In a past life I worked in 2-way. You can't beat a quarter wave for value and durability. They're cheap, wideband, efficient, and nearly indestructable.

In the case of dual band there is form of 2M/440 antenna that is simply a 2M quarter wave that's loaded with a coil, which becomes a phasing coil on 440 giving you a quarter wave plus half wave on that band. "Purists" will cite the lack of gain on VHF but I consider the simplicity and durability of a short antenna that's just a whip with an air coil in it. Examples are the Comet B10NMO and Tram 1181.

If you don't care about gain or pattern on 440 you could just run a 2M quarter wave, which will give you about a 2:1 match on 440 and for basic communication works just fine. The advantage to either the dual band or 2M quarter wave used for 2M and 440 is having a single, inexpensive, durable antenna.

Finding a decent groundplane on an RV is a challenge at best. Quick and dirty is a corner bracket screwed to something already on the roof like the air conditioner. Not ideal, but since it's 10+ feet off the ground even a mediocre antenna can get some decent coverage. My RV came with a small rooftop satellite dish which I will never use for satellite, but face down makes for a handy groundplane for a V/U antenna. Another groundplane idea I've seen I'd like to try is a large baking sheet or pizza pan glued upside down to the roof using dicor sealant with an NMO mount in the center.

Through the glass antennas can work OK but have some disadvantages. They have to be used on glass that isn't passivated, meaning tinted with metal. Through-glass antennas are half waves meaning they're longer than quarter wave antennas so there's a clearance question with tall vehicles. And due to the nature of their feed they tend to have a lot of feedline radiation. Which can sometimes cause overload problems with the transceiver or other equipment in the vehicle. I would consider success with them to be more a matter of chance than skill.

Since GMRS and amateur 440 radios are in the same band you're looking at separate antennas for each radio. I would recommend doing that anyway for the sake of simplicity. There are any number of antennas cut for the commercial UHF band from quarter wave to colinears you can use on GMRS but the same issue of having an adequate groundplane applies.

If you haven't run across it yet my buddy Alan, K0BG has a website dedicated to mobile installation information. There is a section pertaining to RV's.

https://www.k0bg.com/

Mark B.
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Old 02-13-2020, 10:45 PM   #8
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What about using a dipole antenna like this one:


http://www.panorama-antennas.com/sit...frequency-band[143%20(138-148MHz),H4%20(141-151MHz)]


From what I understand, this style antenna provides it's own ground, so the ground plane is not required to complete the equation.
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Old 02-14-2020, 08:53 AM   #9
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Wire dipoles aren't the most convenient antenna to install on a vehicle. It wouldn't be a dual band antenna either. That notwithstanding, stretched along a roof it would be horizontally polarized which is opposite the convention of vertical polarization for VHF/UHF FM operation.


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Old 02-14-2020, 09:04 AM   #10
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I have an idea..it may prove to be crazy due to signal loss.

The jeep will always be towed behind me. I have room on the jeep for a very large whip and a place to install a duplex to allow 2 antennas. One for the 2m and another that would cover 70cm or gmrs.

The cable would be approx 60ft, and attach to the jeep at the front. I would use an a/b switch to change between feeding the jeep equipment vs the rv.

In the rv, another a/b switch to select between gmrs and the dual band ham.

Would rg58 be adequate in this situation?

The cable run would be primarily alongside all the rv wires which include 12v accessory items and other stuff...a target rich environment for RF contamination, especially at 40ft.

Would ferrite core spaced every 10ft along the cable help?
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Old 02-14-2020, 03:28 PM   #11
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Larsen makes a shorty dual-bander with a base spring. Perfect for a RV roof. I have mine mounted to an aluminum box screwed to the roof with some 1/2" aluminum stock screwed to the box for radials. This setup works great. I've beat it up pretty good a few times on trees with no ill effects. You can also swap the shorty out for a larger antenna when you're parked, but this thing works so well I've never bothered. The aluminum stock radials are rigid enough that they don't flop around in the wind.
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Old 02-14-2020, 05:08 PM   #12
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Larsen makes a shorty dual-bander with a base spring. Perfect for a RV roof. I have mine mounted to an aluminum box screwed to the roof with some 1/2" aluminum stock screwed to the box for radials. This setup works great. I've beat it up pretty good a few times on trees with no ill effects. You can also swap the shorty out for a larger antenna when you're parked, but this thing works so well I've never bothered. The aluminum stock radials are rigid enough that they don't flop around in the wind.
Do you have a part or model number of the antenna ?
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Old 02-14-2020, 09:27 PM   #13
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Well, I get using the antennas on the toad from the RV gives you more utility from one install. 60ft of 58/U is about 6dB attenuation at 450MHz, so a reduction of signal by a factor of four. Not unusable but not ideal, so only you know if that degree of degradation will work for you. i wouldn't worry too much about common mode but depending on the shielding of the line and how well isolated the antennas are you might get some ingress/egress RFI. I've never had to put ferrites on any mobile installation that weren't part of the antenna design, so if you go that route I'd try it without first to see if it's even a problem.

I think once you work through the logistics of routing that much coax through the RV and deal with connections you'll figure out putting a modest dedicated antenna on the roof of the RV is the more practical solution, but you can do whatever you decide will work for you. Having antennas on each vehicle gives you the most operating options.

Something I did on one of my RV trips is run my APRS box on a mag mount in one of my interior cabinets. The cabinet was a good 10' off the ground and the RV being essentially RF transparent I figured an antenna inside wouldn't work much different than one outside. Seemed to work OK but without a comparison to an outside antenna just how much difference couldn't be determined. Just planting the seed that any antenna is better than no antenna, even if it's a temporary one wherever you can put it. You have to be mindful of the RF exposure limits with whatever you end up with, even roof mount when the roof is something other than metal.

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Old 02-15-2020, 05:31 AM   #14
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Do you have a part or model number of the antenna ?
https://www.dxengineering.com/parts/lsn-nmo2-70sh
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