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GMRS / FRS - Discussions related to GMRS (General Mobile Radio Service) and FRS (Family Radio Service) communications

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  #21 (permalink)  
Old 04-03-2018, 11:27 AM
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Originally Posted by nafai View Post
Aren't those 5-6db gain more suited for open road / interstate travel and flat terrain?
ill bet its really a dBi rating
and as far as whipping tie it off to keep it from doing that
but running through trees is really 1/4 wave whip territory
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Old 04-03-2018, 11:30 AM
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Thus why I only run a 1/4 wave.
one might be surprised how happy a UHF radio is feeding a VHF 1/4 wave whip
several folks around here run like that with no trouble at all
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Old 04-03-2018, 11:31 AM
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I know some will cringe at this post .... I'm just throwing this out as an alternative for mounting antennas on Wranglers...

I use a mag mount for my Larsen NMO 2/70 ham antenna. I stuck the antenna on the passenger side cowl a year and a half ago. It was only meant to be a temporary measure, but it's worked so well, that it has kind of became permanent.

I live in the country and the nearest repeater is over 30 miles away. It's flat prairie here and I can reach many repeaters. We also have an APRS digi about 40 miles away on a 300 foot building. I have no trouble getting position fixes when I broadcast my location traveling around out here in the boonies.. If I check my tracks on aprs.fi, it shows that that very few of my position broadcasts aren't received by the digi as travel about.

This method obviously isn't 'best' method, but I does exactly what I want.
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Old 04-03-2018, 11:46 AM
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I don't know enough yet to cringe jwt873, but I do have a couple questions.

- Does your passenger mind, and I wonder about RF exposure that close from a 40-50w transmitter?
- Do you have any issues with noise due to proximity to the AM/FM antenna?
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Old 04-03-2018, 2:06 PM
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Originally Posted by k9wkj View Post
one might be surprised how happy a UHF radio is feeding a VHF 1/4 wave whip
several folks around here run like that with no trouble at all
Agreed. Often people new to the hobby/industry don't understand the trade offs regarding higher gain antennas. 1/4 wave are my go-to antennas for anything mobile. I'll run 1/2 wave antennas where I've got a lack of ground plane, but that's about it.
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Old 04-03-2018, 2:07 PM
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Originally Posted by nafai View Post
I don't know enough yet to cringe jwt873, but I do have a couple questions.

- Does your passenger mind, and I wonder about RF exposure that close from a 40-50w transmitter?
Good passengers should be considered expendable, especially when off road.

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- Do you have any issues with noise due to proximity to the AM/FM antenna?
Probably more like desensing the AM/FM radio when transmitting.
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Old 04-03-2018, 2:17 PM
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Just so I'm clear, when you all say 1/4 wave, are you talking about a 1/4 wave for some other frequency band than GMRS? My tuned GMRS antenna is a 5/8, but for 462 MHz and is only about 12 1/4" tall, perfect for offroading. A 1/4 wave for GMRS would be teeny, eh?

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Good passengers should be considered expendable, especially when off road.
Ha!
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Old 04-03-2018, 2:50 PM
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Originally Posted by nafai View Post
Just so I'm clear, when you all say 1/4 wave, are you talking about a 1/4 wave for some other frequency band than GMRS? My tuned GMRS antenna is a 5/8, but for 462 MHz and is only about 12 1/4" tall, perfect for offroading. A 1/4 wave for GMRS would be teeny, eh?



Ha!
My Laird 1/4 wave tuned for 450-470 MHz is 6". Nice and tight...
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Old 04-03-2018, 3:01 PM
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Gotcha, so a 1/4 is the same as a "unity" then? I was going to go that route but thought when I'm out of the mountains in the desert that I'd appreciate a little gain. First radio, first antenna, so I'm really just guessing.
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Old 04-03-2018, 3:14 PM
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Originally Posted by nafai View Post
Gotcha, so a 1/4 is the same as a "unity" then? I was going to go that route but thought when I'm out of the mountains in the desert that I'd appreciate a little gain. First radio, first antenna, so I'm really just guessing.
I'm in the hills of TN and I can talk to repeaters that are 30+ miles away (granted on mountains) loud and clear with a 40w Kenwood. You are correct on no gain/unity. Given attempting to getting up a little higher, I'd have probably gone with a slightly taller antenna, like you did, not so much for the gain, but for the height.
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Old 04-03-2018, 9:44 PM
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1/4 wave antennas are very broad banded, so they work well when you have a lot of spectrum to cover and want to keep reflected power reasonable (low SWR).
An approximately 6 inch tall 1/4 wave UHF antenna will work surprisingly well.
They are also a great solution if you have a commercial UHF radio and are using it to cover GMRS, LMR as well as the 70 centimeter amateur radio band.
Not hard to get 50MHz or more of useable bandwidth out of a UHF quarter wave.

I ran a 5/8ths wave vertical for a while back when I had my GMRS license. Worked well, but SWR was a bit high on 70cm. Mine didn't have a spring, so it was a little stiff. On the roof of a full size 4x4 truck, it was a bit of a hazard with low branches.
I replaced it with a 1/4 wave, and I couldn't tell any difference in performance on GMRS, but I lowered the SWR when on 70cm. On the flat lands, I had no issues getting 15-20 miles of range simplex on GMRS. Since I have a lot of hills and mountains around me, quarter wave tends to be the preferred choice when repeaters are above you and the horizon isn't that far off.

1/4 wave antennas, especially on UHF, 800MHz, etc. are -really- low profile. On top of a full size SUV or truck, they just about disappear. Plus, they are really inexpensive.

I run quarter wave VHF antennas on my truck now. With a commercial radio I can easily use it on the 2 meter amateur band as well as my work stuff (up above 158MHz). Putting the antenna on a proper analyzer shows low SWR from 144MHz well up towards 170MHz.

Here's the plot from a VHF quarter wave. 2.0:1 is a pretty good SWR number to stay under, so that's where I placed the markers. You can see 2.0:1 is easily done from 138-170MHz:


Here's a 5/8th's wave VHF antenna, you can see it's narrower useable bandwidth (and rises pretty fast):


If all you are using is GMRS, then a higher gain antenna is fine, if you don't have clearance issues. Any issues with repeaters being above the primary lobes of the radiation pattern are probably not going to be any sort of issue. The benefit of lobes launching off towards the horizon can be a benefit out on the plains, but I suspect you'd have a really hard time hearing the difference unless you were really right on the fringes of coverage. Another slight downside of longer/higher gain antennas is that they'll bend back when driving a highway speeds, which doesn't help radiation patterns much. A VHF or UHF quarter wave won't really do that to any noticeable effect.

The big benefit is that you used an NMO mount, which gives you the widest selection of antennas. That makes it really easy to swap out different types of antennas depending on what you need. Considering you can pick up a 1/4 wave UHF antenna for about $6.00, it might be a fun experiment.

Last edited by mmckenna; 04-03-2018 at 9:50 PM..
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  #32 (permalink)  
Old 04-04-2018, 9:20 AM
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Quote:
Originally Posted by nafai View Post
I don't know enough yet to cringe jwt873, but I do have a couple questions.

- Does your passenger mind, and I wonder about RF exposure that close from a 40-50w transmitter?
- Do you have any issues with noise due to proximity to the AM/FM antenna?
With any front mounted antenna, either the passenger is exposed, or the driver... The Jeep isn't my daily driver and I only carry passengers in it about 10% of the time, so it's not really an issue.

There is no noise generated by AM/FM antenna. The VHF signal has no affect on the AM/FM reception. (At least not with my factory 430 radio.. I very seldom listen to local AM/FM.
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