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Author Topic: GMRS/70CM Antenna  (Read 3273 times)
KT4NR
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« on: August 16, 2016, 09:25:40 AM »

Doing some planning on a possible add on for the truck. Some of the groups in the area use GMRS/FRS for their trail rides. I am thinking of adding a 70cm DMR rig as well. Is there one antenna broad enough to cover 440-470? Most of the good ones I have checked on seem to Split at 450 or 460. I'd prefer it be a whip and NMO if possible.

I am hoping to just swap radios and not swap antennas as well for the rare times I need to use GMRS/FRS. (Less muss, less fuss)

Thanks for any ideas.

Dan S.
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K0BG
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« Reply #1 on: August 16, 2016, 11:06:20 AM »

You're going to find that swapping things back and forth isn't all it is cracked up to be. The best solution in my opinion is to install both the FRS transceiver, and its own antenna.
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W9IQ
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« Reply #2 on: August 16, 2016, 11:34:24 AM »

Use high quality coax, keep it short, and use a simple matching circuit for the other band.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KT4NR
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« Reply #3 on: August 16, 2016, 11:54:05 AM »

Alan,

I know swapping may not be totally fun, but it might only be once or twice a year that I need GMRS from what I can tell. Other groups use CB so the idea there is use the screwdriver and tune that to the right channel. For the CB I'll have a pigtail for power with power poles off the dedicated radio fuse panel that I can tap into as needed.

Honestly, I am trying to avoid more than 2 or 3 rigs in the truck full time. (Honestly how many can you use at once!) GMRS just is not high on my priority list. Sadly, I can't convince them all to go and be hams. (At $80 for a license you'd think being a ham would just be a financial savings LOL)

Dan S
KT4NR
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KT4NR
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« Reply #4 on: August 16, 2016, 11:55:35 AM »

Glenn, while losses are not really my worry (i need like 2 or 3 miles coverage max) not sure I am totally buying into that idea. Can you explain more?

Thanks!

Dan S
KT4NR
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W9IQ
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« Reply #5 on: August 16, 2016, 12:03:37 PM »

Dan,

Buy a 70 cm amateur band antenna and install it. With an antennalyzer at the end of the coax, measure the complex impedance of the antenna on your GMRS/FRS frequency. Plug those numbers into Elsie or equivalent to yield a simple LC circuit that presents a 50 ohm impedance to your FRS/GMRS radio.

You will experience some loss due to SWR in your coax but using a short and high quality coax will minimize this. Other than that you will not be able to detect the difference in performance from a dedicated GMRS/FRS antenna.

Capisci?

- Glenn W9IQ
« Last Edit: August 16, 2016, 12:29:13 PM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KB4QAA
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« Reply #6 on: August 16, 2016, 01:12:07 PM »

Simple alternative.

Remove your Ham 70cm whip and replace it with a piece of strong solid wire from the junk box.  Trim it to length for the GMRS band.  Swap as needed.
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K6JH
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« Reply #7 on: August 16, 2016, 02:43:32 PM »

Comet has the CA-2x4SR and CA-2x4SRNMO that they claim is under 2:1 up to 465 MHz. YMMV.

I don't believe it's legal to use for FRS, as those radios are supposed to have a permanently attached antenna. GMRS OK? (Yeah I know everyone just programs their ham rig to go out of band, but it would be nice to at least pay lip service to the legality.)


From http://www.cometantenna.com/amateur-radio/mobile-antennas/ma-dual-band/

 CA-2x4SR

BROADBAND VHF/UHF Dual Band Antenna

Dual band, designed to assist Search & Rescue Volunteers and Professionals
1.5:1 or less SWR:
144-148/440-450MHz
2:1 or less  SWR:
140-160/435-465MHz
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73
Jim K6JH
KK4YDR
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« Reply #8 on: August 19, 2016, 11:33:45 PM »

A larsen NMO 2/70 is wide enough to do both bands as long as you trim it so that the 70cm portion is barely within the <2.0 swr range (lowest frequency of 70cm) and GMRS is on the high end of the antennas band width.

it is a very tough to do trim and tune but with an analyzer you get it done quickly.

My larsen is actually tuned to do the mid to higher end of 2m and 70cm and can do MURS and FRS/GMRS. Well it can't do Murs Blue dot and green dot as those are 153mhz and too far out of the safe range for my taste.

But around 146mhz its about 1.2:1 and the low end of GMRS is safe. But I dont have a gmrs license so I don't go there. I only use HAM, FRS/MURS. I dont even use FRS, just monitor it. MURS is far far far superior and we use it for road trips etc...
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KT4NR
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« Reply #9 on: Yesterday at 09:29:43 AM »

Thanks All!

The idea of just cutting a quarter wave piece of wire is rather ingenious. Might be the easiest way to do it.

As for the Comet. I had a couple. Not a fan. They break way too easy. Never had one last more than a year. And no, I am not modifying any radios for out of band. Never have and do not see a need for at this time. Also, FRS and GMRS do share frequencies so you can run GMRS on an FRS frequency but only at lower power.

Glenn: The matching network is interesting but the losses...I might have to tinker with the alternative which would be a coaxial transformer. I have done that on HF but never heard of it done on VHF/UHF frequencies. Should work.



 
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KG4RUL
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« Reply #10 on: Yesterday at 10:09:34 AM »

The Larsen NMO4503CS http://productfinder.pulseeng.com/product/NMO4503CS is tuned for 450-470 MHz and is an NMO mount.
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W9IQ
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« Reply #11 on: Yesterday at 03:08:08 PM »

Dan,

i am not sure what losses concern you with the L network. I would guess any attendant losses would be less than 1 dB. But as you say, several good suggestions have been made.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KB2VUQ
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« Reply #12 on: Yesterday at 11:00:07 PM »

maxrad mwu4505s
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