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GMRS Antenna Questions

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Ranothil

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Hey! This is my second thread on this forum! I greatly appreciate all of your help! :) Anyway, I have a few basic questions about GMRS Antennas. First, I need an omnidirectional antenna of some sort to put on my roof or in my attic. I was also wondering if it might be possible to recieve some UHF or VHF TV stations from such an antenna since the frequencies are so similar. I probably won't be transmitting more than 30-40 Watts from my Base Station. In addition, I would like to get a pair of HT's. I was wondering if there are any antennas that will fit right on to an HT that are like quarter wave dipoles or something. I was thinking something small, maybe under 3-4 feet to extend the range of the units slightly. I may not be able to mount it high, but I am hoping for 3-4 miles of reliable communication and I think a better antenna than a rubber duck may be necessary. The main thing I need though is an Antenna that could recieve at least some TV signals. (If my parents aren't going to get something free out of this they aren't ever going to let me do it) I need them to be impressed. Thanks for your help. If it is impossible then make sure to tell me so as I don't want to disappoint them to much :D
 

mmckenna

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An external antenna for a GMRS radio will likely pick up local/strong TV stations, but not very well. TV antennas are usually directional and have gain that improve performance. A omnidirectional antenna isn't going to work as well as a "real" TV antenna. Best solution would be to install a mast with a UHF/GMRS antenna on it and mount a separate dedicated TV antenna below it.

Also, you would not easily be able to have your GMRS radio and TV connected to the same antenna with out some expensive equipment. Sort of pointless when it won't work as well as separate antennas.

Getting a good UHF omni antenna up on the roof as high as you can and with good quality coaxial cable is going to give you pretty good coverage. Important part is to not go cheap on the coaxial cable. Cheap cable will lose a lot of the signal in the distance it takes to get to your radio.

After market antennas for a GMRS radio will help quite a bit, but you are going to need to research it and find one that fits the connector on top of your hand held radios. Some of the limitations that will come into play is that the antenna connector on top of the radio will not support a very big antenna without damage. An oversized antenna on the radio can damage the connector easily. The limited ground plane under the antenna (as compared to the external antenna) is going to limit things a bit. Personally I'd say put the money into the base antenna first, then decide if you need to upgrade the antennas on the hand helds.
 

Ranothil

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Thanks so much for your help! Do you think it would be better than a rabbit ear antenna on the top of a TV Set or one of those $50 flat antennas that you hide on the wall behind your TV. If it would be better than those I might be able to do it. We could at least get some local stations ;-)
 

mmckenna

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There are way too many variables to determine if it will work or not. Distance from the transmitters, surrounding topology, home construction, etc. The only way to know for sure is to find out what others are using around you, or trial and error.

Usually, you'll find, that getting a good antenna up high and in the clear will work best. Anything that is down low, especially behind the TV, is likely going to be a poor performer.

I use an old style TV antenna tucked up in the rafters of my garage. It works well, but I'm only 20 miles or so from the transmitters.
 

krokus

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Wirelessly posted (Opera/9.80 (Android 2.3.4; Linux; Opera Mobi/ADR-1309251116) Presto/2.11.355 Version/12.10)

You could use the GMRS antenna for DTV reception, but there are issues with that option. (In addition to what has already been mentioned.)

GMRS is vertically polarized, typically, and DTV signals are horizontally polarized. That difference will cost you some signal.

You will have to disconnect your GMRS radio when you want to watch television. (Without the aforementioned expensive equipment.)

DTV coaxial connections are at 75 Ohms, while the GMRS connections are at 52 Ohms. Running 52 Ohm coax and antenna into your DTV receiver will be the least troublesome.
 

RandyKnowles

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Ranothil -

In addition to the excellence comments so far, you need to consider the topography of the area you want to cover on GMRS. Is it flat terrain or hilly? GMRS base station antennas obtain their gain by folding the signal to radiate towards the horizon. Smaller antennas generally equal lower gain. Folded dipole antennas with 4 elements would probably be your best compromise to keep the antenna size limited but still have decent gain and an omnidirectional pattern. If you plan on using a hand held portable, the type of connector on the radio is going to be a big factor.

Of course the higher you can get the GMRS antenna the better your range will be. What antenna transmission cable you will need will depend on the total distance from your radio to the antenna. Antenna cable line loss is measured in decibels (db) and RG8 cable loss at UHF is something on the order of 5.2 db per 100 feet, a totally unsatisfactory amount. 3db in roughly equivalent to 1/2 loss, so that will give you an general idea. Also, the type of connector on a UHF cable is important. PL259 types have a lot of loss, and Type N is the preferred connector. However, I have never heard of a portable radio that has a type N connector on it. Your best alternative there would be a BNC style connector on the radio. if you are going to have 50 feet or more of transmission line, then you will need LMR400 type cable at the least.

The comment about putting a horizontally polarized tv antenna on a mast under neath your GMRS antenna is a good suggestion. But be sure to have plenty of vertical separation otherwise when transmitting on GMRS you can overlaid the tv's front end.

Best of luck and let us know if you have any more questions.

Randy Knowles, KAA 8142
 
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