How far from a GMRS repeater antenna does 2m/70cm antenna need to be?

[OkieBoyKJ5JFG]

When I moved here almost 3 years ago, I brought my GMRS repeater with me and set it up on a 30' telescoping flagpole beside the house with a Comet 712EFC antenna. I got an amateur license in January of this year, and I've been running a 2m/70cm radio with a mag-mount mobile antenna on a pizza pan in the floor of my living room. The repeater antenna is about 20' away and so far, there doesn't seem to be any issue with desensing between the two, but the separation is both horizontal and vertical. I'd like to put up a dual-band antenna outside. I hope to build a tower someday, but for now, it will probably also be on a flagpole. How far apart should the two antennas be to avoid interference? Obviously, more distance is better, but what's realistic?

 

[prcguy]

50 watts into a 6dB gain antenna 20ft from another 6dB gain antenna at the same height will give about 17dBm or 50mw at the other antenna not counting any feedline loss. That's not bad and if you reduce antenna gain or have less power to the GMRS antenna there will be less power into the other radio. 20w into a 6dB antenna with the other antenna being a Discone there would be about 8dBm or 6 milliwatts not counting feedline loss.

 

[OkieBoyKJ5JFG]

50 watts into a 6dB gain antenna 20ft from another 6dB gain antenna at the same height will give about 17dBm or 50mw at the other antenna not counting any feedline loss. That's not bad and if you reduce antenna gain or have less power to the GMRS antenna there will be less power into the other radio. 20w into a 6dB antenna with the other antenna being a Discone there would be about 8dBm or 6 milliwatts not counting feedline loss.

Okay, so 20 feet and they'll probably both perform pretty well. I think I can get them at least 30 feet apart by putting them in different parts of the house. The repeater doesn't really need to be in the same area as any other radio. I don't have to interact with it much; it pretty much just sits there and does its thing.

 

[prcguy]

Okay, so 20 feet and they'll probably both perform pretty well. I think I can get them at least 30 feet apart by putting them in different parts of the house. The repeater doesn't really need to be in the same area as any other radio. I don't have to interact with it much; it pretty much just sits there and does its thing.

I would expect a 70cm ham radio to get some interference but no damage.

 

[KF0NYL]

I have my Comet GP9 dual band antenna and my Comet CA-712EFC antenna spaced about 50 feet apart. The GP-9 is on a mast at the one end of my house while the CA-712 is on a j mount at the other end. The base of each antenna is at the same height above the roof. I don't have any issues or interference with my dual band radio while on 70cm or with my GMRS radio. Both radios are set to high power (50 watts).

I also run a 50 watt dual band radio and a 50 watt GMRS in my vehicle. I leave both radios set to high power (50 watts). I am using a Comet SBB-1 dual band and a Tram 1174 tuned for GMRS. The antennas are only three feet apart from each other. Again no issues or interference between the two radios.

I am using a TYT Th-7800 and Wouxun KG-1000G in the vehicle. I also have a Wouxun KG-1000G in my shack. I was originally using another TYT TH-7800 in the shack but have since switched that one out for an Icom IC-2730.

 

[OkieBoyKJ5JFG]

I have my Comet GP9 dual band antenna and my Comet CA-712EFC antenna spaced about 50 feet apart. The GP-9 is on a mast at the one end of my house while the CA-712 is on a j mount at the other end. The base of each antenna is at the same height above the roof. I don't have any issues or interference with my dual band radio while on 70cm or with my GMRS radio. Both radios are set to high power (50 watts).

I also run a 50 watt dual band radio and a 50 watt GMRS in my vehicle. I leave both radios set to high power (50 watts). I am using a Comet SBB-1 dual band and a Tram 1174 tuned for GMRS. The antennas are only three feet apart from each other. Again no issues or interference between the two radios.

I am using a TYT Th-7800 and Wouxun KG-1000G in the vehicle. I also have a Wouxun KG-1000G in my shack. I was originally using another TYT TH-7800 in the shack but have since switched that one out for an Icom IC-2730.

I had wondered how people ran multiple radios in a vehicle without interference. Obviously, an antenna with no significant gain will interfere less with another antenna, but there's no way on a vehicle to get them very far apart.

 

[KF0NYL]

The Tram 1174 has 3 dBd of gain and the Comet SSB-1 has 2.1 dBi of gain on 70cm which is basically zero gain in dBd.

The Comet GP-9 has 9.7 dBd of gain on 70cm and the Comet CA-712EFC has 6.8 dBd of gain.

I converted dBi to dBd where the dBd value was not listed. You take the dBi value and subtract 2.14 to get the dBd value.

Ideally one will want to keep the mobile antennas at least 1 full wave length apart. But that is not always possible.

My base antennas are higher gain and why I tried to keep them spaced apart as much as possible. I don't get any interaction/interference between 70cm and GMRS base radios with the antennas 50 feet apart.

 

[OkieBoyKJ5JFG]

I left the repeater set at 50 watts "just because", but I have pretty advantageous topography at my house and I typically run the ham base unit at 12 watts because I've discovered -- with only a couple of exceptions -- that if I can't reach it with 12 watts, I can't reach it with 50 watts either. There are a couple of 2m repeaters right at the fringe of their signal which I can hear but I have to bump up to 25 or 50 watts to be heard by them. I certainly may be able to dial down the power on the repeater without any significant change in coverage. I'll have to drive around and check it out at various locations.

If I understand correctly, issues will only occur if both units are transmitting simultaneously, and more power means more chance of feedback. Do I have that right? Also, (again, if I understand correctly) there should be no significant risk of interference between frequencies in the 2 meter band and the GMRS band. True?

 

[KF0NYL]

You should definitely not have any issues between 2m and GMRS. And if your antennas are far enough apart then you won't have any issues between 70cm and GMRS.

I'm running 50 watts on my dual band and GMRS base radios going into high gain antennas without any issues. Granted I do have 50 feet of separation between the two antennas.

I've never tried transmitting on both radios at the same time. But I can be on 70cm simplex or the local repeater at the same time was being on the local GMRS repeater or simplex. I can talk on one radio and do not get any interaction/interference on the other radio.

 

[OkieBoyKJ5JFG]

The Tram 1174 has 3 dBd of gain and the Comet SSB-1 has 2.1 dBi of gain on 70cm which is basically zero gain in dBd.

Just an aside, in my admittedly limited experience, gain doesn't seem to count for much in a mobile antenna. I'm guessing that's because the whole point of a mobile radio is to travel around, in which case the terrain becomes an even bigger factor than with a fixed unit. Also, (I'm guessing) because you just can't get really high gain in a mobile antenna. I suppose it might be different on simplex, but with repeaters, it seems that if I can get it, I can get it, and if I can't, I can't. There's certainly a difference in clarity, but for the most part, I can use the same repeaters with a mobile and a handheld.

I wouldn't say ham is an entirely different world than GMRS, but I am definitely learning some new things. GMRS is mostly used on simplex, and power does make a difference in distance, for example from one mobile to another. I have thus far never used amateur frequencies on simplex, so I can only guess that the same may hold true. GMRS is mostly "pick a channel and press the PTT" and you either reach them or you don't. With multiple bands and thousands of available frequencies, things get a bit more complicated. If it seems like I don't know much, it's because I don't. I appreciate the information you guys and gals provide.

 

[KF0NYL]

I talk on repeaters and simplex on my base radios and mobile radios using 2m, 70cm, and GMRS. Our club repeaters are on a radio station's antenna. The 2m and 70cm antennas are at 900 feet and the GMRS antenna is at 450 feet.

I find that I have the exact same dead zones on both 70cm and GMRS while mobile. Yes terrain and foliage affects UHF more than VHF. I have learned to dislike pine and cedar trees even more since getting my amateur and GMRS licenses. Pine and cedar trees play havoc with UHF frequencies.

You need a taller mobile antenna to get any kind of gain. And unfortunately I am restricted on antenna height since I park my 23 Ford Escape in the garage and also use parking garages when going to doctors appointments. I would love to run my Comet 2x4SR on the Escape but it is way too tall. The Comet 2x4SR is 40 inches tall. It has 1.7 dBd of gain on 2m and 4 dBd of gain on 70cm. It also works great for MURS and GMRS along with public safety bands.

A lot of farmers and hunters use simplex on GMRS. But we also have several GMRS repeaters in the surrounding area. And the GMRS repeaters do get used quite a bit.

Each area will be different on what bands are active or not. 2m sees the most use around me with 70cm and GMRS a close second and third. No one uses the 1.25m/220 MHz band here in Mid Missouri.

 

[OkieBoyKJ5JFG]

You need a taller mobile antenna to get any kind of gain. And unfortunately I am restricted on antenna height since I park my 23 Ford Escape in the garage and also use parking garages when going to doctors appointments. I would love to run my Comet 2x4SR on the Escape but it is way too tall. The Comet 2x4SR is 40 inches tall. It has 1.7 dBd of gain on 2m and 4 dBd of gain on 70cm. It also works great for MURS and GMRS along with public safety bands.

A lot of farmers and hunters use simplex on GMRS. But we also have several GMRS repeaters in the surrounding area. And the GMRS repeaters do get used quite a bit.

Each area will be different on what bands are active or not. 2m sees the most use around me with 70cm and GMRS a close second and third. No one uses the 1.25m/220 MHz band here in Mid Missouri.

I have a 2x4SR coming today to mount on my truck, but it's just not practical to put a 40" antenna on my little Chevrolet Trax. I have an NMO mount on the roof, but adding 40" would make it impossible to park in the garage or use a lot of commercial drive-throughs without removing it, and I just don't want that hassle. I don't drive the truck as much, so I don't mind removing it occasionally. If things really went south (tornadoes, fires, etc.) I'm more likely to use the truck than the car, so better communication in that vehicle makes sense.

My repeater gets almost no use, but that's because I keep it pretty private. I put it up because I had it, but also because it enables me to communicate from the family farm several miles from the city. There are a handful of people I'd like to be able to contact in the event of an emergency, but it's not really available for public use. In Alaska, I kept it in a cabin in the middle of nowhere and only turned it on when I was there.

I see the same pattern of use here. There's a 1.25 meter repeater in Oklahoma City and I usually try it when I'm close enough, but so far, I've never gotten a response.

 

[KF0NYL]

I run the Comet 2x4SR on my SxS with a 20 watt radio and it gets out great. I have the antenna mounted on the back rack of my Honda Pioneer 500. I do have one of those hitch mounted expanded metal cargo carriers that I modified to fit the Pioneer 500. It works pretty good for a ground plane.

It's too much of a hassle to try and run the 2x4SR on my Escape. I know myself and I'll forget to fold it over and tear things up.

As far as your base station antennas go, either separate them vertically or keep them as far apart horizontally as possible.Mine are 50 feet apart and I don't have any issues.

 

[OkieBoyKJ5JFG]

As far as your base station antennas go, either separate them vertically or keep them as far apart horizontally as possible.Mine are 50 feet apart and I don't have any issues.

Separating them vertically is definitely an option. The 2m/70cm repeaters I use regularly are accessible from my house with an HT. Almost any antenna at any elevation will work for them, but they have very good altitude on their end. The GMRS repeater needs to be higher because my friend across the river lives in a simplex dead zone. Getting the antenna up 30' makes it possible for him to talk to me.