Antenna for GMRS repeater?

[WRTC928]

I know I'm likely to get a lot of differing opinions, but that's okay.

If you were putting up a GMRS repeater, what antenna would you use, assuming that for the time being, you're going to put it on a 30' telescoping flagpole?

[WRYZ926]

While the Comet CA-712EFC is a good antenna, it might be overkill and too big for what the OP needs. Especially since he is using a 30 foot telescoping flag pole. Plus we don't know what the local terrain is like where he lives. He might be better suited with something with a little less gain than the CA-712EFC.

And yet again, not everyone lives in a flat and wide open desert where they can tx over 200 miles. That kind of distances is quite rare for most of the country.

[OffRoaderX]

I DID put up a GMRS repeater and I used the Tram 1486.  I chose it because of its low price, low weight, very low wind-profile and because it does not look like a space-ship antenna.  It's also very simple to tune.

 

[WRTC928]

16 hours ago, OffRoaderX said:

I DID put up a GMRS repeater and I used the Tram 1486.  I chose it because of its low price, low weight, very low wind-profile and because it does not look like a space-ship antenna.  It's also very simple to tune.

 

If I am reading the specs correctly, the Tram 1486 is a dual-band amateur radio antenna. Did it require any adjustments to function well on GMRS? Would it serve as both a GMRS repeater antenna and an amateur 2m/70cm antenna? By that, I mean, did you have to do anything to it that would make it unsuitable for 2m/70cm use, or is it usable on all three bands?

[SteveShannon]

19 minutes ago, WRTC928 said:

If I am reading the specs correctly, the Tram 1486 is a dual-band amateur radio antenna. Did it require any adjustments to function well on GMRS? Would it serve as both a GMRS repeater antenna and an amateur 2m/70cm antenna? By that, I mean, did you have to do anything to it that would make it unsuitable for 2m/70cm use, or is it usable on all three bands?

There are variants of the 1486, including the -B variant:

https://www.amazon.com/200-Watt-Pretuned-Fiberglass-Antenna-Connector/dp/B092DWP4NG

Edited to add: I haven’t found a dual band Tram 1486.  All that I have found in a quick google search have been a tunable single band (UHF not VHF) antenna that comes with a tuning guide for whatever frequency you wish to use.

[OffRoaderX]

1 hour ago, WRTC928 said:

If I am reading the specs correctly, the Tram 1486 is a dual-band amateur radio antenna.

You're not reading the specs correctly.. it is a UHF antenna that requires (simple/easy) cutting for best performance on GMRS, OR it can be cut for 70cm - not both.

[WRYZ926]

The few Tram antennas that I have dealt with all need tuned. And they come with the directions on how much to cut off for the frequencies you want to use. It's pretty simple especially if you have a NanoVNA or antenna analyzer. Though you can use the SW-102 also, it will just take a little longer since you have to test at different frequencies to see where the SWR dip is at.

I always cut slightly long and then test the SWR and adjust from there in small increments. The last Tram antenna I cut/tuned was the Tram 1174. I cut it close (long) and then made 1/16" to 1/8" cuts until I got it where I wanted. I tuned mine for 467.600. My SWR is 1.4 at 467.600 and 1.2 at 462.600. 

[WRTC928]

6 hours ago, OffRoaderX said:

You're not reading the specs correctly.. it is a UHF antenna that requires (simple/easy) cutting for best performance on GMRS, OR it can be cut for 70cm - not both.

Ah. Thank you. I've looked at a lot of antennas. I must have confused it with something else.

[WRTC928]

And here's sort of an ancillary question: If I used, for example, a BTech GMRS-RPT50, I can use the mic on the repeater frequency, but it doesn't transmit on the simplex channels. What sort of duplexer would I need to run a repeater and a GMRS base unit on the same antenna? 

[OffRoaderX]

25 minutes ago, WRTC928 said:

What sort of duplexer would I need to run a repeater and a GMRS base unit on the same antenna? 

Not sure if one exists (the 'experts' will educate us both) but for this exact shituation I use a high-quality antenna switch.

[WSHH887]

37 minutes ago, OffRoaderX said:

Not sure if one exists (the 'experts' will educate us both) but for this exact shituation I use a high-quality antenna switch.

Are these, in principle, the same thing as the old coax switches one used for multiple inputs to a TV with a single input?

[OffRoaderX]

1 hour ago, WSHH887 said:

Are these, in principle, the same thing as the old coax switches one used for multiple inputs to a TV with a single input?

Yes, only way, WAY more expensive ...
 

 

[SteveShannon]

1 hour ago, WSHH887 said:

Are these, in principle, the same thing as the old coax switches one used for multiple inputs to a TV with a single input?

Yes, but they differ in two ways. They’re 50 ohms instead of 75 and the better ones ground the center conductors of any antennas that are not selected for use. 

[WSHH887]

Thanks Steve.  

[WRYZ926]

3 hours ago, WRTC928 said:

And here's sort of an ancillary question: If I used, for example, a BTech GMRS-RPT50, I can use the mic on the repeater frequency, but it doesn't transmit on the simplex channels. What sort of duplexer would I need to run a repeater and a GMRS base unit on the same antenna? 

You will definitely want a coax switch. A duplexer will not work for using a repeater and a mobile radio on the same antenna.

I have had good experiences with the Diamond brand of manual coax switches. I have a 2 port and 3 port switch. While they are not top of the line, they work well. The Diamond switches are definitely cheaper than Alpha Delta switches.

Here is the 2 port coax switch I have.

Diamond Antenna CX-210A Heavy Duty Wideband Coaxial Switches CX210A

[SteveShannon]

10 hours ago, WRYZ926 said:

You will definitely want a coax switch. A duplexer will not work for using a repeater and a mobile radio on the same antenna.

I have had good experiences with the Diamond brand of manual coax switches. I have a 2 port and 3 port switch. While they are not top of the line, they work well. The Diamond switches are definitely cheaper than Alpha Delta switches.

Here is the 2 port coax switch I have.

Diamond Antenna CX-210A Heavy Duty Wideband Coaxial Switches CX210A

But @WRTC928, if you do use a coax switch you must keep in mind that one or the other radio (repeater or base) will be transmitting into a very high SWR if activated when the other radio is connected to the antenna.  So, when you set the switch to connect the base station to the antenna, I would want to temporarily disable transmit on the repeater.  Although it’s unlikely to be activated by a weak signal from another station, it could still attempt to transmit an ID if it was recently in use or perhaps even in response to the use of the base station.

[WRYZ926]

1 hour ago, SteveShannon said:

f you do use a coax switch you must keep in mind that one or the other radio (repeater or base) will be transmitting into a very high SWR if activated when the other radio is connected to the antenna.  So, when you set the switch to connect the base station to the antenna, I would want to temporarily disable transmit on the repeater

I forgot to mention that fact. Thanks for bringing that up. You definitely do not want to transmit on the radio that is not directly connected to the antenna. So in the case of the repeater, I would make sure that transmit is turned off or just turn the repeater completely off when using the other radio.

I know most radios will cut the output power down when the SWR gets too high but why take the chance of burning up the finals.

[WSHH887]

Okay, while I am not interested in setting up a repeater, I find the topic interesting. So, other than cost what is prohibitive about using two antennas fairly close to one another?

[SteveShannon]

Just now, WSHH887 said:

Okay, while I am not interested in setting up a repeater, I find the topic interesting. So, other than cost what is prohibitive about using two antennas fairly close to one another?

Both will affect the propagation pattern of the other. Also, there’s always the possibility of desensing one receiver when the other radio transmits. Other than that no problem.

 

[WSHH887]

3 minutes ago, SteveShannon said:

Both will affect the propagation pattern of the other. Also, there’s always the possibility of desensing one receiver when the other radio transmits. Other than that no problem.

 

Okay, now I have to learn about propagation patterns. 
Just a thought, but would changing the elevation between the two antennas help?

 Recall seeing Mount Wilson, where all the broadcast antennas are, as a kid. It looked like a darned porcupine with all the antennas. I wonder how they dealt with propagation pattern issues. 

[OffRoaderX]

FYI - this is what I use:

I always fully power-off the other device when its not switched to use the antenna.

[SteveShannon]

16 minutes ago, WSHH887 said:

Okay, now I have to learn about propagation patterns. 
Just a thought, but would changing the elevation between the two antennas help?

 Recall seeing Mount Wilson, where all the broadcast antennas are, as a kid. It looked like a darned porcupine with all the antennas. I wonder how they dealt with propagation pattern issues. 

Absolutely!  Mounting one above the other would help immensely. 

[WSHH887]

2 minutes ago, SteveShannon said:

Absolutely!  Mounting one above the other would help immensely. 

Explains why the antennas on Mount Wilson are different heights. 
 

You know this is turning into one of those "the more you know, the more you know what you don't know" situations.  Turning into quite an interesting rabbit hole. 
 

Again, thanks Steve. 

[SteveShannon]

28 minutes ago, OffRoaderX said:

FYI - this is what I use:

I always fully power-off the other device when its not switched to use the antenna.

That one is especially nice because it shorts all of the unused center conductors to the shield (which is typically grounded) and it provides surge suppression for the center conductor of the selected antenna. 

[SteveShannon]

22 minutes ago, WSHH887 said:

Explains why the antennas on Mount Wilson are different heights. 
 

You know this is turning into one of those "the more you know, the more you know what you don't know" situations.  Turning into quite an interesting rabbit hole. 
 

Again, thanks Steve. 

Antenna physics are fascinating but I’ve just barely started learning. Every so often I receive an inch thick densely printed periodical called Proceedings of the IEEE Antennas and Propagation Society. Most of it is so far over my head that it’s indecipherable. The majority of it deals with microwave and higher frequencies up to around 10-11 gigahertz. Almost none of it is written for the frequencies of ham radio, although recently a couple of ELF articles appeared. I struggle to even skim through it. I mean it’s so far over my head that I don’t even know where to start, but there are people who actually understand this stuff!

[WRTC928]

17 hours ago, SteveShannon said:

Absolutely!  Mounting one above the other would help immensely. 

Do you mean one higher than the other on separate masts, or one directly above the other on the same mast? My understanding is that the latter can be done, but it requires some fancy calculations.