GMRS duplexer?

[ClemsonSCJ]

Hey guys I’m reading and studying up to get my tech license. I’ve currently got an Icom 2730A in my car and I’ve got all the GMRS channels programmed into it. We’ve also got a GMRS repeater in our area that I hear people on quite often, so my plan is to get my GMRS license in addition to my tech since this radio has the ability to be modded to transmit 50W in GMRS. The issue is I know my SWR on those channels would probably fry my radio eventually so I’d like to get a duplexer and have a designated GMRS antenna. But I’ve done a little google searching and am having a hard time finding a duplexer that will split 460MHz and up from everything below 460MHz. Do they make such a thing?

 

[mmckenna]

Hey guys I’m reading and studying up to get my tech license. I’ve currently got an Icom 2730A in my car and I’ve got all the GMRS channels programmed into it. We’ve also got a GMRS repeater in our area that I hear people on quite often, so my plan is to get my GMRS license in addition to my tech since this radio has the ability to be modded to transmit 50W in GMRS. The issue is I know my SWR on those channels would probably fry my radio eventually so I’d like to get a duplexer and have a designated GMRS antenna. But I’ve done a little google searching and am having a hard time finding a duplexer that will split 460MHz and up from everything below 460MHz. Do they make such a thing?

So, a couple of issues I feel the need to point out. What you do with this information is totally up to you. I'm not the radio police and not trying to play that role.

The Icom 2730 is an amateur radio. While they can be modified to transmit out of band (often referred to as a MARS/CAP mod), the radio does not have transmitter type certification to transmit outside the amateur radio bands.
It is a violation of the GMRS rules to use a radio that does not have type acceptance for the service.

It's up to you to do what you want with that info.


As for the antennas...
I used to be active on GMRS. I had an appropriate UHF radio that had Part 95 certification that made it legal to use on GMRS. I also have my amateur license, so I'd use it on the 70cm ham bands. I used a 1/4 wave UHF antenna tuned for around 460MHz. That landed it between the lower end of the 70cm repeater segment and the GMRS repeater inputs. A 1/4 wave antenna is very broad banded and will provide acceptable SWR across those frequencies and will not result in the fryage of your radio.

Since it's a dual band radio, you could install a 1/4 wave VHF antenna tuned for the 2 meter band. That will act as a 3/4 wave on UHF and will cover 70cm and GMRS nicely. Drawback is that radiation pattern is less than ideal for UHF, but it'll work fine, I did it for years with a dual band ham radio.

 

[sallen07]

The issue is I know my SWR on those channels would probably fry my radio eventually so I’d like to get a duplexer and have a designated GMRS antenna.

What antenna do you currently have on your car? Something like this will cover 2m/70cm/GMRS:

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=191_192_200_450_453&products_id=8128

You are unlikely to find a commercial duplexer that will split GMRS from 70cm.

 

[Project25_MASTR]

There are antennas out there that will cover what you need. I don't recommend the Comet sallen linked to though. It's a very heavy antenna and can cause damage to the roof a vehicle when used with a standard NMO mount.

I'd typically run something like a Panorama triband antenna (146-174/430-480/764-894 MHz) or the antennas using a diplexer and run a standard VHF 1/4 wave and something like a PCTEL wideband knob (430-480 MHz).

 

[sallen07]

There are antennas out there that will cover what you need. I don't recommend the Comet sallen linked to though. It's a very heavy antenna and can cause damage to the roof a vehicle when used with a standard NMO mount.

Just curious ... do you mean heavy or high wind load?

I was unable to find weight anywhere, but it *is* pretty long (40"). I've got it on a NMO magmount (sorry mmckenna) since I need to be able to put it on my truck only when I need it on long trips, which are few and far between nowadays. As long as I have 10' clearance under bridges I'll be good.

 

[ClemsonSCJ]

So, a couple of issues I feel the need to point out. What you do with this information is totally up to you. I'm not the radio police and not trying to play that role.

The Icom 2730 is an amateur radio. While they can be modified to transmit out of band (often referred to as a MARS/CAP mod), the radio does not have transmitter type certification to transmit outside the amateur radio bands.
It is a violation of the GMRS rules to use a radio that does not have type acceptance for the service.

It's up to you to do what you want with that info.


As for the antennas...
I used to be active on GMRS. I had an appropriate UHF radio that had Part 95 certification that made it legal to use on GMRS. I also have my amateur license, so I'd use it on the 70cm ham bands. I used a 1/4 wave UHF antenna tuned for around 460MHz. That landed it between the lower end of the 70cm repeater segment and the GMRS repeater inputs. A 1/4 wave antenna is very broad banded and will provide acceptable SWR across those frequencies and will not result in the fryage of your radio.

Since it's a dual band radio, you could install a 1/4 wave VHF antenna tuned for the 2 meter band. That will act as a 3/4 wave on UHF and will cover 70cm and GMRS nicely. Drawback is that radiation pattern is less than ideal for UHF, but it'll work fine, I did it for years with a dual band ham radio.

Yeah I assumed it probably wasn’t legal to use the same radio for both, but my logic is if I’m legal to do both, but the only thing making it illegal is the fact that I’m using one radio and they want me using separate radios. At that point in my eyes the FCC had become overly redundant. And I get it, it opens the door for people using illegal power on FRS but that’s something that should be dealt with when it happens. We shouldn’t create laws to prevent people from breaking other laws.


Anywho, I did the MARS mod on my radio when I got home. Super easy. With my current antenna (Larsen NMO2/70B) my SWR is in the high 2’s to mid 3’s on all the 50W GMRS channels. It’s about the same on the 5W channels. For kicks and giggles I checked it on the 0.5W channels and its in the 7’s (yikes!). Not worried about that though. That is puzzling though why those particular frequencies have such a high SWR when they’re right in the middle of the band.

 

[hill]

You can use one radio for both ham and GMRS, but your radio is not type accepted for GMRS.

Most likely you need to find a commercial radio with the correct Part 95 acceptance and then you can just add any ham 70 cm repeaters and simplex channels you wish. McKenna is good is good on which radios can be used for this.

 

[mmckenna]

Yeah I assumed it probably wasn’t legal to use the same radio for both….We shouldn’t create laws to prevent people from breaking other laws.

Yeah, I'm not the "radio police" so it doesn't matter to me what you choose to do.

But for the record, it goes beyond just simply making laws for the sake of making laws. There's some really good reasons for the type acceptance rules that most choose to overlook.

Anywho, I did the MARS mod on my radio when I got home. Super easy. With my current antenna (Larsen NMO2/70B) my SWR is in the high 2’s to mid 3’s on all the 50W GMRS channels. It’s about the same on the 5W channels. For kicks and giggles I checked it on the 0.5W channels and its in the 7’s (yikes!). Not worried about that though. That is puzzling though why those particular frequencies have such a high SWR when they’re right in the middle of the band.

The issue with the itinerant channels likely isn't anything to do with the antenna. It's probably an issue with your SWR meter calibrating correctly to such a low power. The SWR should be the same no matter what the power output is. SWR is a ratio of power being sent to the antenna compared to what is reflected (not radiated) by the antenna.

So, you need a more appropriate antenna. Not sure what you are using now, and if it'll work on GMRS.

The VHF 1/4 wave whip is a cheap/easy option, but will have less than perfect radiation pattern on UHF.
Here's a VHF 1/4 wave whip on top of one of my trucks. This is showing the UHF spectrum, you'll see that the SWR is just fine on the GMRS channels and more than acceptable on 70 centimeter band:

You do have some other options that will work:

Option 1:
Get a wider bandwidth dual band antenna.
Kind of my least favorite option, but it'll work if you find the right one….
Here's the UHF plot for a Larsen NMO-2/70SH. Good SWR on 70cm and good SWR on GMRS:

And it'll work just fine on VHF.

Option 2:
Get a diplexer (different than a duplexer) to split your radios antenna output to a separate VHF and UHF antenna. Connect the VHF port to a VHF antenna suitable for 2 meter band. 1/4 wave antennas work well and are cheap.
Connect the UHF port to a 1/4 wave UHF antenna. Tune it for around 460MHz. That'll give you more than acceptable SWR on both 70cm and GMRS. The 1/4 wave UHF antenna will have a good radiation pattern and they are broad banded enough to work just fine over a wide swath of the UHF spectrum.

 

[Project25_MASTR]

Just curious ... do you mean heavy or high wind load?

I was unable to find weight anywhere, but it *is* pretty long (40"). I've got it on a NMO magmount (sorry mmckenna) since I need to be able to put it on my truck only when I need it on long trips, which are few and far between nowadays. As long as I have 10' clearance under bridges I'll be good.

The CA2x4SRNMO by itself weighs right at 1 lb where other antennas weigh less than a half pound. It is a very stiff antenna and provides little to no flexibility. It also has an extremely high center of mass (about 12 inches from the bottom of the antenna). Compared to other antenna designs, (such as the Larsen NMO-2/70) it transfers a lot more stress to the mount itself. A good antenna strike (50% up the antenna or higher) is very capable of ripping the brass NMO mount itself out of the sheet metal it is mounted in (or shearing the two locating tabs on the mount).

 

[ClemsonSCJ]

The CA2x4SRNMO by itself weighs right at 1 lb where other antennas weigh less than a half pound. It is a very stiff antenna and provides little to no flexibility. It also has an extremely high center of mass (about 12 inches from the bottom of the antenna). Compared to other antenna designs, (such as the Larsen NMO-2/70) it transfers a lot more stress to the mount itself. A good antenna strike (50% up the antenna or higher) is very capable of ripping the brass NMO mount itself out of the sheet metal it is mounted in (or shearing the two locating tabs on the mount).

They sell the spring to go right above the base to mitigate that.

 

[sallen07]

They sell the spring to go right above the base to mitigate that.

Yep. I added that to mine. Plus it's on a magmount, so worst case it will tip over. I have to remember not to go to the carwash with it on!

 

[tweiss3]

For what it's worth, the Larsen 2/70SH and Larsen NMO150/450/758 antennas work well on 2m ham(unity gain) , 70cm ham and GMRS below 2:1 SWR. Personally, I've switched to the 2/70SH for better 2m operation (marginally), and reused the NMO150/450/758 for scanner duty.

Both hold up to multiple car washes, parking deck hits and trees, though the bottom 2-3 inches is protected from a base hit by the roof rack bars, so it can't be sheared off at the base in my installation.

 

[bharvey2]

I have both ham and GMRS licenses and use the same radio for both in one of my vehicles. I use the 1/4 wave antenna shown below and it gives acceptable SWR and performance on both.

https://www.amazon.com/Nagoya-NMO-72-Antenna-440Mhz-Requires/dp/B0744QPSG8/ref=pd_di_sccai_3/143-3870974-0774123?_encoding=UTF8&pd_rd_i=B0744QPSG8&pd_rd_r=5f7c32b4-423b-4a19-b311-f8e46901d589&pd_rd_w=NHsID&pd_rd_wg=rcOPa&pf_rd_p=c9443270-b914-4430-a90b-72e3e7e784e0&pf_rd_r=Y652SZQ2FZGKEJWED5Y6&psc=1&refRID=Y652SZQ2FZGKEJWED5Y6

While I generally prefer Laird or Larsen, I haven't had any troubles. I just keep a spare.

 

[Project25_MASTR]

They sell the spring to go right above the base to mitigate that.

Now they do. When the antenna was first released they did not (took them a few years actually). I'm also speaking from experience here...I had one that ripped the mount out of the roof of a vehicle. I repaired the roof damage and replaced the mount...and antenna. A week later Comet announced the spring.

With the spring you relive some of the shock loading from striking something but all of that mass is still swaying around up there flexing and straining the mount.

 

[ClemsonSCJ]

I spent my day at work today being a worthless POS and researching various antennas. I almost pulled the trigger on the Comet but then stumbled across the Browning BR-137. Spent a good while looking at reviews and everyone seemed to be super impressed with the performance. Found several reviewers who swapped from the antenna I now have and said it seemed to perform at least as good with the added benefit of extended band coverage. Plus it’s half the length of the Comet so I don’t have to worry about any potential issues with the weight.

 

[hill]

Wouldn't buy the Comet antenna, as they are lower quality ham antennas from overseas. Plus the one linked on this thread has a gimmick fold over feature and these have been know to fail with folding over when driving at speed.

For the price of the Comet antenna you most like could ourchase a Larsen or Laird for less money that will out last the more the a few vehicles it is mounted on

 

[mmckenna]

Tram/Browning are the Baofengs of the antenna world.

They are cheap Chinese knock off antennas, copies of professional antennas. I've looked at some and never been impressed, but I've yet to fully dissect one. I have taken apart some of their other antennas and connectors and it's not high quality stuff. You've done some nice work on your truck with your installs, I'd say take some time to make the decision on this and make sure you get a good antenna that will meet your needs. Tram/Browning is pretty low tier stuff.

You need to be careful of the ratings, as they are often from hams/hobbyists that don't have quality antennas to compare to, or don't have appropriate test equipment to give you anything other than a "my ears say…." review.

For a bit more, you can get the real versions of those antennas from PCTel or Laird.

If I was doing this, a better option would be to go with a diplexer and split your VHF and UHF off into separate feeds. Use a 1/4 wave antenna for VHF. Use a separate UHF 1/4 wave antenna. That'll give you better performance and more useable bandwidth for 70cm and GMRS.

The other issue with those big fat antennas is there is zero flex at the base. If that's going to be on the roof of a truck, you'll need to carefully avoid low tree branches.