Antennas have to be tuned.

[WRFV510]

 Iv noticed alot of talk on GMRS forums of people asking about antennas and replies of this antenna will work between 4**Mhz and 470Mhz and never mention that it needs to be tuned.  By reading what people post it sounds like people dont know that they have to tune every antenna.  You have to trim the antenna to get a SWR below 2.0 or better. 1.5 or lower is ideal.  for GMRS you need to tune your antenna at 465.00000Mhz if you use repeaters.  if simplex only, 462.60000Mhz. you dont just install a antenna because it list that it works between. 420Mhz and 470Mhz. That means it can be tuned between those channels.  you tune at the center of the band you are using. or you can use the channel you will use the most.  your frequency range will be the channels you can get below 2.0SWR.  

[Jones]

True.  And the higher gain they are, the more narrow the bandwidth will be, and more difficult to tune.  There are no antennas that have enough bandwidth to be used from 420 MHz to 470 MHz without tuning, but the most bandwidth coverage is from quarter-wave antennas, which are about 6 inches long.  A quarter wave can cover from 433 to 468 with good results and fairly low SWR.  ...and watch out at ham fests and flea markets for quarter wave UHF antennas that seem too cheap.  I saw a guy once selling UHF quarter-wave NMO antennas for $1 each.  I measured one, and it was only 5-1/4 inches long.  Those are to cover 470-512MHz, and won't work for GMRS or ham.  If they are too long, you can grind them down. If they are too short,         .  .  .    _  _  _    .  _  .  .

 

Guys, if you are looking for a super high gain antenna for both Ham and GMRS use, just give it up.  There will be a compromise on one end, or the other.  If you must use just one antenna for both UHF ham and GMRS, then use a quarter wave tuned to 450.

[Lscott]

Guys, if you are looking for a super high gain antenna for both Ham and GMRS use, just give it up.  There will be a compromise on one end, or the other.  If you must use just one antenna for both UHF ham and GMRS, then use a quarter wave tuned to 450.

For a really high gain antenna that is likely true. For a modest gain one the answer isn't so clear cut. I recommended a Comet CA-2x4SR dual band antenna for a friend at work, who got his GMRS license, and looks forward to getting his Ham license as soon as local clubs open up for testing again. You can get it in either a UHF or NMO type mount.

 

http://www.cometantenna.com/wp-content/uploads/2013/09/CA-2x4SR.pdf

 

He mounted the antenna on the fender of his pickup truck. It would have been better on the roof but at 40 inches tall it made things difficult going into parking garages, drive thru windows at banks and fast food joints.

 

I used a Rigexpert AA-1000 antenna analyzer to do an SWR sweep of the operating range.

 

https://rigexpert.com/products/antenna-analyzers/aa-1000/

 

On UHF the SWR was OK. As long as the SWR is under 2:1 most radios should have no issues and work OK.

 

432 MHz - 2:1

438 MHz - <1.1:1

454 MHz - 1.9:1

464 MHz - 1.2:1

467 MHz - 1.6:1

469 MHz - 2:1

 

The SWR on VHF was even better.

 

140 MHz - 1.5:1

152 MHz - <1.1:1

160 MHz - 1.6:1

 

This is close to what the published spec's from Comet are for the antenna. While the UHF band has some notable dips and peaks in the sweep it is still very usable. The antenna was mounted close to the roof pillar. I would guess if it was mounted further away the SWR sweep would have looked a bit better.

 

You're right about a 1/4 wave having a large bandwidth. I built a couple out of PCB mount type female BNC sockets and some 1/16 inch stiff buss wire for the elements soldered to the center pin and the flange, bent down at 45 degrees for the ground plane. All about 6 inches long. Sitting on a table top the SWR scan showed a max SWR of 1.4:1 at 430 MHZ and 470 MHz after some slight trimming. The minimum of almost 1:1 was at  445 MHz.. I'm using this on top of a bookcase at the office with a handheld UHF radio. It works better than the typical rubber dummy load, called a rubber duck antenna, that the radios ship with from the manufacture.

[marcspaz]

This will give you an idea about typical usable bandwidth of a stacked phase dual-band VHF/UHF antennas.  I will shoot another video with a single band 5/8 wave a little later, so you can see the differences.

 

This is a Diamond NR7900A 2m/70cm.  https://www.diamondantenna.net/nr7900a.html

 

https://youtu.be/Rh6w46VM_Ng

[Lscott]

This will give you an idea about typical usable bandwidth of a stacked phase dual-band VHF/UHF antennas.  I will shoot another video with a single band 5/8 wave a little later, so you can see the differences.

 

This is a Diamond NR7900A 2m/70cm.  https://www.diamondantenna.net/nr7900a.html

 

https://youtu.be/Rh6w46VM_Ng

Very good! We could use more real world info like this. Far better than saying “I can hit such and such repeater x miles away.” I see questions always coming up asking if such and such antenna can be tuned for Ham and GMRS. Nothing like having the measurements which speak for themselves.

[marcspaz]

This is the other video... hopefully this will help.

 

https://www.youtube.com/watch?v=P5GiPLzVzbg

[Lscott]

This is the other video... hopefully this will help.

 

https://www.youtube.com/watch?v=P5GiPLzVzbg

For a quarter wave antenna it has some rather odd behavior. There seems to be several peaks and  dips in the SWR over the range. I would have expected to see just one dip at the resonate frequency and smoothly rising to either side, which is what I saw with the antenna I built. Also the EZNEC+ antenna simulation I ran for it showed the same thing.

 

I'm wondering if part of what you're seeing is caused by the UHF (SO-239 to PL-259) adapter used on the antenna analyzer. Of course those adapters are nowhere near a 50 ohm impedance. The simple 1/4 wave i made used BNC adapters which are 50 ohms. Have to be careful because there are 75 ohm versions too and hard to tell apart since the size is very similar.

 

The interface between the two adapters is likely around 1/2 inch, however due to the velocity factor of that part electrically it looks longer. Teflon has a dialectic constant or 2.1 and the velocity factor is the square root of the reciprocal of the dialectic constant. So for Teflon that works out to 0.69. At 462/467 MHz the RF travels through the connector interface at 69 percent the speed of light, or you can consider the connector looks longer by the factor of 1/0.69, or 1.45 times longer. For a straight barrel adapter, female to female SO-239, I measured the center OD and the shell's ID and using a basic formula to calculate the impedance I got around 28 to 29 ohms.

 

https://www.electronics-notes.com/articles/antennas-propagation/rf-feeders-transmission-lines/coaxial-cable-characteristic-impedance.php

 

So if we consider the adapter interface is around 0.72 inches long at 28 to 29 ohms we could use the lossless transmission line equations to figure out what the impedance will be with a given load on the other end. I did that for various frequencies for my own education and the results were "interesting." At the frequencies we are using for GMRS that impedance bump is rather large. I have a PDF of some calculations a while back along those lines just to see how it affected the SWR. If somebody wants them I would have to PM the file. I can't seem to attached anything to my messages in a long while, I keep getting an error that says I only have 666 some bytes left to attach a file. If I could I would.

 

I know this might be getting a bit deep for some people, but what really counts is the results.

[marcspaz]

Actually, its a type-A to so239.

 

Some of the wavering could have been from the cheap cable and mag mounted. My analyzer doesn't read cable impedance or capacitance on UHV, so we can't see what's happening on that side of things.

[Jones]

For a quarter wave antenna it has some rather odd behavior.

That is likely due to capacitance coupling of the ground plane.   That is a mag-mount antenna, so the ground plane is not DC coupled at the base of the radiator.

[marcspaz]

That is likely due to capacitance coupling of the ground plane. That is a mag-mount antenna, so the ground plane is not DC coupled at the base of the radiator.

Good point. I dont have an NMO mount on my vehicles and I don't have a UHF 1/4 wave antenna. Otherwise I would redo the test to see the difference. That would be another point of consideration for antenna tuning as well.

[Lscott]

That is likely due to capacitance coupling of the ground plane.   That is a mag-mount antenna, so the ground plane is not DC coupled at the base of the radiator.

This is a valid point. The antenna impedance is not a simple resistance when off resonance. Combine that with some capacitive coupling in series I’m sure will also generate some weird SWR characteristics.

[DOCSGMRS]

Another antenna that may work for dual band use ... Tram 1181 ... covers amateur and GRMS freqs ... I’ve seen a SWR analysis which gave acceptable ratios across the range, not ideal but acceptable.