Antenna

[Othergrampa]

If I set the swr on my truck(magnetic mount), and move it to my wife's car do I have to reset?

[Jones]

If your asking about GMRS/UHF, then once you get the antenna cut to specification, is should make no difference if you move the magnet mount to the top of a car, truck, train, or Maytag washer, it should be about the same SWR, since the quarter wave ground-plane is only about 6 inches.

 

If you use a mag mount, it just won't matter that much which vehicle it is on, it is a compromise, sure; but it should get the job done just fine in most cases.

 

If you want a great mobile antenna, I would suggest a quality permanent NMO mount type antenna, but most mag-mounts will do a very respectable job... with no need to re-tune after changing cars.

 

Basic unwritten rule of magnetic mount antennas:  --  Have at least a quarter-wave of metal all around the magnetic base for the ground plane.

 

This means:

 

About 6 inches (in radius) on UHF.

About 18-20 inches (again, radius) on MURS, VHF, 2-Meters

and about 8 feet on CB antennas,which is why mag-mounts don't work well there.  (hahaha.. Partial joke, partial truth)

[Othergrampa]

Thanks for the info.i sort of thought that may be the case but wanted to check.

[MIKEROD67X]

ok im new to the game  how do you set the swr on a gmrs antenna? i have a ut-72  mag mount

[Jones]

ok im new to the game  how do you set the swr on a gmrs antenna? i have a ut-72  mag mount

 

 

Any tunable antenna should have come with a cutting chart that shows how long the whip should be for a given frequency.  Check the chart, measure it, and cut it off to the correct length.  If you do not plan on using repeaters, then set the antenna length for 462.600 MHz.  If you plan on using repeaters also, then you might want to split the difference, and set the antenna tuning for 465.000 MHz.  That should give you good coverage for the GMRS band.

 

If you want to get picky, and actually check and set the SWR, then the first thing you need is an SWR meter that will work on UHF frequencies. The typical cheap CB type SWR meter will not work at UHF.  To get an accurate reading, you must also use a 1/2 wavelength cable going into your SWR meter.  This is frequency dependent, will vary depending a cable velocity factor, and IS critical.

 

NOW... All that being said, With your UT-72, I believe you have a fixed, non-tunable antenna.  It doesn't look like you can set the SWR on it.  It is just a quarter-wave whip on 2 Meters, and will function as a 3/4 or 5/8 wave on 440.  Either way you look at it, it isn't going to be much good on GMRS, as from what I can see, it is a Ham-band antenna.

[PastorGary]

Has anyone here tried either the    MFJ-874   OR   the  Diamond SX600   HF, VHF, UHF meters?

[n4gix]

I've used the Diamond SX600 for many years now and have been very pleased with it.

[Elkhunter521]

I have an MFJ-874. For accuracy, I have verified it on HF and it reads as expected for pwr and SWR. With uhf, it reads expected SWR after tuning two antennas by their trim charts. (Browning 1170 mag mnt, and Browning Br450 nmo). I have no verification of pwr readings on uhf however it read low for a Btech gmrs-v1 (3 watts) and acceptable for a Midland XMT400 (38.5 watts) only observed issue is poor quality PL239 connectors. Ive replaced both on the HF side. Not hard but should not have to.

[Jones]

For checking antennas, I use a Bird 43 THRULINE directional Watt meter with the frequency-correct insert slug for whatever band I am testing.  But then, I do this kind of work professionally.  These meters are very nice, but a bit out of price range for the typical radio enthusiast. a new Bird 43 will set you back about $350, and the slugs run from $100 to $300 a piece, depending on power level and frequency range.

 

...and they don't read SWR.  They read forward, and reflected power in Watts.  You then have to do the math to figure out the Standing Wave Ratio (SWR). (There's an APP for that)

[WRPP884]

Can a CB antenna be trimmed/tuned to work for GMRS? I have a Wilson Little Wil magmount base. The antenna is a stainless steel rod.

Trusting the internet I converted 465.000 MHz to wavelength of 25.38248 inches. I cut the CB antenna whip off at about 27". Then started cutting little pieces off and checking with a surecom swr meter. I never got a reading under about 2.5.  My antenna length is now about 20". And the swr reading is 2.9 to 3.0.

Obviously I started my experiment with very little knowledge.  Do you have any suggestions / corrections or where to go to learn?

[gortex2]

No. The base has a load in it also to make it 108" per say. You need a 6" antenna for a 1/4 wave antenna. Very reasonably priced on amzon. 

[UncleYoda]

6 hours ago, WRPP884 said:

Can a CB antenna be trimmed/tuned to work for GMRS?

You can trim the antenna but you don't want a full wavelength, use 5/8.   And mount it on center of roof if possible.  And the cable is probably an issue.  Small cables like RG58 and even smaller ones they use on these mag mounts are not good for UHF.  You'd have to cut the cable near the mount, put on a new counter, and connect it to some better quality cable.  Buying a new antenna would have been a better choice because you'd still have your CB antenna for a time when it may be useful again.

[gortex2]

Uh, since when is RG58 bad for UHF on a mobile radio ? There are hundreds of thousands of Police, Fire, EMS radios thru out the US that have an NMO with RG58 on them in UHF, 700, 800 Mhz. I have 6 NMO Mounts on my truck and 4 of them are RG58 standard Motorola NMO mounts. 2 are LMR as the new multiband came with it. 

[marcspaz]

On 2/11/2023 at 7:47 AM, UncleYoda said:

You can trim the antenna but you don't want a full wavelength, use 5/8.   And mount it on center of roof if possible.  And the cable is probably an issue.  Small cables like RG58 and even smaller ones they use on these mag mounts are not good for UHF.  You'd have to cut the cable near the mount, put on a new counter, and connect it to some better quality cable. 

 

To to be sure no one else reads your post and trashes an antenna or cable...

 

No.. you can't trim a base-loaded CB (HF) antenna to use on GMRS for more reasons than I feel like typing.  Antennas with a coil and/or capacitors have a very, very narrow usable frequency response.  On the wide side, it would be 3 or 4 MHz. On the narrow side, it may only be usable for a few hundred KHz. There is no way you're jumping from 27 MHz to 465 MHz (approximately the center of GMRS).

 

Also, there's no reason someone can't use RG58 cable in a mobile installation. Most mobile applications would only experience about 1.5dB of losses due to the cable.  Basically, that means 20w out of the radio would be 14w to the antenna. Not a big deal in the grand scheme of things. 

[bd348]

On my mobile I did as Uncle Yoda suggested, and cut the 20 feet of thin RG-174 from the mag mount down to just one foot in length, then adapted it into RG-58 for 6 feet, then into the radio.

If the mag mount comes with 20 feet of RG-58, that can be 2db, and so it may be worth cutting it back to what is actually needed.

[marcspaz]

22 minutes ago, bd348 said:

On my mobile I did as Uncle Yoda suggested, and cut the 20 feet of thin RG-174 from the mag mount down to just one foot in length, then adapted it into RG-58 for 6 feet, then into the radio.

If the mag mount comes with 20 feet of RG-58, that can be 2db, and so it may be worth cutting it back to what is actually needed.

 

2dB of loss is almost nothing in the GMRS world.  You would have to lose 6dB before you even see a difference on a properly calibrated S-meter on your radio and I would be willing to bet money you would never hear the difference while you are having a conversation.

 

In a situation like yours, I would be more upset about the connectors you added 1 foot from the magnet, that is now beating up my paint job while I'm driving around.

[bd348]

In my case, the thin cable goes right underneath the hood, and the connector is sealed in a layer of elec tape, sealer tape, then more elec tape.  I like that the bitty Midland mag mount is almost unnoticeable, and I can tuck it under the hood and cowl if i won't use it for a while.  It's one of those Jeep compromises, see.  By the way, that cable routing you mentioned whenever ago worked well.