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Antenna


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#1 Othergrampa

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Posted 30 May 2018 - 04:00 PM

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

#2 Jones

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Posted 30 May 2018 - 08:21 PM

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)


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#3 Othergrampa

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Posted 31 May 2018 - 04:48 PM

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

#4 mikerod67x

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Posted 02 June 2018 - 08:10 PM

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



#5 Jones

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Posted 03 June 2018 - 10:13 AM

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.


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#6 PastorGary

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Posted 03 June 2018 - 03:27 PM

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


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#7 n4gix

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Posted 04 June 2018 - 11:11 AM

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



#8 Elkhunter521

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Posted 04 June 2018 - 01:21 PM

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.
Be vewy vewy quiet.
I'm listening to my wadio!

#9 Jones

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Posted 04 June 2018 - 11:31 PM

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)


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