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Tuning a mobile rig.

Nov 2nd 2012, 18:47

Stan_TheGunNut

Joined: Nov 2nd 2012, 16:46
Total Topics: 0
Total Posts: 0
I am a new ham and have recently purchased a mobile radio to install in my truck. The radio is a Kenwood d710a. I also purchased a Comet 146/440 MHz magnet base antenna. I'd like to ensure the setup is properly tuned prior to using it which leads me to my question. I have a Radio Shack SWR meter that was purchased to tune a CB. Will the same meter work with ham radio or do different frequencies require different meters? Also, how does one go about turning this antenna if tuning is required. It does not appear to be adjustable.

Thanks for your help.
Nov 2nd 2012, 19:56

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

Yes, SWR meters typically have frequency ranges.
http://www.radioshack.com/graphics/uc/rsk/Support/ProductManuals/2100534_PM_EN.pdf
Radio Shack 21-534 is only rated for 2 to 30MHz--this does not cover the 146/440MHz.

Sometimes, a meter will work over more than its rated range. You can test this by connecting it to a dummy load known to have a low SWR at the frequency of interest--it should measure the dummy load as having a low SWR.

The use of an antenna analyzer has become popular for antenna adjustment. These devices can look at the antenna over a wide frequency range--which can be quite helpful if the antenna resonance is out of band.

http://www.eham.net/reviews/detail/3143
one of these reviews includes a tuning procedure

Zack Lau W1VT
ARRL Senior Lab Engineer
Nov 3rd 2012, 18:42

Stan_TheGunNut

Joined: Nov 2nd 2012, 16:46
Total Topics: 0
Total Posts: 0
Thank you for taking the time to reply and for the links. I may see if one of my coworkers has the appropriate meters to check things out. I'd hate to mess up a new radio by not having things set up correctly.
Nov 7th 2012, 23:13

KM3F

Joined: Mar 6th 2008, 13:50
Total Topics: 0
Total Posts: 0
Just to comment, a 50 ohm load that is near flat at the test frequency will show little to no swr with any meter used if there is no reflected power.
It's when there is reflected power, that accuracy vs design counts.
With a RS meter, on 2meters you may get the match down within acceptable limits but it may stiil be in some error as compaired to a meter designed for the frequency.
Said another way there is much more to SWR meter design than ,meets the eye.
On 70 cm. all bets are off using a RS meter designed for 11 meters.
On both bands the length of the sample line in the 11 meter unit is a substantial fraction of the wave length being measured so it makes a noticable difference in accuracy.
To sample reflected power over a 'small length' is the better way or use s torriod transformer designed for the frequency.
I bring this up to help a new Ham to know there is more to this than to offer a difference of opinion.
It's a learning hobby.
Good luck.
Nov 8th 2012, 11:32

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

The most important property of the directional coupler used to separate forward and reflected power is its ability to show zero reflected power when there is no reflected power. A Bird 43 wattmeter is designed to have at least 25 dB of directivity--when you are transmitting with 400 watts--the meter will show less than 1 watt of reflected power.

http://www.bird-technologies.com/~/media/Bird/Files/PDF/Products/manuals/920-43.ashx

see page 4

But, the cheap meters introduce another complication--they use inexpensive diode detectors with a power threshold--which means you get more accurate measurements of SWR as the test power level goes up. Or, you will see an abnormally low SWR when you make low power measurements. Not that you should use high power when measuring antennas, but these measurement issues can be very confusing to the newcomer--why does the SWR with the wattmeter differ from the measurement with the SWR analyzer?

Zack Lau W1VT
ARRL Senior Lab Engineer

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