Maxtrac VHF Output: How low...?

kc7gr · Post by **kc7gr** » Mon Aug 14, 2006 2:54 pm

More specifically, and to re-re-use a somewhat (in)famous quote: 'How Low Can You Go?'

I know about the rule-of-thumb which says never reduce a Class C RF amp in an FM transceiver below 50% of its rated output, but I'm seriously wondering if anyone on the board has ever taken a Maxtrac below that point, and gotten away with it?

I'm asking because I'm still getting a bit of a desense problem with one of my Spectras when the Maxtrac (which is dedicated to APRS) keys up. It's at half-rated power right now (15 or so watts from a 35-watt rated radio), and I'm wondering if I can get away with taking it down any lower (perhaps as low as 7-8 watts).

Thoughts?

Thanks much.

Will · Post by **Will** » Mon Aug 14, 2006 3:06 pm

Mr. Bruce,

Time to get out your spectrum analyzer and "take a look"....
Then get out your antenna analyzer and "take a look" at the antenna.....

Antenna mismatch does contribute to spurious transmitter conditions.

Post by **escomm** » Mon Aug 14, 2006 3:25 pm

I've seen 45watt M1225LSes turned down to about 10 watts without any problems (besides the radios not being used for hand warmers)

kcbooboo · Post by **kcbooboo** » Mon Aug 14, 2006 4:42 pm

I've run both 45w and 25w UHF radios as low as 6-7 watts to drive a 10/100 external power amplifier, which fed a Bp/Br duplexer. I did check the final signal and saw nothing out of the ordinary, but I'm not sure it's a stable combination. I wouldn't want to go any lower, and if I had to do it again, I might try running the radio at least at half power and use an attenuator to reduce the signal going into the amplifer.

No matter what you do, a MaxTrac will still get hot.

I agree with Will - use a spectrum analyzer and verify clean operation.

Bob M.

kc7gr · Post by **kc7gr** » Tue Aug 15, 2006 11:18 am

Experiences and suggestions noted and logged. Thanks much to all responders.

I will definitely do as Will and others have suggested, and monitor the results with a SpecAn. I'll also double-check the antenna to be sure it's a good match.

Keep the peace(es).

George · Post by **George** » Wed Aug 16, 2006 5:02 am

If power out is really a problem in this application, I would get an LPI radio that puts out, normally, about four watts.

Then I would turn it up all the way or close to it and find that you will get about six watts out. It's good and stable with little on the heatsink and lots of space to get rid of the heat. I have run some of these for years and it's not a problem.

George

Maxtrac VHF Output: How low...?

Got Qwoob?

Maxtrac VHF Output: How low...?

Thanks!