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Re: Coax



-> I understand the need for low loss coax.  There are a number of
-> choices: 9913F, LMR400, Aircell and others. Are there good reasons to
-> choose one over the other?  Any advice from the old hands to an utter
-> newbie?
->
-> Robert Rice
-> WB5PKN

I assume you'll be running mast mount preamps??
Try and run the best coax you can between the preamp and shack. It's not 
really needed on the receive side if you use a good preamp, but you'll loose 
on the transmit side.
One place to not cut corners is in your rotor jumper. If you use a good 
adjustable gain mast mount receive preamp, it will typically have a NF of 
~.8dB. A good rule of thumb is to keep the loss between the antenna feedpoint 
and the preamp to less than the NF rating of the preamp. Remember, additional 
losses in front of a receive preamp can add to the NF destroying all the good 
money you spent on it. For example, I'm using Aircom Plus for rotor jumpers. 
Losses are .3dB on 2M and .45 dB on 70 cm. My preamps are rated .8dB NF. From 
the preamp to the shack, (75') I run 1/2" Hardline on 2M and 7/8" Hardline on 
70 cm. Seems to work real well.

One final thought, too much preamp receive gain is not a good thing. I've 
always optimized the gain on my preamps. Here's an excellent way to do it 
courtesy of G3SEK. (Thanks Ian!)
-----------
Here's a method that requires no test equipment at all. It comes from
G4DGU, who designed all the original muTek transverters and outboard
preamps to have adjustable gain. This method uses the sharp threshold
effect of FM detectors at low S/N ratios, and it allows you to optimize
the preamp/transverter gain for your local band noise conditions.

1. Turn the transverter/preamp gain well up. 

2. Find a very weak but steady unmodulated carrier (off-air, not from a
signal generator or a local birdie). Rotate the antenna until you can
just detect the signal in FM mode.

3. Reduce the preamp/transverter gain until you hear the noise increase.
The FM threshold is sensitive to a small fraction of a dB in S/N.

4. Increase the gain just a little,to the point where you can't hear the
quieting improve much. 

5. Switch back to a real DX mode.

Remember that every dB of unnecessary preamp/transverter gain will
probably subtract almost 1dB from your system intermod intercept!

The penalty of adjusting the gain correctly is that you're living just
above the "knee" where S/N will begin to deteriorate rapidly if
something changes. It's worthwhile to repeat this test every few months
- especially just before a contest.
 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
------------
73,
Mike, N1JEZ
AMSAT #29649
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