[amsat-bb] AIDC-2733 Gain Measurement & the Stub

[BobsImsai8800@xxxxxxx]

Art and all:

REVISITING THE GAIN MEASUREMENT: I measured the gain of a few more 
downcoverters using various tests. The gain that you measured at 2400 MHZ 
that was about 15 dB low can be attributed to the 2400 MHZ stub and possibly 
the RF tap's insertion loss if you were using one.  Cut the stub, and the 
gain will be closer to what is indicated on the downconverter. 

After checking a few downconverters, I found that the stubs can notch right 
on 2400 MHZ or far enough off to not affect a measurement at 2400 MHZ.  Some 
showed proper gain, some were up to 10 dB low at 2400 MHZ

MEASURING THE RF TAP POWER INSERTER'S INSERTION LOSS: I measured the through 
loss on various taps on an HP 8753A network analyzer -- the loss is between 3 
and 4 dB AT 123 MHZ and 144 MHZ.  These numbers were also duplicated with an 
HP 8660C signal generator and an HP 436A power meter.  I checked the ones for 
Pacific Monolithics, the Emerson and ones made for these downconverters -- 
they were all within this range.

With the stub's insertion loss at 2400 MHZ and the RF tap's insertion loss at 
4 dB, your gain numbers could be 15 dB or so down. This loss would depend on 
the stub's actual insertion loss at your frequency of measurement (try the 
same test at about 50 -150 MHZ higher) and whether or not you were using an 
RF tap.

Once again, the stub's loss occurs after 99% of the NF has been determined 
and does not really affect NF.  It only reduces the amount of attenuation you 
will put in a VHF receiver to get the noise level to a reasonable level.  
However, as I previously posted, it is an easy task to cut the center 
conductor of the stub's connection at the preamp's output and the hole goes 
away.

TNX & 73,

Bob Seydler
k5gna@aol.com
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