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Re: Ruining $4 Attenuators



Bob,

At 12:38 PM 8/6/02 -0400, BobsImsai8800@aol.com wrote:
>>From K5GNA:
>
>Putting a Radio Shack 6 dB attenuator (model 15-1257, $3.95) in line with 
>your converted MMDS type of downconverter will reduce the no signal noise to 
>a more reasonable level.  Although there has been some debate on this BB on 
>reducing excess gain and noise in the RX, Dick Knadle, K2RIW, and others
have 
>presented papers on the benefits of eliminating this excess gain, so the no 
>signal noise is reduced.

Agreed.  I found that too much excess gain would eventually drive my IF
receiver into non-linear levels with resulting distortion of weak signals
(not my AO-40 equipment; but my LNA + 1420/144 radio astronomy convertor
gain totalled over 60 dB into the FT-847 on 144.  I had to add 20 dB atten
to get the levels tamed!

One should be aware that addding loss (attenuation) into your receiving
system can degrade overall noise figure, if you are not careful.  I would
use ao40rcv software to check the effects on your system NF (and thus your
system sensitivity).  In the example above, any atten. over 22 dB degraded
my NF.  Of course the best solution is to remove gain stages starting from
the IF end.

But a simple attenuator is certainly simpler.

>However, the main benefit of the attenuator is for you to sacrifice a $4
unit 
>instead of a downconverter (or another expensive device) when you key your 2 
>meter TX into the downcovnerter. You will probably key up accidentally no 
>matter how careful you are, as I have heard many stories and even done it 
>myself.  Because of the berivety of most of the accidental transmissions, it 
>may not burn something up every time, but it does happen. Even the step 
>attenuators can be ruined and they usually cost a lot more than $4.  The 
>first thing that your 2 meter transceiver should see is the $4 attenuator or 
>somthing else cheap that will can be sacrificed.

This is true!  I used a Alan Microwave (1-100 dB) step-attenuator in my
initial investigation with the radio telescope system, but then replaced it
with a 20w 20dB attenuator.  The Alan is much too expensive ($500) to
sacrifice to "errant RF fingeritis" {hitting the PTT}.

The RS atten is certainly a good buy...or just get some 1/2w resistors and
make your own pi-atten circuit!  Find attenuator resistor values in the
ARRL Handbook!

Ed, AL7EB 

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