[FoxHunt] Attenuator

Tue, 16 Apr 2002 15:01:10 -0400

I would be interested to hear from anyone who has tried to calibrate an offset attenuator. I suspect it might not be as straightforward as one would assume. 

Resistors are extremely linear devices. If you build an attenuator out of them, they will attenuate a signal by the same amount regardless how big the signal is. That is, a 3-dB pi network resistive attenuator will attenuate a signal by 3 dB whether you are 3 miles from the transmitter, or 3 feet from the transmitter.

A diode is a non-linear device. In fact, that's why they are used as mixers. They behave differently depending on the strength of the signal(s) going into them. Might it be that, if you calibrate your offset attenuator using a signal 3 miles away, your calibration marks would be invalid when you try to use it 3 feet from the transmitter? If so, how how far off might they be? What about when there are strong out-of-band signals present? What about when the temperature of the diode changes? 

I don't know the answers. Does anyone else?

73,
Charles
NZ0I

---------- Original Message ----------------------------------
From: "Michael J. Miller" <[email protected]>
Reply-To: [email protected]
Date: Tue, 16 Apr 2002 10:51:43 -0700

>I have an active attenuator design you may have, upon request.
>Interesting idea about using a step attenuator to guage distance ... I see no
>reason why active attenuators could not have a calibrated dial.  I do think
>they would need to be individually calibrated. And the step attenuator is just
>the thing to do it with.
>
>On another note, I have had good results with a step attenuator and a HT with
>extra shielding.  Simply wrap the HT in aluminum foil, or drop it into a 'tube'
>attenuator shield (waveguide beyond cuttoff).
>
>Mike
>WB6TMH
>
>John Munsey wrote:
>
>> A lot of "good advice" about the attenuator issue but for my part I agree
>> completely with Dale... build an active attenuator!  The it doesn't matter
>> how well the coax is shielded or if the receiver is shielded (NONE ARE!) or
>> not.  You tune to a frequency at least 1 meg away and the receiver never
>> hears the actual hunt frequency.  Complete control and it doesn't cost any
>> more than fooling with various resistive  attenuators that never work.
>>
>> Just my thoughts... others may disagree.
>>
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