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Re: [Amps] Dummy Load

To: "Ian White GM3SEK" <>, <>
Subject: Re: [Amps] Dummy Load
From: "Mark Marsden" <>
Date: Mon, 27 Jun 2005 10:00:34 +0100
List-post: <>
I have now built a 30dB attenuator using non-iductive Vishay parts. 
It's documented on 

73, Mark G4AXX

-----Original Message-----

David Kirkby wrote:
>If you do build an attenuator, I would suggest using the minimum 
>attenuaton you can get away with using the lower quality resistors, and

>making the last 6~10 dB or so from a decent quality attenuator. That 
>way any test kit you connect will see 50 Ohms over a very wide range of

>frequencies. It means your attenator has an input and an output 
>terminal, but that is common - just don't connect it back to front!!

30dB seems to be a "magic number" where it's very practical to make a
high-power attenuator based on an existing dummy load. For example, you
can easily make a 30dB T-pad using an existing 50 ohm dummy load for
"R1". The low shunt resistance "R2" can be made from a fan of 10-20
resistors (I forget the value - work it out) arranged on a PC board like
spokes on a wheel. Then connect two 100ohm 3W MF resistors in parallel
for "R3".

This makes a very easy mod for an existing dummy load... but if you
change the target attenuation to 20 or 40dB, the resistor values become
a lot less practical.

When the 30dB power pad has reduced your 1.5kw (let's say) to 1.5W, you
can then finish off with a second 10 or 20dB pad to bring it down to the
levels that power meters and spectrum analysers like to see.

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
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