[Amps] Dummy Load

[Mark Marsden]

I have now built a 30dB attenuator using non-iductive Vishay parts. 
It's documented on http://granta.digital-crocus.com/Attenuator.php3 

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)
http://www.ifwtech.co.uk/g3sek