David Kirkby wrote:
>Mark Marsden wrote:
>> Hi Stefan & Will
>>
>> That's very interesting to me. I'm looking at a possible 50R 40dB
>>attenuator based on the new thick-film-on-steel planar resistors by
>>TT-electronics/Welwyn. They aimed at the automotive dynamic braking &
>>welding industry, with a very low £/kW ratio.
>>
>> Farnell part numbers are
>> 465-9569 50R 2-8kW £13.93
>> 465-9600 50R 5-16kW £23.52
>>
>> It'll be interesting to see if the meander-line construction can be
>>matched to give a good return loss across the HF spectrum.
>>
>> Best 73 Mark G4AXX
>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
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