Roofing Filters

[Don Rasmussen]

Can someone explain the meaning of "roofing filters".
I have searched everywhere and can't find a good
explaination. Lisa recommended the FAQ secftion of
Elecraft and there they just mention them. Maybe I
should order an updated Ham bible from ARRL.

Thanks, Alan KA9UCP

http://www.zerobeat.net/mediawiki/index.php/K3_Roofing_Filters

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[Mark KN7T]

I'll take a stab at it...  In the real world application, what they try to do is limit the amount of stuff (signals) that shows up in the receiver passband.  The more narrow the filter, the less amount of signal in the passband.  Effectively, what it serves to do is to greatly reduce or eliminate interference from close-in strong signals (i.e. - contesting or if you have a bunch of locals who all like to operate on the same band at the same time, etc.)

For instance, let's say you're trying to work a weak signal DX station and suddenly a very strong signal a couple of kc's away shows up.  Without the roofing filter, you typically get lots of AGC pumping, receiver de-sense and it may become virtually impossible to work the weak signal you want.  With the roofing filter switched in, depending on the width, it can greatly reduce or eliminate the interfering signal and allow you to still work the weak signal station.  In some earlier transceivers that don't have built-in IF DSP, it can also improve the effective noise floor of the receiver.  I can tell you that the addition of a roofing filter to an IC-765 transceiver that I own worked miracles for it and made it a much more effective tool for working weak signal DX and CW contesting.  My overall impression is that it has a lesser effect on radios that have built in IF DSP but there is still benefit to having the roofing filters installed.