[1000mp] Ten-Tec Orion,roofing filter

[Tom Rauch]

> How tough would it be to change out the roofing filter in the MP to
> something narrower?  I could easily do without FM. I know this has
> been asked before, but I have never really seen an answer as to why or
> why not.

It will make the noise blanker stop working, or not work as well. It 
will increase IM performance and blocking performance OUTSIDE the 
window of the roofing filter quite a bit, but won't do anything for 
signals inside the roofing filter unless there is a concentration of 
modest level signals.

In other words, if you change from a 20kHz to a 5 kHz filter the 
receiver will be much more immune to stuff 2.5-10 kHz away. 

Also, an accumulation of many signals can cause IM or blocking. The 
best way to visualize this is a receive has tiny power amplifiers 
that can only handle a given amount of power. It doesn't matter much 
if the overload comes from five slightly weaker signals, or one 
really strong signal, if the peak voltages and currents are the same.

A narrower roofing filter can improve performance in contest 
situations but might not seems to be that dramatic in a two tone 
test, because in contests you can have dozens of extra signals in a 
10 kHz window that you might be able to eliminate.

The noise blanker deteriorates because the rise and fall time of 
noise pulses is "stretched" by the limited frequency response. With a 
slower rise and fall, each noise pulse looks more and more like a 
signal with less peak and more average energy rather than a sharp 
strong but narrow time pulse.

I use 600Hz roofing filters in my heavily modified Drake R4C's, and 
it is a very dramatic change. Too bad it is impossible to get a 
really narrow filter at 70MHz.

73, Tom W8JI
W8JI@contesting.com