Topband: RE: dedicated topband transceiver

Tom Rauch W8JI@contesting.com
Fri, 18 Jan 2002 11:18:09 -0500


Hi Doug, 

> So when I installed the INRAD 455/ 125hz IF filter, it was not
> surprising to find that it rings excessively on 80/160m.  That was

Generally very selective filters have group delay problems, where 
the time for a signal to pass through the filter is different for different 
frequencies. The effect of that is "ringing" or stretching of the signal 
you hear, because you perhaps hear the upper passband noise 
before the lower passband noise as the same broad noise pulse 
hits the filter. The effect of that is the noise peak amplitude is 
reduced but the duration of a noise pulse becomes stretched.

That's why mechanical filters, although they have poorer skirts, are 
often better than crystal filters when digging weak signals out of 
"rough" noise.

The second effect is signals have to change level slower when 
selectivity is increased. All filters show that effect, there is no way 
around it.
 
That's why I use 250Hz filters when the band is quiet with only 
white noise, and 600Hz filters when there is QRN or "rough" noise.

> problem, and I was lucky to find this.  Now if I can just get these
> experimental, floating shield phase lines to quit receiving noise on
> the flag, I will be in business.  Steelwool and Moxon linear RF 
chokes
> on the floating coax shield looks promising........mebbe even
> permanent magnet rings........

I hear that rumor from time-time, but it isn't the least true! Even 
some of our Handbooks get that wrong, but eventually most correct 
the mistake. 

Steel wool has no effect at RF at all as a choke, because the 
strands of even the finest steel wool are much too large. You'll 
notice the ARRL Handbook removed the "steel wool balun" that 
never did a thing!

It is also electrically and physically impossible to "shield" an 
antenna for noise and still have it receive signals, because the 
noise is at the same frequency as the radio signals. The noise is, 
in effect, a signal exactly like the one from any intentional 
transmitter or radiator. If you surround a conductor with a shield 
and place an effective choking material over that shield, and you 
have what you might call an "antenna" inside the shield, the 
antenna will go dead. 

When we add a shield to a conductor and the shield has a gap or 
is open at one or both ends, the shield itself simply becomes the 
antenna! That's why it is electrically impossible to use a hunk of 
coax to shield a ground lead, use only an earth ground and shield 
the drop lead of a Beverage, or shield a loop antenna.

That is also why when you short or close the gap in a "shielded-
loop", it goes dead. The only thing the shield can actually do is 
change the electrical balance of the antenna, and it can make it 
either better or worse depending on how you build the antenna.
73, Tom W8JI
W8JI@contesting.com