Topband: To clamp or NOT...that IS the question

[Tom W8JI]

> Indeed. I've had problems with ICE 196 receiver protectors for several 
> years. I assume the intermod is generated by the diodes in those devices, 
> though I do not know that for a fact. The problem occurs on nights when 
> propagation is very good both in the AM BC band and shortwave in the 5-7 
> MHz range. I get some extremely strong SW BC stations on good nights. All 
> are well below the threshold but the combined affect of many signals is 
> too much.  Removing the ICE 196 clears up the intermod, so I am reasonably 
> certain that is where it is generated. I need to either find another way 
> to protect my front end or use band pass filters.

Sometimes people do not recognize IMD, even when they have it.

IMD is also not the only issue. There are multiple problems besides IMD, 
some are not even noticeable to the station owner.

For example, in almost all transceivers the receiving antenna port is common 
to filters and stages used in the transmitter function as a predriver. Most 
transceivers do not have disconnect relays for the receiving antenna port, 
they depend on diodes to switch  the antenna port off. When levels get too 
high, the RX port isolation diodes allow TX RF back into the transceiver 
predriver stage. This creates all types of wideband spurious signals up and 
down the band.

That is not a rare problem, and some external clamps still allow the RX port 
feedback to affect the transmitter signal quality.

Giving general advice, or selling a universal product, I focus on something 
that should generally work for everyone and not cause problems for anyone, 
especially if it can pollute the band for others.

While sometimes excused or dismissed as "bad radios" or setup anomalies, if 
we all do not plan properly and understand the limits, the accumulation of 
"unimportant" planning errors damages the spectrum for everyone. This is 
why, in any quiet location, a large percentage of signals that could be 
copied on nights of low activity cannot be copied with high activity. The 
band fills with spurious stuff unrelated to receiver defects.

The last thing we want is to not make things as good as we reasonably can. 
Poor planning, marginal systems or methods, and bad advice eventually 
catches up with all of us in one way or another.