Topband: Beverage Taxes and radio survey

[George Johnson]

Regarding antenna port feedback

At 11:13 AM 1/14/2006, Tom Rauch wrote:

>This leaves us three solutions...
>
>1.) We depend on reports and observations
>
>2.) We all just assume we have a problem and use something
>will know will correct and never add a problem. Sort of a
>"Beverage Tax".
>
>3.) We compile data on radios and determine which radios are
>susceptable.
>
>I think we should do numbers 1 and 3 and help those who
>don't pay their Beverage Tax.
>
>
>I think we need a list of good an bad combinations. Sort of
>a "I had this problem" survey.
>
>Many buzzes are not antenna related, by the way. Many of
>them are equipment problems that have nothing to do with the
>antenna port.

I can add the Icom 756PRO and PRO II to a list of radios that are very 
susceptible to receive antenna RF feedback via their receive ports.  I am 
on a small (80 ft x 150 ft) lot without much room to separate my inverted L 
from a receiving loop.  My K9AY loop spacing is 45 ft from the 
vertical.  Transmit power levels over 250 watts causes the Icom to "sense 
reflected power" and retard the transmit drive level to the external power 
amp.  The resulting output drop and reduction of "reflected power" (read 
feedback via the RX port) created a obnoxious full power "buzz" for a 
transmitted signal.  I finally verified the cause by removing the RX 
antenna and the problem disappeared.  My solution was to build an outboard 
RX relay to disconnect the receive port and short the port to the port 
ground.  My circuit is similar to what Ten Tec used to have in their Triton 
IV rig years ago.  I see that a similar circuit is in the ON4UN book and is 
probably what some have referred to in recent postings.

As Tom mentioned, chasing the circuit in the Icom 756 schematics yielded a 
very strange way of antenna T/R switching and I concluded that I had no 
choice but to use an outboard box ... Beverage Tax.

In my case, transmitting with 100 watts did not cause the port feedback to 
be a problem.  Now, if I could only hear KL7...
73, George .. W1ZT