[Amps] PIN diode mods

[Paul Christensen]

> I think we are looking at sample variation, and the PIN diode mod likely
> has no effect, for better or worse.

Recall that the mass impetus for the move to PIN diodes came from a series 
of articles in QST in the mid-90s and were authored by the president of a 
well-known and prominent test equipment manufacturer.

I am not convinced we can make wholesale, condemning statements concerning 
the use of band-switching PIN diodes.  I believe the author of those 
articles emphasized the need for diode re-biasing in the event the owner 
wished to make the PIN diode modifications.

Based on data gathered from those articles, it does appear that only 
marginal improvement results under the best of circumstances, although I 
seem to recall that the focus of the articles was on improved IP2 
performance.

In my own case, I performed a PIN diode upgrade on a Ten Tec Omni Six, just 
as the author suggested but I needed to substantially increase diode biasing 
current.  Was it worth the trouble and expense?  Probably not.  I honestly 
did not measure the "before and after" effect of key receiver performance 
parameters.  But what I did measure was port-to-port isolation between Main 
and Aux receiver ports.  Isolation with standard Si switching diodes was in 
the order of 40 dB.  A +40dB signal on the main antenna S meter would read ~ 
S9 on the aux antenna port.  Changing the T/R diodes to high-power PIN types 
(MicroSemi UM 2110), resulted in spectacular, measured port isolation of 
more than 80 dB. (The S-meter was NOT used as the measuring device.)  So, 
with respect to that part of the circuit (T/R switch), the upgrade to PIN 
diodes was well worth the effort.

Paul, W9AC