On Wed, 3 May 2017 12:26:15 +1000, Clifford Heath
wrote:

Ok, well in that case, just use an imposed DC level (+ve or -ve)
to enable one of two switch diodes. Whichever diode is forward
biassed passes that antenna's signal. Simpler and cheaper than
relays.
Clifford Heath.

I used 4 PIN diodes. I had a bag of Motorola MPN3401 diodes handy.
http://images.ihscontent.net/vipimages/VipMasterIC/IC/FSCL/FSCLS06284/FSCLS06284-1.pdf
As you suggested, I used a bipolar switching arrangment. On each
antenna, a pass diode would conduct the signal from the antenna to the
receiver, while the other diode was reverse biased and effectively
disconnected. At the same time, the other antenna did the opposite.
The pass diode was reverse biased off, while the other diode would
short the antenna to ground. Something close to this, but with
bipolar power arrangement:
http://www.analog.com/-/media/images/analog-dialogue/en/volume-44/number-1/articles/driving-pin-diodes-with-op-amps/pin_diode-fig-05.jpg?la=en

However, it didn't work. I wasn't interested in climbing the roof,
dropping the mast, and dragging a pile of test equipment to the roof
for troubleshooting the problem. So, I just built a relay
replacement, which didn't sweep so good on the bench, but worked well
enough when installed. There were various reasons for this approach.
I vaguely recall that I was scheduled to go sailing the next day or
something similar.

Thinking about the problem, my guess(tm) is that either I used the
wrong ferrite beads and/or chokes to isolate the PIN diodes from the
applied DC, or I used the wrong value coupling caps. I also made the
capital mistake of not sweeping the circuit before attempting to
install it. It was such a simple circuit. What could possibly go
wrong? Anyway, if I had to do it again, I would use the same approach
with the possible addition of FM and cellular RF notch filters.




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Jeff Liebermann
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