Topband: Re: [Towertalk] Back-To-Back Diodes - Careful!!

[Michael Tope]

Hey wait a minute, Ward, my posting you snipped was to Topband not
Towertalk.You are getting me all confused :)

Yes you are correct, that is a danger with back-to-back diodes. In our case
we were doing the 160 meter contest, so interstation QRM was not an issue.
We also didn't have any BC QRM that I could detect. Interestingly
enough, even without back-to-back diodes BC QRM can be a real
problem for the ANC-4 as its external receive antenna input preamp is very
broadband and extends down into the BC band. I have used it in two
locations where BC stations were close-by, and in both cases, I had to use
a sharp hi-pass filter in front of the ANC-4's external receive input to
keep its pre-amp from overloading. Not sure how the MFJ unit compares
as we didn't have strong BC in the location where I was using it (either
that or its so good we didn't notice).

A better solution for a single transmitter environment, is to use the
front-end
saver circuit like the one KD9SV sells. This just disconnects the receive
antennas when you are transmitting. For multitransmitter environements,
bandpass filters and stubs are a good approach. If the limiter approach
is used, you can do as N4KG suggests and series several diodes in each
direction to increase the holdoff voltage of the limiter. The key is to make
an acceptable tradeoff between intermod on low-end and damage level
on the high end (classic limiter design tradeoff).

The two diodes was an 11th hour trip to Radio Shack that allowed us to
make it thru the weekend :)

73 de Mike,
W4EF...............................................................

----- Original Message -----
From: "Ward Silver" <hwardsil@centurytel.net>
To: <towertalk@contesting.com>
Sent: Wednesday, December 04, 2002 9:37 AM
Subject: [Towertalk] Back-To-Back Diodes - Careful!!


> After we replaced the bulb and put a couple of back-to-back diodes across
the receive antenna input, the unit worked fine.

Danger, Will Robinson!  The back-to-back diodes are terrific harmonic
generators and mixers.  If you have strong local BC stations, you will cause
yourself a lot of grief as these little guys generate their mixing products.
Also, if you're in a multi-transmitter environment, the local interference
they generate will also cause a lot of trouble.  If you have neither of
these to worry about - no sweat - but remember they are there.