[Skimmertalk] RP-16 front end tenderness

[Ted Gisske]

Front-end protection for your new Red Pitaya is a worthwhile investment. RP
specs the maximum input voltage at 1V P-P. This is probably so low due to
the 1:14 input transformers. My guess is that older RP-14 and RP-10 boards
have a similar maximum voltage rating, if you are using the external
transformers.

I am using a DX Engineering receiver protector, which I recommend, as it has
lightning protection, as well gross overload protection, built-in. It's not
enough, though. I hooked up a scope to my skimmer antenna and ran some
high-power transmitting test. My skimmer antenna is a couple of hundred feet
from my 40M Delta-Loop and, when running full power, it puts 15V P-P across
my 50 Ohm scope input. The DXE protector does a great job of limiting this
voltage to 2V P-P. That, however, is twice the rated max voltage of the RP.
Not good enuf...

The easiest solution is to disconnect, or short, the antenna with a T/R
relay activated by your PTT line. That will work, but isn't ideal. I'm a
pretty hard-core contester and likely am transmitting at over a 50% duty
cycle in a contest, which would result in a lot of chopped-up decodes. My
skimmer is in a pole-shed quite a ways from the shack, so running a PTT line
is inconvenient, in any case.

I'm working on a more elegant design to work in conjunction with the DXE
protector, which will do the heavy lifting. It is an adaptive PIN-Diode
attenuator that will reduce the 2V P-P out of the DXE protector to below the
1V P-P max RP spec. the trick is to do the limiting in a "soft" way that
does not generate a lot of harmonics that result in false decodes on higher
bands.

I'm modeling the design at the moment. It's a few discrete parts driving a
PIN-Diode bridge. I'll publish the design to this list when I get it up and
debugged.

73,
Ted
K9IMM