[Skimmertalk] RP-16 front end tenderness

[Ted Gisske]

I’m going to build mine into my old-school DXE protector-in-a-mini-box. Plenty of room. I also have a new-issue DXE protector in a die-cast box, which may fit, too.

 

One other thing about receiver protection…If you are using a pre-amp, you need a limiter both before and after the preamp. The limiter before the preamp protects the preamp and the one after the preamp protects the receiver. The pre-preamp limiter sets the max input voltage to the preamp to 2V P-P. That 2V, multiplied by the preamp gain, is then presented to the receiver input. Some preamps saturate pretty early, but the good ones will club your receiver like it was a baby seal, with 8-10V P-P.

 

If you just put one protector in your antenna line, the preamp will likely live, but the front end of the receiver could well get zorched.

 

From: N4ZR [mailto:n4zr at comcast.net] 
Sent: Thursday, July 11, 2019 9:50 AM
To: Ted Gisske; skimmertalk at contesting.com
Subject: Re: [Skimmertalk] RP-16 front end tenderness

 

Thanks, Ted.  Very interesting, and timely, with IARU coming up.  Looking forward to your design - it would be even better if it incorporated the first-stage limiting - one fewer box hi.

73, Pete N4ZR
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On 7/11/2019 10:06 AM, Ted Gisske wrote:

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
 
 
 
 
 
 
 
 
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