On Thu, 24 Sep 2020 13:41:43 -0700, (Dave
Platt) wrote:

In article ,
Jeff Liebermann wrote:

Masthead amps make good targets for lightning hits. I've seen a few
where every semiconductor in the amp was fried.

Yeah... probably need to consider them to be a potentially-sacrificial
component.

I had one mounted on a redwood tree. It wasn't hit by lightning.
Instead, it was invaded by ants, dripping formic acid, which ate the
copper traces on the PCB. I was prepared to replace all the active
components, but not the entire amplifier.

The really troublesome ones I've heard/read about, are the
cheapies... sometimes using just a single broadband transistor as a
gain element. Some of these have been known to break into
oscillation, for some reason (or no reason) and blast broadband noise
all over the place.

https://www.gpsworld.com/the-hunt-rf...nce%20in%20fog.

This might fill in a few details:
http://www.learnbydestroying.com/jeffl/GPS%20Jamming%20Moss%20Landing/
Note the photo of the antenna. The cloverleaf pattern is NOT a
broadband device and will not present a 75 ohm impedance to the
amplifier input. Designing a broadband amplifier to be
unconditionally stable and provide a low NF (noise figure) as well as
high gain into such an antenna is difficult. There will be some
frequency, where the antenna presents the perfect impedance to cause
the amplifier to oscillate. Also, such an antenna construction is not
very tolerant of the corrosive effects of a marine environment and
probably should have been potted or conformal coated.

talks about one such case, in which several on-shipboard "active TV
antenna" systems were emitting enough RF crud to blank out GPS
reception in the harbor in Moss Landing, CA.

Yep. That happened near me. Although I knew some of the
participants, I didn't know that there had been a transmitter hunt
until two years after it was over and the article appeared in GSP
World.

Not a good thing to lose
your GPS when you're trying to come into a foggy harbor at night.

GPS selective availability was finally turned off on May 1, 2000,
navigating a 150 ft wide harbor entrance channel by GPS would have
been theoretically possible. At the time, MBARI did operate their own
DGPS transmitter on Mt Toro, but that was for precision vessel and
buoy location in the bay. Today, even with WAAS, GALILEO and GLONAS
satellites added to improve GPS precision, I'm told it's still tricky
due to reflections from moving metal masts and rigging, plus a very
large steel building at the power plant. Once into the channel, I
would probably switch to navigating by the harbor lights:
https://geographic.org/nautical_charts/map_img/18685_3-t.png

This is one reason why which buying a well-tested, professional-grade
mast-head amplifier would probably be a better idea than buying a
generic cheapie.

Yep. In my never humble opinion, there was probably nothing wrong
with the amplifier. Instead, it was the design of the antenna, which
had the misfortune of not being properly matched to the RF amplifier
causing the amp to oscillate. A properly designed Yagi, Bow-Tie,
LPDA (log periodic dipole array), or other broadband design, would
worked better, and not oscillated.
http://www.hdtvprimer.com/ANTENNAS/comparing.html
http://www.hdtvprimer.com/SIMS/



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558