Date: Fri, 30 Jun 2017 15:41:29 +0000 (UTC)
From: Roger Parsons <ve3zi@yahoo.com>
To: "Tower and HF Antenna Construction Topics."
<towertalk@contesting.com>
Subject: [TowerTalk] Another Lightning Question
<I have a 900Mhz link between my home and remote station - it is approximately
10 miles and not LOS. This has meant that I have had to use high gain antennas,
relatively high power and high antennas - the one at the remote site is ~300ft
AGL. (The tower is a 325ft former AM broadcast tower so is very well grounded.
The base insulator is bridged by a wide copper strap.)
The link antennas have amplifiers a few feet from them, and the antennas have
folded driven elements. The coax outer is securely connected to the tower at
top and bottom. In 12 years I have had two amplifiers fail, presumably due to
lightning. I have not had any failures of the equipment at the base of the
tower.
There is no surge protection at the amplifier/antenna, but there is at the base
of the tower and in the equipment room. I am debating whether I should try to
fit a protector at the amplifier? I guess it shouldn't hurt, but it also adds a
failure point and changing the discharge tube is nearly as bad as changing the
amplifier - climbers are very expensive - and I would probably have to bring
the whole lot to ground level to confirm what had failed.
Comments welcome.
Roger
VE3ZI
## Is the center conductor of the coax DC grounded to the braid of the
coax.... via the folded driven element ? IF it is, any surge protector
located between output of amp..and input to 900 mhz ant, is probably not going
to buy you anything....esp with the
braid of coax being bonded to top of tower. Some lightning arrestors will not
only use the typ gas tube... but also a series cap on the input side..to
provide for a DC blocking function. Some will also use a coil or torroid
between center conductor..and chassis ... braid. With said coil, I believe on
the output, ant side of the gas tube. The coil will DC ground the center
conductor of the coax. The coil + DC block cap can be used..and gas tube
removed. If the coax center is ultimately bonded to the braid... via folded
driven ele, the coil will do nothing. Dunno if just a DC block cap by
itself will provide additional protection vs no cap at all. Whether any
of these items will function at 900 mhz is another issue. Climbing a 300 ft
tower to either swap out a gas tube or an amp..is a tough slog. Another
thought is perhaps using a gas tube with a higher peak V rating... something
high enough to still protect the amp, but avoid...nuisance failures.
The german HOFI brand lightning arrestors are good up to 6 ghz..and come
with either SO-239, Type N , or 7-16 DIN connectors, and come in various
power ratings.
Jim VE7RF.
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