[TowerTalk] Another Lightning Question

[Jim Thomson]

Date: Fri, 30 Jun 2017 15:41:29 +0000 (UTC)
From: Roger Parsons <ve3zi at yahoo.com>
To: "Tower and HF Antenna Construction Topics."
<towertalk at 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.