Mike and Jim
Many thanks for your helpful comments. You confirm my feeling that there is
very little point in a surge suppressor between the amplifier and the antenna -
and yes, the inner and outer conductors of the coax are 'connected together'
via the antenna folded driven element.
I am intrigued by the shorted stub protectors you have described, but I haven't
been able to find any detailed specs for them. Obviously, a quarter wave
shorted stub would be very effective at the input to the amplifier whether or
not a discharge tube was used as well. However, what I don't know is if there
is DC blocking? The amplifier is powered up the coax so clearly a DC short
would not help! (I was probably a bit misleading when I described the
amplifiers as 'high power' - they are actually 25W which is a lot for a 900MHz
data link but not really otherwise!)
Thanks again
73 Roger
Roger: Jim's comments are right on -- the gas tube suppressors not very
effective on DC grounded antennas/front ends. Andrew (before CommScope)
used to make a shorted stub protector specifically for 900 MHz. Known as
the "Arrestor Plus T-Series, Lightning Surge Protector" . They are
frequency selective and still on ebay. You probably want one with the -1 or
-3 suffix to cover 900 MHz. The NM-NM model is APT-NMNM-* where * is the
suffix. Similar products are still available. Check
https://www.tessco.com/products/displayProductInfo.do?sku=477764 for a
typical. They seem to be effective 450 MHz (Specific bands) and up. I
suspect below that, the stub gets too long. --Mike, WV2ZOW
On Fri, Jun 30, 2017 at 12:42 PM, Jim Thomson <jim.thom at telus.net> wrote:
> 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.
>
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