## I assume some sort of 'bias-T' is used to inject..and extract the DCV
to feed the 900 mhz amp ? If that is the case, then it would already have
DC blocking on the coax ( and at both ends)... between shack and input to
amp.... ( IE: up the tower). What I was talking about, was DC block cap
between output of amp..and yagi. But dunno if that would buy you
anything.... or not. The ICE brand arrestors use a gas tube in parallel
with a torroid, then a series cap on the shack side. The back emf on the
torroid is what fires off the gas tube.
## a gas tube on the input to amp... if blown, will then just dc short your
vdc that feeds your amp.
## I',m wondering if you could install a DC block cap on the input..and
perhaps output of the amp, but inside the amp itself ? But things get a
bit dicey at 900 mhz..and unforgiving. 300' is a long piece of coax..and a
long way to climb the tower.
Jim
-----Original Message-----
From: Roger Parsons
Sent: Saturday, July 01, 2017 5:21 PM
To: Tower and HF Antenna Construction Topics.
Cc: wv2zow@gmail.com ; Jim Thomson
Subject: re: [TowerTalk] Another Lightning Question
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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