On Thu, 24 Sep 2009 15:15:01 -0400, "Robert11"
wrote:
Saw a picture somewhere of an in-line lightning protector for a HF radio
receive only antenna.
Coax style.
Apparently has the typical gas tube, and when activated shorts the center
conductor to the braid.
See:
http://www.polyphaser.com/productdata.aspx?class=coax
This is what I use on the local mountain top sites.
http://www.polyphaser.com/cms_spol_app/Translations/English/B50.pdf
Note that we don't have much lightening on the left coast, so I don't
have much history on how well it works. The one site where we did
take a nearby hit, managed to blow up some unprotected ethernet to
coax translators, and some cheap sacrificial ethernet switches.
Also, not all lightning protectors use a gas tube. I have an older
Polyphaser with ZnO ceramic elements. When hit with sufficient
energy, the ZnO expands and closes a gap. Polyphaser places 4 of
these in series from the center conductor to ground. My guess(tm), is
that when hit by lightning, one of them might stay shorted. 4 hits,
and it's totally shorted and must be replaced. However, I'm not sure.
I'll post a photo when I find it.
Was wondering a bit about this.
Articles on the subject.
http://www.polyphaser.com/technical_notes.aspx
Ham Radio protection:
http://www.polyphaser.com/cms_spol_app/techdocs/Ham%20Radio.pdf
See Pg 6 for some interesting comments on receiver protection. Quoting
in part:
Coax protectors should be units that have dc blocking on the center
pin. This serves as a high pass filtering that prevents the
lightning's low frequency energy from continuing to your equipment.
The strike energy is picked off and diverted into the ground system
in a controlled way. The dc blocking ensures the operation of the
protector regardless of the input circuitry of the equipment.
Did you know that spark gap protectors with dc continuity will not
work on receivers and shunt fed duplexers? The shunt to ground
inside a receiver (coil to ground for static draining) prevents the
low frequency lightning energy from turning on the dc continuity
protector. The coil shunts the energy to ground all right, but it
is at the wrong place. If the coil can't handle the energy (half
the coax surge energy is on the center pin), the coil will open up
and the current will translate to a large open voltage source
capable of arcing anywhere within the radio.
I seem to remember in the old days, there was always a third tap on these
sort of things that you ran a solid ground to, e.g., a water pipe.
Yep. The current needs to go somewhere to get to ground. Best
through a big fat wire ending in a good ground than through your
equipment. Incidentally, a water pipe is not a decent ground for
lightning protection.
Does merely shorting to the braid provide "good" protection ?
No. If lightning came down your coax cable and to shorted braid it
must have a path to continue to ground. Just a shorted coax will send
it through your equipment, probably through the AC power cords, and
then through the house wiring. I've seen photos of the wall blown out
when that happens. Bad idea.
Polyphaser also suggests that a DC short in the radio is inadequate:
http://www.polyphaser.com/cms_spol_app/techdocs/Built-in%20Coax.pdf
Any thoughts would be appreciated.
Also want to ask: what about voltage surges of a few hundred volts or so
induced on an antenna lead from a nearby lightning strike perhaps
.
The levels way below what would trip a gas tube I would imagine, but still
more than enough to ruin a front end of a receiver.
A high value resistor should bleed off any charges. As others have
suggested, a neon lamp and/or back to back diodes, should also work.
How does one protect against these without breaking the bank doing so ?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558