Topband: lightning grounds for elevated radial antennas
k3ky at radioprism.com
k3ky at radioprism.com
Fri Jan 15 18:25:54 EST 2016
Seems like everyone is preoccupied with K5P and the deterioration of
160m DXCC standards, etc. Hi!
Here's what I have picked up over the years. My comments address
lightning mitigation only, and not issues such as RF ground quality,
reduction (choking) of common mode currents on feedlines, etc.
1. Coax feedline laying on the ground or buried is better than coax
in aerial runs from antenna to station
2. Frequently the shield conductor of the coax is connected to a
ground rod and/or short radials at the antenna end *plus* at the
3. A good quality coaxial arrestor such as a Polyphaser where the
feedline exits the station is vital
4. A massive ground panel ('bulkhead') where all conductor shields
entering the building are tied together- ideally *everything* is
tied together here, house electrical service, phone lines, CATV-
even the rotator cable (often 8 conductors) crosses gas tube
arrestors on this same bulkhead
5. Some sort of impedance in series with the coax shield at this
point of egress, two good examples being a large coil of hardline,
solenoidal wound (single layer), large diameter, or else a
'waveguide below cutoff' such as passing the coax through about
ten feet of EMT tubing- grounded to bulkhead at station end, not
terminated on the side towards the antenna where the coax exits.
Could probably be oriented vertically or horizontally or bent.
BTW even a big hank of coax is better than nothing here, but a
proper solenoidal wound inductor is said to be better.
FWIW any of the various common mode chokes can only help in this
lightning mitigation (many examples being made from large coax
wrapped around large ferrite toroids, multi turns)- but the two
methods outlined above are probably way better from a strictly
lightning mitigation point of view. Further, those cores might
even be toast after a really big direct hit on your antenna
system (relatively rare event). Induction currents from nearby
strikes are probably not an issue, however...
***Encourage the lightning pulse to discharge from the
coax shield into the ground as soon and as often as possible;
***Provide as high as possible an impedance to lightning currents
in the coax shield right at the point of egress from your station
(for coils, inboard from a ground rod on the antenna side but prior to
the entry bulkhead);
***Bypass any remaining differential mode lightning energy
from center conductor to ground right at the bulkhead using a
good quality arrestor.
Final point: lightning can laugh at all your efforts and
sneak into your house anyway if you have overlooked anything.
All it takes is the oddball telephone cable or extension cord
you ran through a window and forgot about- or multiple service
entries for CATV, power, telephone, ham feedlines, etc...
Polyphaser has a great little book I got a lot out of from reading
and re-reading, called (from memory) "The Grounds for Lightning
and EMP Protection" (or something close to that, anyway). Turns
out the ideal protection scheme follows the single point egress/
massive ground bulkhead scheme *slavishly*. No exceptions allowed.
And a massive perimeter ground is put all around the foundation
of the building, with massive copper strap connections, cad-
weld connections, ground rods every ten feet or so, etc. Most
hams are not going to take it that far. I never did- but the
very well grounded copper bulkhead is well within reach for
many/most ham homeowners. See W8JI's excellent photo presentation
of how he did this on his website.
73, David K3KY
A lengthy thread re grounding on towertalk prompts me to have concerns
about protection of my T top (85' to top plus two 33' top wires) wire
160m vertical with eight 125' elevated radials. Since that covers more
than an acre (it's mostly among 100' tall conifers) it has a lot of
target area, radial routing close to trees, and high ground
capacitance. I did DC ground the vertical via a 3 core 240-31 choke,
about 5k ohms to a single ground rod but that was for static protection
of radio front ends. The antenna is fed via a 50:25 ohm TLT so the
radials have a DC connection to the choke.
What is best practice for lightning protection of elevated radial
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