antenna "DC" grounded or not?
Rudolph M. Marcelletti
claimed his Cushcraft R7 was DC grounded "because you get continuity between
sheild and center conductors." Cushcraft says its little 2m/440 3 element
beam is DC grounded. I have one. No continuity between center and sheild.
I get continuity between center and sheild on my Comet mobile antennas. And
also use to on my Comet GP-15 until lightning blew the top off it! None of
my other antennas have continuity between sheild and center (Cushcraft X-7,
X740, 5 element 6 meter, trap dipole, Hustler 5btv) My R7 is in storage so
I can't check that right now.
So--how do you tell if an antenna is DC grounded or not? Polyphaser says a
DC grounded antenna is more likely to survive a direct hit that one that is
not.
What's the scoop?
Rudy, K8SWD
Bob Lewis
times this means that you read near zero ohms between the coax center
conductor and shield with a multimeter. Your 3 element beam may use a
gama match so that even though the driven element is DC grounded, the
coax center conductor is not. The gama match has capacitive coupling
between the coax and the driven element. On the R7 the DC ground is via
the matching transformer. The primary benefit of DC ground is to drain
off static charge build up and prevent the resulting noise. I doubt that
the small wire in the R7 transformer is going to survive a direct
lighning hit. Probably your beam won't either. My one experience with a
(presumed) direct lightning hit was a roof mounted aluminum tower at a
friends house. The tower legs split (blown outward) and the whole thing
ended up comming down putting the yagi boom through the roof.
tom
matching box literally exploded....the traps exploded...melted droplets of
aluminum just like a candle....not pretty...but the ground wire/rod system
protected the house.
Tom
N2LDE
Bob Lewis wrote in message <37BC5887...@erols.com>...
Mark Keith
>
> My R5 certainly did not withstand a presumed direct hit. The complete
> matching box literally exploded....the traps exploded...melted droplets of
> aluminum just like a candle....not pretty...but the ground wire/rod system
> protected the house.
> Tom
> N2LDE
Sounds like a direct hit alright. We had some mean lightning this
evening. I was almost sure I was going to take a hit, but got lucky. It
was booming all around. That stuff can make a mess. I saw an A/C
condensing unit not long ago that took pretty much a hit someway or
another though the power line. From the power line junction inside the
unit, it blew a dime sized hole though the metal to ground. Looked like
someone took a torch and blew a hole though it. Blew the compressor of
course. They had to replace the unit. I think the lightning hit the
power pole in their backyard. I worry about what would happen if I took
a hit, and someone was touching the plumbing or something connected to
ground. I had to go around the house tonight chasing people away from
the sinks etc. Makes me paranoid they will be grabbing a faucet handle
,and I'll take a hit on my mast. Which is grounded to everything
including the water pipes. I'm not sure what would happen, but I hear
it's dangerous to touch grounded stuff when a strike takes place. MK
--
http://web.wt.net/~nm5k
Dennis C. O'Connor
meter repeater antenna at the top of the tower had 50% of it just vanish...
Gone... Puff...
The rigs that were disconnected from the antenna switches and unplugged from the
ac, but had ground wires still attached, fried... The rotor control box arced
across to the nicely grounded rigs...
The power company "expert" who examined our pole pig, fried well motor, clocks
physically blown off the wall, the vaporized antenna, etc., estimates it was a
30,000 + ampere hit, in the 95 percentile range... He complimented my gound grid
between the tower and the shack... His opinion was we would have had burned
buildings but for my extensive grounding..
My rule now for the rigs/amps is totally unplugged when not in use... This means
everything has to be isolated... Of course, the antennas are grounded... Nothing
handles a direct hit... Where polyphasers, etc will help are nearby strikes with
large induced currents in your antennas and house wiring..
Denny
Richard Harrison
The rigs were disconnected from the antenna switches and unplugged from
the ac, but had ground wires attached, fried.
This is possible if the ground wires on the fried rigs were a better
path than that being traveled by the stroke which is cose enough to
flash-over to the rig. The rig must have an entrance and an exit for the
energy passage.
Dennis also wrote:
Of course, the antennas are grounded. Nothing handles a direct hit.
Dennis has much company in his opinion that "Nothing handles a direct
hit".
As a contrarian, I would say that everything directly hit handles it one
way or another. Some handle it with disasterous results. Other things
handle strikes well. Most broadcast stations are directly hit many times
a year. Most microwave repeaters suffer the same fate. Their towers are
the highest things around. Most suffer little or no damage. Most suffer
only hiccups in their continuous operation. But, some poorly built
stations have been burnt to the ground.
I worked 50 years with all types of commercial radio stations, from l-f
aircraft beacons to microwave. I`ve had hundreds of stations and towers
all over the world, and never had a lightning problem that wasn`t solved
so that all but infrequent trivial damage resulted from lightning,
It is as simple as providing an easy bypass around what you want to
protect AND providing an effective impediment in the path you don`t want
the lightning to take, namely through your equipment or yourself.
Best regards, Richard Harrison, KB5WZI
Tom W8JI
<ru...@net-link.net> wrote:
>So--how do you tell if an antenna is DC grounded or not? Polyphaser says a
>DC grounded antenna is more likely to survive a direct hit that one that is
>not.
>
>What's the scoop?
Save you money and time sorting antennas that way Rudy. The fact the
antenna is "DC grounded" means next to nothing so far as surviving
lightning hits.
Metal support antennas, like multiple dipoles on a metal mast, do a
good job at surviving hits not because the dipoles are "dc grounded"
but because the mast is large and extends past the fragile cables and
small elements.
The single most important thing is mounting and grounding OUTSIDE the
element, and having a large conductor that can carry the lightning hit
without vaporizing.
DC grounding means about zero.
73 Tom
Richard Harrison
Polyphaser says a DC grounded antenna is more likely to survive a direct
My experience agrees with Polyphaser that grounded antennas provide
better protection. With hundreds of tower-top VHF, UHF, and microwave
antennas and many trials of simple antennas that are ungrounded for
lightning, both ground-planes and gain-antennas, we experienced
lightning damage to the antenna and connected equipment nearly every
time we tried the ungrounded types. The only way we could tolerate an
antenna of this type was to put a quarter-wave, at the operating
frequency, stub across its base and make sure we had a coax ground
connection to the tower at this point.
To the contrary, antennas using folded driven elements, grounded to the
tower at the mounting point, and by the coax to the multiple tower
grounds at its base, almost never suffered any damage.
We always used Andrew heavy-duty stainless steel antennas for VHF and
UHF. It works, even though many posters on this newsgroup don`t believe
in the protection given by the folded element antennas. All I can say is
they are dead-wrong.
Tom W8JI
(Richard Harrison) wrote:
>My experience agrees with Polyphaser that grounded antennas provide
>better protection.
SNIP
>The only way we could tolerate an
>antenna of this type was to put a quarter-wave, at the operating
>frequency, stub across its base and make sure we had a coax ground
>connection to the tower at this point.
All the things you describe are meaniingless if the antenna element
conducts the bulk of the energy and it is a flimsy element.
DC grounding means nothing, it is HOW grounding is accomplished and
how wide a frequency range the ground(ing) is good that is important!
>To the contrary, antennas using folded driven elements, grounded to the
>tower at the mounting point, and by the coax to the multiple tower
>grounds at its base, almost never suffered any damage.
Mostly because the large conductor they are mounted on conducts the
bulk of the energy.
>We always used Andrew heavy-duty stainless steel antennas for VHF and
>UHF. It works, even though many posters on this newsgroup don`t believe
>in the protection given by the folded element antennas. All I can say is
>they are dead-wrong.
The only serious antenna damage I ever had was with a dc grounded
element. My shunt fed tower, with the tower solidly bonded to a large
earthing system, suffered a hit and the energy destroyed a T4XC, a
vacuum variable, and several relays.
The "DC Grounded" fiberglass VHF antenna on the tower top exploded.
Everything you describe DOES improve lightning damage immunity, but
NOT because of "DC grounding".
DC grounding is a sales pitch more than anything really important.
73 Tom
Gary Coffman
>My rule now for the rigs/amps is totally unplugged when not in use... This means
>everything has to be isolated... Of course, the antennas are grounded... Nothing
>handles a direct hit... Where polyphasers, etc will help are nearby strikes with
>large induced currents in your antennas and house wiring..
If nothing handles a direct hit, then broadcast stations would be off the air
repairing damage after every thunderstorm. But they aren't, because hits
can be handled. Many amateur installations aren't properly done to handle
hits, but that doesn't mean that they can't be properly done so that they
can handle hits.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Bob Lewis
off the air
> repairing damage after every thunderstorm. But they aren't, because
hits
> can be handled. Many amateur installations aren't properly done to
handle
> hits, but that doesn't mean that they can't be properly done so
that they
> can handle hits.
Gary, doesn't the size and strength of the antenna and tower material
also improve the survivability of commercial installations? In my
case, the aluminum tower itself failed. Perhaps it would be good to
have a heavy ground wire run the length of the tower bypassing all
the joints. It seems unlikely that the typical amateur antenna (T3
aluminum, traps, etc) would survive a *direct* hit undamaged. It
makes sense that proper installation would improve your odds -
especially because the majority of lightning damage is probably not
due to a *direct* hit.
Richard Harrison
DC grounding is a sales pitch more than anything important.
I agree that "DC" grounding is non-essential, but may be a practical
method for better suppression.
Lightning is not d-c. It is a transient pulse lasting less than a
second, with a fast turn-on time and a pretty fast turn-off time. This
pulse produces a wide spectrum of frequencies. Witness the static in
your radio.
However, direct-coupling was often used in low-frequency amplifiers, not
because it supported d-c signals, but because it was effective at very
low a-c frequencies.
What`s true for d-c coupled amplifiers may be true of d-c grounding for
lightning suppression. Of course, it may just be thought that it sells
well or eliminates a lot of questions.
Lightning is an intense wideband interference. Techniques for
lower-powered noise and interference elimination are applicable,
provided they can withstand the stress. Apparatus which rejects as much
energy as possible outside the energy spectrum you want to work with
eases your lightning protection job. A small spectrum slice arriving at
your equipment is easier to take than the whole stroke.
A 2-meter folded monopole is inefficient at gathering energy in the AM
broadcast band. That makes the energy to be handled by a gas-tube at the
receiver`s input more manageable. Grounding the coax at the top and
bottom of the tower reduces the common-mode current in the coax. The
large cross-section tower, with its own end-to-end copper ground cable,
has lower resistance and reactance than the coax.
Tom also wrote:
The "DC Grounded" fiberglass VHF antenna on the tower top exploded.
The same thing happened to me. Andrew got us to try a ground-plane
consisting of 5 fiberglass tubes which were coil forms for copper strip
conductors. The antenna was compact at 33 MHz, as compared with our
usual stainless steel monopole. The first lightning strike just days
after its installation left shards of fiberglass and black barber pole
scraps.
I believe that I am in substantial agreement with Tom.
Tom W8JI
wrote:
>Gary, doesn't the size and strength of the antenna and tower material
>also improve the survivability of commercial installations? In my
>case, the aluminum tower itself failed. Perhaps it would be good to
>have a heavy ground wire run the length of the tower bypassing all
>the joints.
That's an old false improvement that is meaningless with the type of
towers ham's use.
There is so much pressure in pounds per square inch on the bolted
joint, that there is no way resistance could be high. Not only that,
even if the joint did become "insulated" (which would never happen) it
would break down and conduct with the slightest voltage applied.
Even AM BC towers, used as antenna elements, almost always no longer
have jumper wires at leg joints.
>It seems unlikely that the typical amateur antenna (T3
>aluminum, traps, etc) would survive a *direct* hit undamaged. It
>makes sense that proper installation would improve your odds -
>especially because the majority of lightning damage is probably not
>due to a *direct* hit.
It generally isn't the aluminum that has a hard time, except right at
the point of the arc. My DB products dipole array for two meters,
placed above the beacon light of my 318 foot tower, actually has an
area melted out of the tip (about 2 inch diameter and pretty thick
wall up there) from lightning hits.
That is because the heat of the actual arc.
Elsewhere on the structure even a fairly small conductor will handle
the current, as long as there is no arc.
Any series coils, like in phasing sections of VHF antennas or matching
coils, drop so much voltage (from the series impedance) they arc and
explode.
That's why antenna DESIGN is more important than antenna current path
TYPE (ie, DC grounded). DC grounding is virtually meaningless except
for the gradual build up of charge that might prodce a "tic tic" when
something breaks down.
If the DC grounding is through a path that has high impedance even at
very high frequencies and/or if it won't take extreme amounts of
current, it will never help during a strike.
73 Tom
>
tom
I agree that commercial broadcast stations must sustain hits all the time,
but my R5 was pretty flimsy in comparison! The thin aluminum, traps, and
incredibly fragile matching network with tiny wires and toroids was simply
no match for the strike. That is the price I paid for the convenience of a
mutiband, no-tune, no-radial antenna. In may residence, a vertical is my
only real option. Now, if it was a single-band solid galvanized steel pole
with no traps or matching network, it probably would have survived.
Tom
N2LDE
Gary Coffman wrote in message <38240675....@news.atl.bellsouth.net>...
>On Sat, 21 Aug 1999 13:33:29 GMT, "Dennis C. O'Connor" <k8...@midmich.net>
wrote:
>>My rule now for the rigs/amps is totally unplugged when not in use... This
means
>>everything has to be isolated... Of course, the antennas are grounded...
Nothing
>>handles a direct hit... Where polyphasers, etc will help are nearby
strikes with
>>large induced currents in your antennas and house wiring..
>
>If nothing handles a direct hit, then broadcast stations would be off the
air
>repairing damage after every thunderstorm. But they aren't, because hits
>can be handled. Many amateur installations aren't properly done to handle
>hits, but that doesn't mean that they can't be properly done so that they
>can handle hits.
>
Gary Coffman
>On Sun, 22 Aug 1999 13:55:36 -0400, "Bob Lewis" <aa...@erols.com>
>wrote:
>
>>Gary, doesn't the size and strength of the antenna and tower material
>>also improve the survivability of commercial installations? In my
>>case, the aluminum tower itself failed. Perhaps it would be good to
>>have a heavy ground wire run the length of the tower bypassing all
>>the joints.
>
>That's an old false improvement that is meaningless with the type of
>towers ham's use.
>
>There is so much pressure in pounds per square inch on the bolted
>joint, that there is no way resistance could be high. Not only that,
>even if the joint did become "insulated" (which would never happen) it
>would break down and conduct with the slightest voltage applied.
That's basically true for a *steel* tower. It isn't necessarily true for
a light weight aluminum tower. The aluminum oxide skin of the tower
leg materials is sufficient to withstand the static loading, and can
insulate tower sections one from the other. The lightning currents
then have to arc across the joints, and as you point out, that
generates great heat at that point. A straight run of heavy copper
wire, top to bottom of the tower, can be helpful in such cases.
>Even AM BC towers, used as antenna elements, almost always no longer
>have jumper wires at leg joints.
Correct. They are almost always steel, and heavy steel at that. The pressure
in the joints is quite sufficient to ensure good electrical contact. In fact it will
almost always be greater than the pressure exerted by an approved lightning
grounding clamp on a ground rod.