Richard,

Thank you for the exhaustive reply!

[...] this works for RF but for lightning that
is another matter (and at different frequencies simultaneously).

LOL!

Your need for ground when you are using a dipole is unwarranted in
terms of RF.

Yes and no. A "stand alone" antenna such as a dipole is not worked
against a RF ground. (BTW, I remember seeing Hertz's original dipole
in a museum in Wuerzburg... & I swear it wasn't grounded). But a
ground is a good pulse sink if it is very permeable across the
spectrum. In that sense, a pulse sink should be a decent RF ground
too.

Make sure the ends stand off
from the supports by a good amount - I can only guess, and that would
be 10 feet.

Absolutely!


Insofar as safety ground goes, it is hard to believe that
is not supplied in your apartment - somewhere. Further, with regard
to lightning, you are probably already living inside a faraday shield
(the building's steel, skeletal structure), but make sure you ground
the coax coming into the apartment at the window or where it
penetrates a wall. To answer my earlier question, connect the coax
shield to the building frame before it begins its descent to your
apartment.

I wish...

In this part of the world steel structures are unheard of 'cept in
industrial buildings. The tallest building in the country is only 30
floors, and its structure is reinforced concrete. Also, walls are
always some form of brick & mortar ot at least gypsum. This explain
why fire safety & property insurance are almost an afterthought here.
We do have disasters, but very few in post 1930's high rises - mostly
it's cheap built 19th century buildings collapsing after a gas
explosion. Otherwise, one flat may burn out but others won't be
damaged.

Also, a power grounding is very effective for low frequency events. If
you get zapped by a pulse, all bets are off if your ground is high
impedance - and an end-fed thin ground wire is. In my case, the
shortest route to the building ground connection on my floor is almost
100 ft with lots of zigzagging.

Think of a Gamma or Delta match. Although these connect to the metal
of the structure they drive, they are inductive loops. You could as
easily build the loop and place it in close proximity to the structure
to accomplish the same thing. This gives you isolation except for the
frequency the loop is tuned to (and would limit you to a narrow band
of operation unless you have some means to remotely tune it).

I had never thought of that. So a broadband / omni antenna puts a bit
more punch into the coax than a narrowband / mispointed beam. But I'd
still rather put up a simple, forgiving, uncritical broadband, even if
known inferior.


Keeping leads apart and away from conductive structures replaces all
the exotic insulation you could imagine. Besides, if we are talking
about your T2FD and coax, I cannot imagine this is a problem in the
first place. You are going to want to drive the coax shield to ground
as soon as possible, not isolate it with insulation. You are going to
want to isolate the T2FD ends, but any induced potentials will have to
reckon with the built in resistor snubbing them. Such potential does
not merit heroic insulation efforts.

OK, I'm abandoning the heroic insulation efforts. I'll ground the
coax up on top, and introduce an insulating transformer to decouple
grounds only if double grounding introduces noise - that can be done
later at the shack end of the coax.


What is the spectrum of an EMP?
It is mostly Pulsed DC with very little RF over 1MHz. The pulse shape
is characteristically described as several µS rise and about 20 µS
fall. If you consider the strike contains 100,000A and integrate this
pulse over one second, then the duty cycle reduces this to 10 - 100 A.

Now I know what to expect when I wile away a thunderstorm sucking an
antenna plug.



I NEVER saw baluns with windings atop each other
To maintain a wide bandwidth and to preserve balance.

Got it!


I have repaired many such sets, professional and consumer alike. Yes,
some included the NE-2. That practice was discarded just as many
years ago too. There are probably a billion TVs out there, and not
one of them with this kind of protection.

Things change. Just as many more people now enjoy the other side, most
TVs get their storm thrills through the power socket nowadays -
especially if served by cable :0. Open a PSU, and you'll likely find
MOV's. Not good for RF work because they tend to have too much
capacitance....

And TV antennas tent to be 1) tiny 2) narrow beam, 3) tuned to UHF. No
protection againt a direct hit, but non that sensitive to near hits.

The market for gas discharge devices for many kinds of RF applications
is still quite lively though, and it's not for protection against
nearby transmitters.


mine the archives of rec.radio.amateur.antenna
for Richard Harrison, KB5WZI

I'm grabbing my steel hat as I type!

73, thanks again.

Filippo N1JPR/I2