I know a capacitive hat (or capacitance hat) lowers the resonant frequency
of a whip by electrically lengthening it, but by how much? When I was in
the service, one of our techs loaded an AFRTS BCB transmitter (1520 kHz, I
think) into a 35-foot whip that had a capacitive hat. The hat was a five
foot diameter ring of #10 wire with four spokes off the top of the whip. I
don't recall (or never knew) what other matching he did. 50 watts went
about ten miles, so it worked OK, not great.

The ARRL antenna book makes it look like the physical length is about 2/3 of
the electrical length (60 degrees physical length plus 30 degrees from the
hat).

On Thu, 27 Jul 2006 21:43:15 -0700, "Sal M. Onella"
wrote:

I know a capacitive hat (or capacitance hat) lowers the resonant frequency
of a whip by electrically lengthening it, but by how much?

Hi OM,

The hat replaces roughly double its length in height - or so goes one
proportions of one magic formula. So, for your example of:
a 35-foot whip that had a capacitive hat. The hat was a five
foot diameter ring of #10 wire with four spokes off the top of the whip.
hat would say it is equivalent to a 40 foot radiator (35 + 2 · 2.5).
I've often wondered why anyone would go to the trouble to wrestle with
the mechanical details of keeping a top hat aloft, when they couldn't
manage what would be 5 additional feet of whip in this case. Top hats
built out of guys is another story, but free-standing hats seems more
like adornment than being necessity driven.

You may note this doesn't come even remotely close to resonant for
1520 KHz - if that was the implication in your posting. Further, it
would be an amazing top hat that could for a 35 foot whip.

73's
Richard Clark, KB7QHC

Sal M. Onella wrote:
I know a capacitive hat (or capacitance hat) lowers the resonant frequency
of a whip by electrically lengthening it, but by how much?

What I do to answer questions like that for myself
is to model the antenna system using EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp

Sal,

A capacitance hat allows you to drive your car into the garage without
breaking off a whip of 15 feet equivalent height.

A capacitance hat loads a short vertical without suffering the loss in
a loading coil.

To calculate capacitance of a hat, above an antenna of given height,
with N spokes of given length, surrounded by a halo, plus resonant
frequency, download program TOPHAT from website below.

The program also calculates L and C values of the tuner. The whole
job can be done in a couple of minutes.
-----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Sorry! The correct name of the program is TOPHAT2.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

"Reg Edwards" wrote in message
news
Sal,

A capacitance hat allows you to drive your car into the garage without
breaking off a whip of 15 feet equivalent height.

A capacitance hat loads a short vertical without suffering the loss in
a loading coil.

To calculate capacitance of a hat, above an antenna of given height,
with N spokes of given length, surrounded by a halo, plus resonant
frequency, download program TOPHAT from website below.

The program also calculates L and C values of the tuner. The whole
job can be done in a couple of minutes.

That's outstanding! I see the original antenna was only about 15% efficient
and the capacitance hat improved it by about 2 dB. I will play with that
program. Thanks.

"Sal"
(really KD6VKW)

"Sal wrote
That's outstanding! I see the original antenna was only about 15%
efficient
and the capacitance hat improved it by about 2 dB. I will play with
that
program. Thanks.

=======================================
Sal, enjoy yourself with it. I do not wish to dampen your enthusiam,
but 2 dB is only 1/3 of an S-unit. Hardly noticeable! ;o) ;o)
----
Reg.

Reg Edwards wrote:
Sal, enjoy yourself with it. I do not wish to dampen your enthusiam,
but 2 dB is only 1/3 of an S-unit. Hardly noticeable! ;o) ;o)

Hardly noticeable? That's enough to be crowned champion
of a 75m mobile shootout with all the attending honors. :-)
--
73, Cecil http://www.qsl.net/w5dxp

1dB is a human-detectable change.

2dB could make the difference between a QSO and no QSO with marginal
S/N ratio.

1 "standard" S-unit of 6dB can probably make the difference between
marginal S/N and armchair copy.

If copy is already armchair, Reg's totally right. No change. I like to
make a lot of QSO's that are down in the noise, though.

73,
Dan

wrote:
I like to
make a lot of QSO's that are down in the noise ...

I do believe that meets the definition of masochism. :-)
Did you see the T-Shirt? "Life is too short for QRP"
--
73, Cecil http://www.qsl.net/w5dxp