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Sal M. Onella
July 28th 06, 05:43 AM
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).

Richard Clark
July 28th 06, 07:32 AM
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

Cecil Moore
July 28th 06, 01:01 PM
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

Reg Edwards
July 28th 06, 02:26 PM
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
.................................................. ..........

Reg Edwards
July 28th 06, 02:37 PM
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
> .................................................. .........
>
>
>

Sal M. Onella
July 30th 06, 06:52 AM
"Reg Edwards" > wrote in message
...
> 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)

Reg Edwards
July 30th 06, 05:19 PM
"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.

Cecil Moore
July 30th 06, 07:09 PM
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

[email protected]
July 30th 06, 07:24 PM
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

Cecil Moore
July 30th 06, 07:33 PM
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

[email protected]
July 30th 06, 08:29 PM
Hmm... maybe "like" is the wrong word...

Whaddya mean, QRP?? I'm running 100W into my 40m dipole on 20m with 100
feet of RG-58! Only 13dB of loss there! That's only 2 and change in S
units. I mean, 100W/10^(1.3) = 5.01W... uhoh.

Seriously though, a lot of my 6m DX QSO's on CW are within a couple dB
of being unreadable, I bet. Probably none on HF right now that could
be split this finely, because all my tough ones are being covered by
static crashes on 40 and 30m. Back when 15-10m were open, though, I
would have welcomed a "1/3 S-unit" increase on some of the hard stuff.

Of course, fading is much more than 2dB, but having the average signal
level that much higher will still make an *improvement* in readability.

73,
Dan

Reg Edwards
July 30th 06, 10:05 PM
Cec,
With a generous standard error of 1dB in the measurements, and taking
the 3-sigma limits, anybody who has claimed a crown on the strength of
1/3 of an S-unit should be obliged to hand it back to the judges
pending the other participants clamour for a re-count. ;o)
----
Reg.

Cecil Moore
July 31st 06, 12:50 AM
Reg Edwards wrote:
> With a generous standard error of 1dB in the measurements, and taking
> the 3-sigma limits, anybody who has claimed a crown on the strength of
> 1/3 of an S-unit should be obliged to hand it back to the judges
> pending the other participants clamour for a re-count. ;o)

I have a confession to make, Reg. I noticed precipitation
water from the fog dripping from everyone's coils when
the measurements first started. I waited until last when
the sun was shining and won the shootout. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Bill Turner
July 31st 06, 04:16 AM
ORIGINAL MESSAGE:

On Sun, 30 Jul 2006 18:09:58 GMT, Cecil Moore >
wrote:

>Hardly noticeable? That's enough to be crowned champion
>of a 75m mobile shootout with all the attending honors. :-)
>--

------------ REPLY SEPARATOR ------------

Speaking of shootouts, are there any scheduled this summer? I've never
been to one and I'd love to go.

Bill, W6WRT

Sal M. Onella
July 31st 06, 05:37 AM
"Reg Edwards" > wrote in message
...
>
> "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)

Agreed. What appealed to me was that the difference was so easily
calculated with your program. I simply reran the program with the hat
reduced to miniscule size and observed the difference.

This was an AM broadcast station (Armed Forces Radio Service), so the
difference was not likely to have been audible. The tech who put the hat on
the whip may just have been making it easier to match. It was forty years
ago.

Reg Edwards
July 31st 06, 06:26 AM
"Cecil Moore" wrote
> I have a confession to make, Reg. I noticed precipitation
> water from the fog dripping from everyone's coils when
> the measurements first started. I waited until last when
> the sun was shining and won the shootout. :-)
> --
========================================
Cec, now that's really making science work for you.

I can imagine you in a deer-stalker capacitance hat, using a Sherlock
Holmes magnifying glass to read the S-meter.
----
Reg.

Cecil Moore
July 31st 06, 02:11 PM
Bill Turner wrote:
> Speaking of shootouts, are there any scheduled this summer? I've never
> been to one and I'd love to go.

I haven't heard of any.
--
73, Cecil http://www.qsl.net/w5dxp

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