On 14/02/2014 09:48, gareth wrote:
"Sal" salmonella@food poisoning.org wrote in message
...
"gareth" wrote in message
...
In another post, those of us from THIS side of the pond were
shown the Texas Bugcatcher antenna (easily googled), and its
extensive website.
And my question is this, why is it called
a capacity hat, because calculation of the capacity between the hat
and the ground will only give, at a guess, a miniscule fraction of a
puff.
Others have provided essential theory.
In practice: http://en.wikipedia.org/wiki/KFI
See paragraph entitled "Third Tower" and read
second sentence.
75 feet of broadcast tower is a lot of steel and a
fair amount of money.
Pictures of the tower and hat are at
http://www.oldradio.com/archives/warstories/640.htm
and scroll down most of the way.

Thanks, but I do not pursue URLS, my viewpoint being
that if there is something worthwhile to discuss, then
discuss it.

The latter link is a series of photographs. Are you saying you need
somebody to describe them for you?

"Kafkaësque" wrote in message
...


The latter link is a series of photographs. Are you saying you need
somebody to describe them for you?


Please don't pursue this. I would feel terrible if my post inspired yet
another ****ing contest. Let it go, please.

"Sal"
(KD6VKW)

On 14/02/2014 19:07, Sal wrote:
"Kafkaësque" wrote in message
...


The latter link is a series of photographs. Are you saying you need
somebody to describe them for you?


Please don't pursue this. I would feel terrible if my post inspired yet
another ****ing contest. Let it go, please.

Of course. But thanks for posting it - fascinating that a circle of
relatively small diameter, plus a few feet of wire, could save 75 feet
of vertical tower.

It seems that nobody has pointed out the third advantage of capacity
hats; that they reduce the Q of the antenna, and therefore broaden the
bandwidth.

"Kafkaësque" wrote in message
...



It seems that nobody has pointed out the third advantage of capacity hats;
that they reduce the Q of the antenna, and therefore broaden the
bandwidth.

That makes sense when you say it but I wouldn't have thought it. I guess
such a bandwidth broadening affect is akin to spreading out the element(s),
as with a vertical cage or a fan-wire, but without adding full-lendth
elements.

Experiment time! (aka "play time")

"Sal"

On Friday, February 14, 2014 1:09:58 PM UTC-6, Kafkaësque wrote:
It seems that nobody has pointed out the third advantage of capacity
hats; that they reduce the Q of the antenna, and therefore broaden the
bandwidth.

If we define bandwidth in terms of 3:1 SWR(50), a capacitive hat tends to decrease the bandwidth since it reduces the feedpoint impedance. For instance, the 3:1 SWR(50) bandwidth of a 1/4WL vertical is about 1.2 MHz while the 3:1 SWR(50) bandwidth of a 1/8WL vertical with horizontal top hat is about 0.5 MHz according to EZNEC.
--
73, Cecil, w5dxp.com

It seems that nobody has pointed out the third advantage of capacity
hats; that they reduce the Q of the antenna, and therefore broaden the
bandwidth.

If we define bandwidth in terms of 3:1 SWR(50), a capacitive hat tends
to decrease the bandwidth since it reduces the feedpoint impedance. For
instance, the 3:1 SWR(50) bandwidth of a 1/4WL vertical is about 1.2 MHz
while the 3:1 SWR(50) bandwidth of a 1/8WL vertical with horizontal top
hat is about 0.5 MHz according to EZNEC.

What would be the 3:1 SWR(50) bandwidth of the same 1/8WL vertical,
with no horizontal top hat, under the same conditions?

Actually I guess there would be two sets of conditions:

- No base loading coil or matching network (in which case I'd guess
it's outside the 3:1 range anyhow)?

- A theoretical (but handy) zero-loss matching network at the base,
giving it a 50-ohm feedpoint impedance at the desired center
frequency?

I think the latter is what was being referred to by the original
poster.

On Friday, February 14, 2014 7:25:29 PM UTC-6, David Platt wrote:
I think the latter is what was being referred to by the original
poster.

Sorry, I was looking at it from another angle - starting with a full-sized resonant antenna and adding a top hat for the purpose of decreasing its height.
--
73, Cecil, w5dxp.com