Tom Rauch wrote........
Message: 1
Date: Tue, 27 Sep 2005 11:11:36 -0400
From: "Tom Rauch" <w8ji@contesting.com>
Subject: Re: [TowerTalk] Top hat on mobile usefull?
To: <towertalk@contesting.com>
Message-ID: <000e01c5c375$c16a8260$6401a8c0@akorn.net>
Content-Type: text/plain; charset="iso-8859-1"
> If you can mount the top hat 6" or more above the center
loading coil
> the additional capacitance will reduce by approxmately
23-33% the number
> of turns required to resonate the antenna on 80/40m. Less
resistance =
> better performance.
Sometimes, but not always. As a general rule the hat has to be near the top
of the radiator.
Under some conditions when the hat is installed close to the loading coil
and/or if the hat is mounted low on a long whip it can actually not increase
field strength or even decrease field strength!
Two things happen.
The low hat has the effect of shunting current off that would flow to the
top of the radiator, so it can reduce effective height.
Any capacitance appearing directly across a loading inductor, even though it
reduces turns required, will actually INCREASE effective coil resistance.
The increase in effective loss resistance in the inductor can be significant
even though bandwidth and number of turns in the coil decrease.
All those bug catcher sheet metal hats mounted right on the loading coils at
the bottom of a long whip? Bad idea.
73 Tom
End of Tom Rauch's message.
----------------------------------------------------------------------------
Tom,
I had a homebrew bugcatcher style mobile antenna I used on 40m CW. I
placed a tophat about one coil length above the loading coil by using a
second Delrin coil center rod on top of the coil. I did that in order to
reduce the capacitive loading between the tophat and the coil. In your
opinion, do you think that distance would be enough to mitigate the possible
coil losses?
Secondly, in terms of effective height, I am of the opinion that so little
current exists in the whip above the coil of a heavily loaded short vertical
that the effective height is not much higher than the coil itself. So
therefore, unless you have a very big tophat at the top of the whip, such
that you could greatly reduce the inductive loading (thereby increasing the
current above the coil significantly) the position of the tophat doesn't
make much difference to effective height. The mast below the coil, and the
coil itself probably contribute to almost all the radiation. Also, I think
a few feet difference in effective height on 40m is not very important.
Your reasoning might be a better argument for increased radiation
efficiency, rather than increased effective height.
I have an unscientific observation to add. I was easily able to light a
florescent tube with the electric field at a distance of a few feet from any
point on the whip above the coil. I think that is because the tophat that
we usually use, which is about the size of a pie plate, is not enough to
decouple the effect of the whip above it completely. I suppose that leads
us to the question of whether a tophat of that size is worth the trouble at
all?
Perhaps the only way to determine that is through field strength comparisons
with the different configurations or very careful modeling.
It might be better to alter the design as follows:
Traditional Bugcatcher (starting from the bottom up).
- 5 ft solid mast.
- Bugcatcher coil.
- Tophat (spaced above the coil by one coil length).
- 8 ft flexible whip.
Improved Design
- 5ft sold mast
- Bugcatcher coil
- 5ft solid mast (or shorter if necessary).
- tophat at the top.
Given that the improved design is solid, even thought it is shorter, you
could use a much larger tophat.
My mobile antenna was guyed by 60lb test monofilament lines that went
towards the front and opposite side of the car. One could incorporate top
loading wires for part of the length of those guy lines.
Dudley - WA1X
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|