[600MRG] Impedance at the end of a dipole(or vertical).

[Pat Hamel]

Ken & all,

The old books mention antenna "End-Effect" which they claim to be due to the
capacitance of the end insulator. I was fortunate to find some foot-long
insulators years ago that I am still using.

A few Years back, the ARRL included a copy of the first Handbook when I
bought a new handbook. In the old book there is a discussion of
edgewise-wound and flat-wound coils. The difference in self-capacitance is
remarkable. 
With modern lightweight plastics and spray-on tool-dip insulation, it might
be practical to use aluminium, copper, or brass strap flashing material and
make a flat spiral coil with a lot of self-capacitance to serve as a topper
for a vertical (with a "stinger" above it) or near the end of a dipole. 

(Visualize a sloping loading wire from the top of a tower or pole. 
Then add the flat spiral coil on a standoff between the wire and the tower.
The loading wire then provides a place for the current to go and get through
the coil.)

The thing that made my inductive top-loading possible was the realization
that the coil did not need to be structural. 
Since I hung the top-loading capacitive flattop spreader from a tree,
weed-eater line supported the weight of the radiator below the coil for all
those years. "White-board" or other plastics can support the coil and the
loading wire from the tower or wherever.

The formulas for flat spiral coils are in the NBS book extract I put on the
web site in the bottom of the downloads.
73,
Pat /6

-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Kenneth G.
Gordon
Sent: Friday, September 18, 2015 2:07 PM
To: Sparkwireless at aol.com
Cc: 600mrg at w7ekb.com
Subject: Re: [600MRG] Impedance at the end of a dipole.

On 17 Sep 2015 at 23:56, Sparkwireless at aol.com wrote:

> Technically the impedance (at resonance) at the terminus of any antenna ,
in
> free space, is infinity.

Absolutely correct. That is why I said earlier that a loading-coil at the 
terminus of any antenna would require infinite inductance.

>  The figure you give is probably a lot more
> practical.    Aub

I think you're correct on this too, Aub. I got that figure from two
different 
articles by two different folks who know a lot more about this subject than
I 
do. ;-)

Thanks,

Ken W7EKB /24

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