Hi, Dick.
On Wed, 19 Apr 2006, Dick Rollema wrote:
>By chance I came across an interesting article in the Dutch weekly (!)
>magazine Radio Expres of 11 January 1934. It referred to an article in
>the
>Proceedings of the IRE of December 1933 by Nickle, Dome and Brown on a
>procedure to increase of the feed point impedance of a vertical antenna
>by
>means of top loading. A normal way is to use a series resonant circuit as
>in fig.1, consisting of a capacitive hat and a coil. The coil is in a
>awkward position for tuning. The alternative by Nickle, Dome and Brown is
>to use a short circuited stub slightly shorter than an quarter wave that
>at
>its top end produces the proper inductive reactance for series resonance
>with the capacitive hat. Indicated in fig. 2a. The system is tuned by
>moving the short circuit at the bottom up or down. The left leg of the
>stub
>is doubling as the radiator. An easier way of tuning is to make the stub
>too short and lengthening it with a coil(b) or a stub that is too long
>and
>shortening it by a capacitor (c).
It is nice idea! Certanly this should work. Elevated coil is better then
coil near the ground, not too much better but better anyway. And a
tuning problem gone if one use fig.2.
>I modelled a quarter wave vertical for 80 m
>according to (c) using Antenna Optimizer by Brian Beezley, K6STI. When
>in
>resonance the feed point impedance was raised from 36 ohm without top
>load
>to 4925 ohm!
This means you change 1/4 wavelength resonance to 1/2 wavelength
resonance. This is imposible on 136 kHz when vertical is much shorter
then 1/4 of wavelenth.
On 136 kHz radiation resistance will not rise too much when you'll use
elevated coil (or someone the same as in fig.2). But it is worth to note
enviroment loss may decrease significaly.
73 de RA9MB/Alex
http://www.qsl.net/ra9mb
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