Re: LF: Top load coil at ground level?

[Dick Rollema <dickrollema@casema.nl>]

Rik, ON 7YD wrote

At 16:29 19-4-06, you wrote:
> Hi Dick,
>
> I guess you are right with 
> "The stub functions
> as a transmission line to transfer the inductive reactance of the coil to
> the top of the radiator."
>
> But I am afraid that for any reasonable antenna height (< 50..100m)
> the stub is that short (in wavelengths) that it will hardly transfer any
> reactance.

To check  this I took as a stub 50ft of 600 ohm open line. At one
end I connected a coil of 2mH, a typical value I think for a top load
coil. Its reactance at 136kHz is 1700 ohm.
Using a transmission line program by ARRL I find that at the other end of
the line a reactance of 1978 ohm appears, which looks like a coil of
2.32mH.
So it seems to work.

> What is the
> result when you model both quarter wave 80m antennas (+/- 20 m height)
> for 2200m ? 

The antennas and the distance between the legs of the stub (20cm) are so
small in wavelengths that the program does not work.

73, Dick, PA0SE


> At 14:54
> 19/04/2006 +0200, you wrote:
> > To All from PA0SE,
> >
> > 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). 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! The radiation diagram was
> > identical to the one for the unloaded vertical within a few tenths of a
> > dB.
> >
> > It occurred to me that the system could perhaps also be used for a 136kHz
> > vertical. Using system (b) in fig.2 the loading coil between top of the
> > radiator and topload could be at ground level. The stub functions as a
> > transmission line to transfer the inductive reactance of the coil to the
> > top of the radiator. Of course a second coil could be inserted between
> > bottom of the stub and ground for matching and feeding of the
> > antenna.
> >
> > What do you think?
> >
> > 73, Dick, PA0SE 
> >
> >