[TowerTalk] Fwd: Re: vertical ground experiment

[David Gilbert]

I totally agree.  Due to neighborhood considerations, I've used a lot of 
different verticals in my ham radio career and I've done a lot of 
experimenting with them.  It's not difficult to achieve a pretty wide 
range of feedpoint impedances by altering the feedpoint relative to the 
vertical portion versus the radials, or even to some extent by making 
the radials non-equal lengths.  It's pretty easy to achieve 50 ohms by 
making the vertical longer than 1/4 wave and shortening up on the 
radials, or by making the vertical longer than a quarter wave and 
inserting some capacitance below the feedpoint.  As you say, a ground 
plane with a finite ground system is electrically similar to a dipole, 
and an off-center fed dipole gives a higher impedance and can even still 
be resonant (for all that matters).  It doesn't take too big a leap to 
imagine that variability in coupling between radials and ground, or 
possible radiation from the radials, would have a similar skewing effect 
on the relative electrical (as opposed to physical) feedpoint, and 
therefore the antenna's impedance.  I have never been able to accurately 
measure the field strength as a function of those variations, but EZNEC 
models point to the same thing that you, W8JI, and others have 
previously reported ... namely that feedpoint impedance by itself is not 
an absolute indicator of performance.

73,
Dave  AB7E



Rick Karlquist wrote:
> One of the reasons why the feedpoint impedance doesn't necessarily
> correlate with the efficiency is that there isn't necessarily
> any well defined location that represents a line of demarcation
> between the "antenna" proper and the "counterpoise".  You can
> build a thin dipole, put it in free space, and insert a small
> source in series at the center and expect to get 73 ohms without
> much chance for trouble.  You know exactly where the source is.
> You can expect to get half of 73 ohms impedance for a vertical
> only if it is over a perfect infinite ground.  Otherwise,
> the counterpoise is also an antenna of sorts.  Where does the
> vertical start and the counterpoise end?  At your source?  That
> is an accident of where you chose to put the source.  It can affect
> drive impedance either by adding loss or by redistributing current.
> It may even radiate to some extent.  You can't tell how much
> of each effect is in play, unless you measure field strength
> at a distance.  Also, in real situations, the coax may radiate
> if you are not careful.
>
> Rick N6RK
>
>   
>