Rik Strobbe wrote:
At 10:16 9/03/00 -0000, G3XDV wrote:
>> Due to its directivity, a short vertical monopole has a gain of 2.6dB over
>> a dipole (4.77dBi versus 2.15dBi for a dipole).
>
>Careful! Surely this assumes perfect ground, doesn't it? Even at
>HF, I think most amateurs would prefer to use a vertical dipole than
>a ground-plane antenna.
Good point. I have been breaking my mind about that for some while. But it
seems to me that we take the inperfect ground into account already with the
ground-loss resistance, so do we have to take it into account a second time ?
In theory, for a perfect loss-free situation, a short vertical monopole
has 100% of the power applied, whereas the dipole effectively has 50%
applied to each side. The directivity of the single element short
vertical monopole is slightly less than the dipole (effectively a two
element monopole), so from a basic consideration one can see that the
gain figures from theory (see above) are intuitively about right. The
gain difference between 4.77dBi for the short vertical monopole versus
2.15dBi for a dipole is 2.62 dB (a power ratio of 50% versus 100% is 3
dB). Also note that the input resistance of the short vertical monopole
lower than that of the (balanced) dipole input resistance (in quarter
wave monopoles vs dipoles, the monopole resistance is half that of the
dipole).
In the practical LF situation, the short vertical monopole is
unavoidably located over lossy ground. Consequently the input
resistance is always HIGHER than the theoretical case for a loss-free
ground plane. I am less sure about what to do when applying this to the
regulatory situation where a limit is set on e.i.r.p. and whether it is
fair to bundle the 2.62 dB figure into the e.i.r.p. calculation. My
suggestion is to not include it, as the input resistance of a practical
short monopole is always a lot higher than the theoretical loss-free
model, so it is invalid to directly apply the theory applicable to loss
free situations.
I am aware that taking this approach means that amateur transmitters
could be increased to about double the applied power to comply with a
given regulatory limit. Amateurs would be very happy to use a loss-free
ground plane, but the reality is that they are not available in
practice. Thus I question the applicability of loss-free theory being
used to estimate radiated power.
Bob ZL2CA
|
