[TowerTalk] 40 meter interaction

[Maurizio Panicara]

Hi Tom,

in my opinion this is a modification with an high risk to ruin the typical
pattern performance of the vertical array, I mean nulls and high angles
rejection .
Actually, if the horizontal antenna is placed over the verticals, the
coupling is not so neglectable as one might hope.
Speaking of vertical and horizontal polarization in this case is quite
unproper, not only because of near and far fields as already explained by
W8JI, but because oriented with the dipole "endfire" direction and along the
imaginary line that goes from the vertical base to the dipole center, the
antenna fields have also the same polarization and are, more or less, in
quadrature (and this even neglecting the story with radials)  .
Using a geometrical analogy, if the "projection" of one antenna above the
other is not a point, some degree of coupling exist and this fact screw up
things, not only because of re-radiation but also because of the mutual
coupling involved.
I believe that everyone tried once the directional array made with 4 half
wave dipoles slooping from a tower where one is feed and the others are
tuned as reflectors. If the dipoles are 45° sloped, they are 90° to each
other in the XY plane but they are coupled and equally polarized along the
ZY and ZX planes. Well, more or less this is what happen, in a tilted plane,
between a vertical and an horizontal dipole thats not exactly centered over
it, and still neglecting the radials coupling.
If to cure the interaction of the low horizontal antenna, some dipole
detuning is obtainable (best is probably with a resonant line, left open),
less predictable and manageable are the common mode currents flowing in the
array composed by the dipole halves and its feed line.
I don't believe even an high impedance choke is enough, probably the most
correct idea is to individually resonante the chokes (wound coax and a
capacitor) on 40m and placing a consistent number of such devices along the
line.
Finally, isn't any other place to erect such low horizontal dipole ?

73,
Mauri I4JMY




>
> Hi TT'ians.
>
> I now have 3 phased verticals on 40.  They work good for dx'ing
> but suck for domestic contesting.  I'd like to put up a dipole
> here but the problem is it would go right over the top of the
> verticals.  The dipole would be at about the 70 foot point up the
> trees putting it about 30 feet or so above the verticals.  Would
> that screw up the pattern on the verticals by having a dipole
> right above them?  Probably wouldn't help things.  73 and HNY
> Tom W7WHY



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