Topband: Reality check request

[Dan Zimmerman N3OX]

> When I added a 2" diameter aluminum tube spaced about 6 inches from the wire
> on the side opposite the top wire there seemed to be essentially no change
> in the calculated source impedance and field strength. It made no difference
> if I grounded the tube or left it ungrounded. I made a second run with the
> wire spaced 1/8" from the tube and again there seemed to be no proximity
> effect. These results were not the ones I expected.

Tod,

I think you're OK.

The tower is very much shorter than a resonant length AND the tip of
the antenna is very far away from the tower.

The coupling between the high current portion of the inverted L and
the tower would be largely through the intense magnetic field circling
them both.  But, since the tower doesn't particularly want to support
large currents at the base (not resonant), you don't get much coupling
that way.  If you had a 120 foot tower it would be a much different
prospect. Try it in your model... you'll see the difference... view
the antenna to see the current profile on the tower and the wire.
With a 57 foot tower and the inverted L you described, I see a peak
current of about 35mA in the tower when feeding the inverted L with
1A.

I think you'd run into more of a problem bringing the tip of the wire
near even a nonresonant metallic structure, but even then it would
probably just affect the tuning... it's the high current portion of
the antenna that does most of the radiating, and you don't get any
appreciable current in a nonresonant parasitic element, no matter how
tight the coupling.

By the way, it might be interesting to check for a resonance around
80m.  The tower with a beam is basically coupled to your inverted L
like an "open sleeve" monopole.  You might actually have a useable
dual band antenna as long as you don't mind using the tower as the
radiator on 80m.

It's good to be cautious (for example, I accidentally had the source
in the TOWER when I first ran the model like you described... the
impedance looked right for an inverted L but the pattern showed a peak
gain of 11.79dBi !  Absolute nonsense

There are certainly pitfalls in modeling but I don't think you've
found one.  A nonresonant tower away from the high voltage bit of the
inverted L acts an awful lot like it's not there.

73,
Dan