I went through a modeling exercise like this a while back:
* A Tee-top vertical of #12 wire, 70 ft. vertical and +/-44 ft. horizontal
has a non-reactive feedpoint (resonant) in the 160M band.
* Lengthening the top wires to +/-85 feet raises the maximum current point
to the top of the vertical section.
* The resistive part of the base impedance rises from 22 ohms to 33 ohms
with the longer top wires.
* EZNEC with the NEC-2 engine shows a small increase in peak gain, but with
an elongation of the pattern shape in line with the top wires -- resulting
in slightly reduced gain off the sides.
I concluded that:
1. As described by Bob W7SX in QEX, the increased radiation resistance
reduces ground losses, especially given the sizeable change.
2. However, in line with Eddy VE3CUI's comments, the change in pattern shape
may negate some (or all) of the improvement.
3. It is quite possible that centering the point of current maximum in the
vertical portion is a good compromise, increasing the radiation resistance
somewhat with less effect on pattern skew. Also, changing from a Tee top to
several spokes will restore an omni pattern, but is more complicated to
build.
4. Unrelated to radiation efficiency, but important for system
effiiciency -- raising feedpoint impedance from 22 to 33 ohms makes it
easier to match, with lower losses in the matching components and broader
matching bandwidth.
There are similar issues with a lengthened Inverted-L.
73, Gary
K9AY
------------------
> Hi All,
>
> I guess I'm just plain stupid, or something, because I still can't seem to
> be able to wrap my head around this one...!
>
> One of the advantages of the "T" antenna---vs. the inverted "L"---is that
> horizontally polarized/high angle radiation is supposed to be cancelled by
> virtue of the opposing tee tops...correct?
>
> So---by lengthening the tee tops, we are---as has been stated here
> before---moving the current peak up the antenna, away from the base, and
> toward the tee...still correct?
>
> And if THAT'S still so, aren't we then creating an ineffective antenna, by
> virtue of the facts that:
...etc.
>
> ~73~ Eddy VE3CUI - VE3XZ
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|