> I've seen references in ON4UN's book regarding the use of "linear loading"
> to compensate for short physical vertical antenna heights...and I recall
> reading somewhere once where Bill Orr (W6SAI) claimed that a 1/4-wave
> inverted "L" may be bent by as much as 2/3 of its "natural" length before
> there's any really significant negative impact upon the antenna's ability to
> effectively radiate a good low-angled signal...
>
>
>
> With the Bobtail, I'm thinking here of having, say, a vertical height of
> some 60', with the remaining "balance" of each vertical leg (70', or so)
> being made-up of back-and-forth parallel wires, physically elevated above
> real earth by posts some 3' - 4', or so, in height.
>
>
>
> One advantage with a Bobtail, surely, would be THREE vertical elements---but
> at the cost of just ONE radial field! But would linear loading steal away
> these benefits through inefficiency...?
>
> Hmmm...! Food for thought. I, for one, have NEVER seen any reference
> anywhere, in any publication, along this line...have you?
>
> ~73!~ de Eddy VE3CUI - VE3XZ
>
Eddy-
I have seen better approaches to this in the literature. BTW I think
that
linear loading is considered very marginal at lengths 50 percent or less
than normal resonant lengths. What has been done, however, is to hang a
Bobtail or Half Square as high as possible and run the remaining
lengths
above and parallel to the ground at the bottom. Trim for resonance. In
other words, the excess wire makes a right angle bend and parallels the
ground. I have heard it is best to keep all such wires planar- within
the
plane of the array. For a Bobtail, the bend on the middle 'tail' can be
symmetrical, i.e. like a T antenna but inverted. I suppose it would feed
'OK' with voltage feed at the junction of that bottom, center T.
It is a waste to erect these beautiful antennas and then try to corner
feed them aloft with coax. That ruins the pattern. Stick with voltage
feed at the bottom ends of elements.
It is probably important to make sure those current points remain right
in the corners, as with the classical versions. This means that you want
to keep the horizontal phasing wires overhead full length,
no more, no less. If you let those current points migrate around, you'd
probably start to 'dirty up' the pattern. You might lose the overhead
null with the Bobtail especially. It might mess with the end nulls.
All this compromises directivity and thus, QRN/QRM reduction.
Remember that tails run close to the ground introduce losses. I've heard
that on topband, a good minimum height for any bottom horizontal wires
ought to be at least 15-20 feet above ground to cut down on those
losses.
So you're needing on the order of 90+ ft elevation on the upper
horizontal
phasing wires in order to keep the losses down. The overall assumption
here is that you would not reduce the vertical runs any more than about
50 percent. Radiation resistance is dropping off fast by that point.
FWIW when I had my full size loop on 160m, I found that my performance
improved noticeably when I raised all my lower wires from around 10ft
to more like 15-20+ ft. It was definitely worth the extra effort to
get it configured that way.
One other hint is to study Cebik's work on these antenna types. You'll
find that the performance is slightly improved if you raise the tail
ends up off of ground a small fraction of a wavelength. Gain and lower
angle radiation improve, as I remember- but go too high and the array
starts growing additional minor lobes at undesirable high angles.
Bottom line for me is that 160m requires 'too much height' for
Bobtails and Half Squares and isn't much worth trying IMO.
73, David K3KY
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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