Edward Swynar wrote:
> ... 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.....
>
>
I thought there might be significant loss due to the shortness of the
elements, but I didn't really know how much, so I put it on EZNEC. I
first started with a full size 160 bobtail with the top at 140 ft. I
fed it at the top of the center element. That had a gain of 6 dBi over
average ground. I used #12 wire and that contributed 0.3 dB of loss.
I then lowered the top to 60 ft and placed the bottom at 3 ft, keeping
the spacing the same, and folded the vertical elements until I got the
SWR back on the right frequency. The vertical wires each required a
total length of 164 ft of wire, folded at 1 ft spacing. I checked the
current phasing in the vertical elements and it was good. The gain went
to 2.28 dBi. The #12 wire now contributed 1.4 dB of loss. Changing to
#10 wire gains 0.27 dB. The feedpoint impedance was 13.5 ohms.
(Feedpoint impedance of the full size bobtail was 41 ohms.) Of course
you can feed these from the bottom of the center element too, but I
didn't simulate that.
Ground quality has a big effect. Average ground (0.005/13) gives 2.28
dBi gain, very good ground (.03/20) gives 5.2 dBi, and very poor ground
(.001/5) gives -1.86 dBi. You might pick up a little gain by adding a
lot of radials under each vertical element. Moving the bottoms of the
wires higher above ground will also decrease the loss, but that may be
difficult to implement if you feed it at the bottom.
Jerry, K4SAV
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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