Feeding the bobtail at bottom / votage feed is a high ohm (several
thousands) and are not very dependant on radial screen, just dc to earth.
As been said the best you might want to experiment with is to slope the
vertical wires down to minimise overall height.
Henrik
OZ1ING
----- Original meddelelse -----
> Fra: K4SAV <RadioIR@charter.net>
> Til: topband@contesting.com
> Dato: Tir, 23. feb 2010 20:41
> Emne: Re: Topband: "Linear Loading" & 160-Meters
>
> 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
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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