Try adding the feedline/pole to your model. When I did this, with a
40 foot vertical run of feedline plus a horizontal run of 62 feet at
one foot above ground (and an assumed ideal choke balun at the
feedpoint), the feed impedance gets pretty close to 50 ohms.
As a "bonus", the added ground losses also increase the bandwidth to
about 140 kHz (and keep the worms warm).
Varying the feedline length, routing, and any grounding will affect
the feedpoint impedance.
73, Terry N6RY
At 09:09 PM 2008-07-24, K4SAV wrote:
>Here is one solution for matching that antenna to 50 ohms, and also
>moving it to the CW portion of the band. Make the top wire 88 ft long
>and reduce the amount of wire in the vertical portion to 56 ft. The
>gain then becomes 3.7 dBi or the same as an 80 meter dipole at 20 feet.
>The 2 to 1 SWR bandwidth then becomes 120 KHz.
>I wouldn't recommend using high power with this because it is really an
>off-center-fed dipole and feedline currents can be a real problem.
> >Don't believe everything you read in print. I just modeled an 80 meter
> >version of that antenna, using the recommended dimensions, top wire at
> >31 ft and 65 ft long, with 82 ft of wire in the vertical portion with
> >the bottom wire 5 ft above ground.
> >It resonated at 3.69 MHz with an impedance of 26 ohms. He claims the
> >longer wire on the vertical side off-center feeds the antenna and raises
> >the impedance to 50 ohms. Didn't work for me. If you matched the
> >antenna to 25 ohms the 2 to 1 SWR bandwidth would be about 70 KHz. (He
> >claims 300 KHz). The gain is 2.9 dBi straight up. (He claims 1 dBi.)
> >2.9 dBi is about the same as an 80 meter dipole at 18.5 feet. He claims
> >it is 80 to 85% efficient. EZNEC calculates 4.13 dB loss. That's
> >equivalent to 38.6% efficiency. He claims it is totally
> >omni-directional. At 30 degrees elevation it has a F/B of 3.6 dB.
> >I didn't include the feedline or any metal support poles in the model.
> >I didn't do the 160 meter version, but I would expect much poorer numbers.
> >Jerry, K4SAV
> >Marinus Loewensteijn wrote:
> >>Long time lurker here but I have a question that I do not seem to
> be able to answer satisfactory.
> >>At the moment I have 44 foot doublet up for 80 and 40 and am
> wanting to move away from a balanced feeder to coax. I am thinking
> of putting a scaled version of the above antenna up for 40 first before 80m.
> >>If it is totally a disaster then I may put up a rotatable dipole
> lke the Cushcraft D40 or Hy-gain Discoverer 7-1.
> >>Due to size restrictions and to the location of TV satelite
> dishes of both myself and close neighbours I cannot put up anything
> larger than this.
> >>I wonder if any of the TowerTalkatonians has opinions or
> experience with this antenna. (
> >>Many thanks in advance, 73 Marinus ZL2ML ..
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