I had to design an antenna with similar boomlenght than a 205 and I found
best to slightly lenghten the boom from 10.4 m to 10.8 m developing a 5
elements OWA with a direct 50 Ohms feed point.
Any attempt to add an extra director, really meaningful to gain and F/B,
required the boom lenght was increased by 1.5 times and so I gave up with
that option.
The calculated gain with a 10.8m boom (35.5 ft) is 9.6 dBi with a F/B better
than 25dB from 14.15 to 14.3 (that yagi have been designed for WWDX SSB
applications) .
Calculated SWR was 1:1.4 from 14.0 to 14.35 while actual SWR at RTX edge was
a bit less but the coax line was really quite long (350 Ft of 7/8" semirigid
cable) and the losses, is known, are bidirectional.
Since an OWA with 50 OHm feed point approaches a lossless feed system but
may lose some fraction of dB if compared with an antenna having a real
"first" director, we can conclude that the absolute gain for a practical
antenna (with reasonable BW, F/B and SWR bw) in the range of 36 ft boom can
probably exceed 10 dBi , even with only 4 elements.
Incidentally, any matching system to rise a low feed point impedance
(hairpin, gamma etc.) is a bit lossy and those losses tend to increase when
the fed impedance is starts to be lower than 30 Ohm.
Once again, nothing real is for free.
73,
Mauri
----- Original Message -----
From: <n4kg@juno.com>
To: <TOWERTALK@contesting.com>; <cqki8fh@juno.com>
Sent: Friday, March 02, 2001 11:41 PM
Subject: Re: [TowerTalk] 20 Meter
> 36 ft is 1/2 WL on 20M.
>
> The W2PV Yagi Antenna Book shows that there is
> virtually NO difference in gain for a 4, 5, or 6L Yagi
> on a 1/2 WL boom. The gain vs. boom length curve
> is NOT linear and if fact, has the smallest slope in
> the region from .35 to .65 WL.
>
> 1/2 WL boom antennas offer little gain improvement
> over 1/3 WL booms (24 ft on 20M), require more elements
> to achieve good F/B and are therefore heavier and have
> higher wind load. To achieve 2 dB more gain than a
> 3L or 4L 20 on a 24 ft boom, you need to go to 5L on
> a 3/4 WL boom (56 ft). At 36 ft you would be lucky
> to get 1 dB over a 24 ft boom, and probably less.
>
> The Telrex 3L20 and HyGain 204BA have virtually
> identical gain and F/B curves, peaking at 8.1 dBi
> in the CW portion of the band and 8.4 dBi in the SSB
> portion of the band. I would be surprised if the CC
> 4L20 or the 205BA gave much more than 9.1 dBi.
>
> I'll let you do the modeling.
>
> de Tom N4KG
>
>
>
>
>
>
>
>
> On Thu, 1 Mar 2001 21:44:12 -0500 dan r goddar goddard <cqki8fh@juno.com>
> writes:
> > What do some of you believe is the best 20 meter, mono band, 36 foot
> > boom
> > antenna on the market. I just can't seem to decide on which one to
> > get.
> > As far as Ham Radio I am what some of you would a button pusher. So
> > I
> > rely on the knowledge of those who are real pro's about these
> > matters.
> >
> > Thanks
> >
> > Dan
> >
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