Jim,
The only problem with a fixed length vertical and adjustable
matching system is that the take off angle goes up in a hurry.
Last I looked, the TO of a ground mounted 33 foot vertical on
21 MHz was above 40 degrees. 12 and 10 meters were similarly
high and the lobe on 17 meters was within 2 dB from 18 to 40
degrees.
The takeoff angles on 15 meters and above are really too high
to be of much use ... the antenna really needs some kind of
phasing or decoupling stubs for operation above 20 MHz....
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
> Sent: Sunday, November 26, 2006 1:11 PM
> To: Rob Atkinson, K5UJ; towertalk@contesting.com
> Subject: Re: [TowerTalk] HF vertical opinions
>
>
> At 09:28 AM 11/26/2006, Rob Atkinson, K5UJ wrote:
> >I don't think the no. of radials and their length affect the angle of
> >maximum radiation in the vertical plain anywhere nearly as
> much as the
> >length of the vertical radiator. the no. of radials and
> their length affect
> > how much rf gets radiated. But to avoid resurrecting
> another argument
> >about radials, I will quckly change the subject and say that
> I also think
> >the SteppIR fluid motion whatever-they-are-calling-it-now
> vertical is the
> >way to go.
>
> A few months ago, I did a modeling analysis comparing an approach
> where the length is always the same fraction of a wavelength vs a
> fixed length radiator and a tuning network at the base to match
> it. I was also looking at trapped verticals (which are sort of a
> special case of the SteppIR sort of thing)
>
> The difference in efficiency is down the sub 1 dB range (i.e. less
> than the variability you'll get from other factors) and varies
> somewhat with frequency and what that fixed length is, and that would
> be a bit more pronounced with "real" tuners.
>
> Probably the only real noticeable difference is that the vertical
> pattern changes (i.e. you radiate the same power, but it's
> distributed differently). The "always resonant" strategy has a
> pretty consistent pattern (depending on what your far field soil
> properties are), while the "fixed length with tuner" gets a variety
> of lobes, particularly as the frequency goes up. IF you know what
> angle of arrival your signals are at, then you might be able to
> optimize, but with random arrival angles, I don't know that it makes
> a huge difference.
>
> I would guess (and that's all it is right now, a guess) that you
> could make the choice based on mechanical or installation convenience
> or cost or aesthetic grounds, and not notice a heck of a lot of
> difference in performance. For myself, I went with the fixed length
> radiator and the tuning network, because it's easier to deploy in a
> portable situation, and I need the tuning network anyway for an
> active phased array.
>
>
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