[TowerTalk] HF vertical opinions

[Jim Lux]

At 07:26 PM 11/26/2006, Joe Subich, W4TV wrote:

>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.


Yes, for longish radiators, this happens, a 1/4 wave on 40m is a full 
wave on 10, after all.  But, consider something a bit shorter, say, 
15-20 ft.  The vertical pattern is pretty consistent until it starts 
to get close to a half wavelength (i.e. 15 ft on 10m).  Sure, you've 
got some potential efficiency issues at very low frequencies because 
the radiation resistance is getting low and matching gets more 
difficult. On 40, a 5 meter long antenna is  1/8th wavelength and has 
a Rrad of 5-10 ohms, not too far from 50.  Getting down to 80, the 
radiation resistance is dropping pretty fast to about an ohm (with a 
huge reactance), so efficiency in the impedance transformation will 
be challenging.

What I haven't looked into (mostly for lack of time) is something 
like two monopoles a few feet apart, one in the 30+ foot range and 
another half that.


>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....

And then you get into trying to figure out what that 
trap/decoupler/stub should look like, and what the efficiency of it is.




>73,
>
>    ... Joe, W4TV
>
>
>
>
> > -----Original Message-----
> > From: towertalk-bounces at contesting.com
> > [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Lux
> > Sent: Sunday, November 26, 2006 1:11 PM
> > To: Rob Atkinson, K5UJ; towertalk at 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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