> 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).
I don't think on half wave is the real problem point ... you could
go as much as 20 or 21 feet (5/8 wave on 10 meters). That's the
point at which the higher angle lobe begins to predominate.
At 21' you're barely under 1/4 wave on 30 meters. 40 meters begins
to suffer but 80 is a real challenge both in terms of matching and
ground losses due to the Rrad around 5 Ohms (ground losses of 6-10 dB
without an extensive radial system).
> 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.
If you're going to do that, look at a central 24' or 33' vertical
with "open sleeve" coupled radiators for 17, 15, 12, and 10 meters.
That is probably the better way to solve both the take off angle
problem and handle the matching for 17-10.
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
> Sent: Monday, November 27, 2006 12:53 AM
> To: Joe Subich, W4TV; towertalk@contesting.com
> Subject: Re: [TowerTalk] HF vertical opinions
>
>
> 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@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.
> > >
> > >
> > > _______________________________________________
> > >
> > >
> > >
> > > _______________________________________________
> > > TowerTalk mailing list
> > > TowerTalk@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/towertalk
> > >
> > >
> >
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> >
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