> I'm sure someone has done a nice NEC analysis comparing the
> two, at least from a loss standpoint. I'll bet just about
> anything is less than 10% loss, so even if you halved it
> to 5%, the forward gain is going to change a tiny fraction
> of a dB. You might do better just wrapping the elements in
> copper foil.
It's rather difficult to do an analysis for more than two
bands due to the various interactions and changing loss
with frequency due to circulating currents, etc. However,
I believe L.B. Cebik, W4RNL had a fairly good analysis for
a two band trapped yagi on his web site.
Practical measurements using the K7LXC/N0AX protocol tend
to show that the two element per band Force 12 designs tend
to show as much "real world" gain as larger (e.g. TH7DXX
and KT36XA) trapped designs. The larger "multi-monoband"
designs (e.g., C31XR) will have somewhat higher gain).
The tunable antennas (e.g. SteppIR) will generate higher
gain than any multiband antenna of the same boom length
with a three element SteppIR "real world" gain within
10% of longer boom "multi-monoband" designs (where the
full boom is not used on every band) and the four element
SteppIR showing "real world" gain 50 to 60% higher than
a comparable length "multi-monoband" design.
Analysis of data from several different tests/testers
all using the same protocols indicate a gain deficit
(measured vs. expected) of 1-2 dB for trapped antennas.
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
> Sent: Wednesday, October 29, 2008 6:56 PM
> To: Dan Hearn
> Cc: Tower Talk; Jerry K
> Subject: Re: [TowerTalk] Lossy traps?
>
>
> Dan Hearn wrote:
> > Did you ever stop to think that the usual tri band trapped
> beam uses a
> > single coax feed for all bands and to get a 50 ohm feed point
> > impedance it is necessary to detune the elements from their best
> > length for gain or f/b. In addition, the element spacing is
> the same
> > for all bands so another compromise there. 73, Dan, N5AR
>
> But how much of a compromise? I think that's what the
> original question
> was getting at.
>
> Let's talk in terms of modern antenna design, so we're not
> necessarily
> talking about designs from the 1960s which were developed
> with a goodly
> amount of cut and try, notwithstanding that they can work well.
>
> I don't think that "detuning" is the right word for trapped
> elements to
> get them to work. Yes, there will be some changes in resonance,
> couplings, and current distributions, and that inevitably
> will have an
> effect on the patterns and feedpoint properties. However,
> de-tuning has
> sort of a negative connotation. I would say "retuning" or
> just "changes"
>
> And, going into the exercise, one is constrained by the physical size
> limitations (i.e. fixed spacing on the boom, and elements).
>
> The real question might be:
>
> Given an overall envelope and mass, is there an important
> difference in
> performance between, say, a 3 element antenna with traps and
> something
> like a Force12, which is basically multiple single band antennas
> interleaved on the same boom?
>
> The former is probably going to be lighter (although, the elements on
> the F12 are smaller in diameter than the elements on the
> trapped beam,
> because they don't have to support the weight of the trap).
>
> It might be cheaper for the trapped beam:less hardware and
> aluminum, but
> you have to pay for the cost of the traps/loading components.
>
> I suspect that in terms of feedpoint impedance bandwidth, they'd be
> comparable: both are essentially arrays of coupled low Q resonators.
>
> The traps might have some additional loss, but inductor Qs of 300 are
> easy to get, which implies an awfully low IR loss. The F12
> style beam
> couples some nonzero amount of power into the other elements,
> so there's
> some IR loss there.
>
>
> I'm sure someone has done a nice NEC analysis comparing the two, at
> least from a loss standpoint. I'll bet just about anything is
> less than
> 10% loss, so even if you halved it to 5%, the forward gain is
> going to
> change a tiny fraction of a dB. You might do better just wrapping the
> elements in copper foil.
>
> What WILL be different is the side and back lobe performance. With
> multiple elements to play with in the F12 case, there's more
> variables
> and degrees of freedom to work on. However, will it be an important
> difference? Are you going to go from -20dB F/B to -6dB? Or
> is it going
> to be a -20dB to -21dB situation.
>
>
> The difference illustrated below between the TA-33 and the C3XLD may
> well be more due to the ability to use those additional degrees of
> freedom, rather than due to any elimination of "trap losses".
>
> Intellectually, I like the F12 design approach over the 3 element
> trapped approach. It's cleaner. And, with modern modeling
> tools, the
> ultimate performance is probably better.
>
>
> >
> >
> > A fixed element non-trapped tribander with only three
> elements appears
> > to be an oxymoron. In the only comparison I can offer, I ran a
> > standard Moseley TA-33 trapped tribander for years with decent
> > results, finally went to a Force 12 C3XLD (3 bands, no
> traps, but F12
> > doesn't call it a "tribander") and the difference was literally
> > astonishing. But the F12 was a $1400 antenna with 10 full-length
> > elements on a 33' boom with separate feeds for each, weighs
> 75 lb, and
> > is essentially three monobanders on a common boom (10M used 4
> > elements). From day one the Force 12 was the proverbial 'nuclear
> > weapon' in a pileup. The difference was dramatic, and yes,
> people 5000
> > miles away and more certainly did notice the difference.
> Just for fun,
> > I also occasionally ran QRP to that antenna and had a blast
> trying to
> > convince Europeans I really was running only 800mw. :) 73,
> Jerry W5KP
> >
>
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