[TowerTalk] Lossy traps?

[jimlux]

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
>