[TowerTalk] Traped antenna for 2:1 freq range

[L. B. Cebik]

On Mon, 11 Aug 1997, Marijan Miletic wrote:
> LB, W4RNL was giving advice on traped dipole for 160m and 80m and sugests using
> trap on 3.4 MHz in order to equalize the gain.  It sounds to me as M$ science.
> One can hardly expect shortened 160m dipole on half the wavelength height to be
> as good as full size 80m dipole so we simply make trap a poor isolator and
> dimish the gain.  The importance of the "same" traps regardless of the resonant
> frequency beats the laws of physics but may fit some simulation programs.
> 73 de Mario, S56A, N1YU.

I rarely reply to such notes, but in this case, perhaps we can dispel
some misconceptions so that no one will think a. that any laws of physics
are being violated (although I do not see any evidence of which laws are
supposedly violated--it seems a handy thing to say when we wish to make
heat rather than light) and b. that traps are anything more than traps,
even in antenna modeling programs.

The theory of traps goes way back into the 40s, and by the 50s, it was
empirically recognized that when the trap frequency is set lower than the
low end of the upper band (assuming a 2-band system), performance on both
bands was relatively equalized.  All that modeling has done via method of
moments calculations (which do not violate any laws of physics, even if
the technique is elusive to many, although reading a chapter in Krauss,
2nd Ed., will introduce the technique) is to reflect what has already been
empirically established.  Modeling is useful in this regard because it
permits the ready development of performance curves that may require large
scale efforts to develop empirically.  Models provide performance
expectations, but are limited in ways that have been fully covered in the
literature.  Everything I have written using method of moments antenna
modeling techniques has given full account to the limitations of the
techniques, and I have written on limitations that appear not to have been
previously recognized with the latest versions of NEC (-4).

The proposed antenna, with or without traps, is limited by height factors.
This is a given in the situation.  Traps do reduce performance, but well
designed traps can minimize the reduction.  If a trap is designed to
resonate within the upper band, the performance will decrease on that
band.  By lowering the trap resonant frequency, the performance improves
on the upper band, but shows a gradual decrease on the lower band.  By
appropriate selection of trap resonant frequency, one can equalize the
reductions so that each antenna loses no more than about 0.15 dB relative
to a full length dipole for each band in the same position as the trap
antenna.  How significant 0.15 dB is to an operator is a judgment call.
Of course, badly designed traps will yield worse performance.  To the best
of my knowledge, coaxial traps have yielded the best trap Qs (lowest
losses), although they tend to be heavy.  Like all antenna parts, traps
need regular preventive maintenance for best performance.

Traps need not reduce the performance on the upper band.  On the lower
band, performance reductions are a combination of a. the losses in the
trap, and b. the reduction of gain natural to the shortening of the
overall length of the antenna on that band.  In a 2-band system, the
performance on the upper band can be slightly sacrificed to elevate
performance on the lower band.  The norm is equalization, but one can
design special purpose antennas which enhance one band to meet a specific
design goal.

Alternative to the use of traps is the dual dipole with a common
feedpoint.  When the second (upper band) antenna is separated from the
lower band antenna by about 3-5' at its end at lower HF frequencies, the
difference in performance from independent dipoles is insignificant
(about 0.05 dB), given the 2:1 frequency ratio of the pair which ensure a
high impedance for the unused band.  However, such antennas require 2 wire
sets and means of keeping the wires separated in varying weather
conditions.

The choice of systems is largely a matter of physical constraints on the
antenna installation.

Nothing here is invented--all can be found in both the professional and
amateur antenna handbooks--although it may be scattered and incomplete in
any one account.

The idea that traps are inherently bad because they have losses is one of
those overgeneralizations that passes for knowledge when extracted from
contexts.  In some applications, traps of inadequate design do hurt
performance needlessly, since alternatives may be available.  For example,
I am leery of antennas employing large numbers of traps to cover lots
(5-6) of bands, since losses are additive. However, in other applications,
traps may be exactly the right design solution to a complex antenna design
problem that includes the physical setting of the antenna as well as all
other mechanical considerations and operating goals.

If everything that produces losses in antenna system is bad, then antennas
would be impossible.  Just as we produce better means of connecting
antenna parts to reduce losses, just as we use lower loss transmission
lines, etc., so too, when traps are called for, we adopt the lowest loss
designs we can.  Like all other components and systems in electronics,
traps conform to the laws of physics.

On the other hand, I am always interested in and ready to learn of new
places within the extant antenna modeling programs where the algorithms
are in conflict with specifically cited laws of physics applicable to the
situations the programs are designed to model.  The NEC handbook, the
commercial inteface package instruction manuals, and various external
writings have tracked a number of such limitations and thereby established
parameters within which decent modeling must be done.  But everyone
involved in developing or seriously using such software stands ready to
hear in detail of new difficulties.  Otherwise, there can be no NEC-5 to
overcome the limitations.

I hope this sheds some light on traps, on modeling, and on good procedures
for raising questions about either.

-73-

LB, W4RNL


L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html






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