R: [TowerTalk] Traps

[Maurizio Panicara]

I've modeled, and later built, a number of multielement yagis using an open
sleeve cell like in the F12 C3.
The results in term of practical compliance between the theorical model and
practical antennas have been extremely satisfactory, so much the real
antennas hadn't ever had any need to be readjusted to meet what aspected.
The modeling software I used is nothing special (a well known NEC derived
product), and the models are only carefully complying some fixed options I
found important not to produce wrong or meaningless results.
Uncertainty of parameters between models and reality is consequently rather
constrained to minor details.
With conventional traps, everything is apparently more complicated since
it's very difficult to model an object, the trap, that's eventually subdue
to changes (non linearity) in its phisical and electrical parameters because
of external causes (i.e. applied power, moist, temperature, etc.)
On the other hand, if the above is a real point against our rather
incomplete modeling software (in this matter), we can't question too much
against modeling compliance to reality of a trapped yagi, at least when
receiving and everything else in the antenna meets what assumed in the
model.
People is generally assuming that traps (and loading coils) losses are only
heat losses, a sort of wasted energy from an antenna whose parameters stay
anyway unmodified. This is fundamentally wrong in a parasitically excited
aerial were elements tuning is worth to obtain the proper phase shift in
conjunction whith certain spacings. Detunings of elements because of traps
or loading coils that vary their constants are not a secondary matter in
losing gain and F/B figures.
In other words, a trap (or a loading coil) can perfectly dissipate a certain
power or sustain a stated voltage across, but can equally well modify its
parameters and the effective antenna gain becomes much less than the
theorical gain minus the sum of the energy lost (dissipated) in each trap.
While transmitting, gain figures and F/B are very much at risk to become
worse than in the ideal conditions and it's not so far from the reality to
assume a real trapped yagi be always, maximum equal or probably worse its
(proper) computer model in terms of gain and F/B.


73,
Mauri I4JMY


----- Original Message -----
From: Tom Rauch <w8ji@contesting.com>
To: <towertalk@contesting.com>
Sent: Monday, July 17, 2000 1:52 AM
Subject: [TowerTalk] Traps


>
> > Neat idea, but I'm not sure the trapped antenna guys would agree.  Good
> > models of trapped antennas (not to mention linear loading) seem to be
> > pretty tricky.  For that matter, NEC-2 has trouble with modeling the C-3
> > type of driven cell -- does NEC-4 do it any better?
>
> Don't know, but this brings up something that is useful.
>
> If anyone has a single old trap from a trapped antenna, even just to
> loan, I'd like to make some measurements of ESR and inductance
> below cutoff and EPR at resonance and above.
>
> How about it folks? Anyone game to see if traps are as bad as
> advertising departments say, or if the antennas just need the
> elements tweaked?
>
>
>
>
>
> 73, Tom W8JI
> w8ji@contesting.com



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