[TowerTalk] Trapped in antenna trap confusion

[Dan Zimmerman N3OX]

>You can use
>the parallel dipole designs which are much more efficient

Much more efficient than a design with total trap losses of 0.24dB?

There's this assumption that paralleling has zero extra loss and that
traps have big losses... but if you are talking about 0.025dB pair
trap loss for each set of off-band traps and 0.25dB in the "working
band" trap, the worst-case for a 9 band trap dipole is 0.5dB or so on
the lowest band...

A good, high Q, well designed trap consumes negligible power, so if
you build good ones, you shouldn't make your decision based on that.
I personally find trap dipole tuning to be much more pleasant than
dealing with a bunch of parallel wires.

- - - - - -

So onto the original stuff:

>I draw  the conclusion that coaxial traps are
>too low in Q, lossy and inefficient for an effective trap. However, are they
>sufficiently poor that they should be removed from consideration?

I think they're less efficient *and also* heavy, a bad combination.
I'd stick with the coil/capacitor traps.  There are all sorts of
situations in which you might choose a lossy trap intentionally as a
design goal.  You build a trap with an inductor on a half inch
diameter #2 powdered iron core with a silver mica across it and you
get an OK trap, but one that will have measurably more loss than
something wound with #12 wire and using big doorknob cap.  The
difference is that the first trap weighs almost nothing and makes for
a great QRP trap antenna, maybe worth a dB or so of loss.  Otherwise,
I'd want to use the better traps.

The thing coax traps have going for them, and in my opinion, the *only
thing* is availability.  I think it's a bad habit to use coax cable as
the capacitance element  in high impedance tuned circuits.  It's not a
capacitor, it's a transmission line.  It works OK if you can keep it
short , and a bunch of paralleled short lengths of coax is much more
like a good capacitor if you have to do it, but if you have another
option, I'd use that.

>Also, how would one ever calculate the correct
>frequency for other bands using W8JI's trap frequency recommendation?

Not really sure but I think 500kHz below the band is probably pretty
OK across the board.  EZNEC would let you directly calculate trap
losses... that's how I'd do it.  Pick an inductor/capacitor
combination that resonates at some frequency with some resistance in
series with the inductor because of its finite Q and check how much
power is dissipated in that load.  But I think it's not so critical.

>However, I can't find a
>reasonably priced source for these old mica transmitting capacitors that
>used to be so prevalent.  One could use ceramic capacitors, I suppose.

Have you dug around Max Gain Systems site?  Check out the Vitramon
things, bet they'd work OK:

http://www.mgs4u.com/RF-Microwave/doorknob-capacitors.htm

I don't know how much temperature variation matters in trap antennas.
The bandwidth of the antenna isn't necessarily narrow and you don't
want to operate the trap as a tank circuit right on resonance (hence
the tuning low) so a little bit of frequency drift in the traps might
not matter much.  I haven't noticed my 10/15m trap dipole drifting
around in impedance with temperature changes.  I built my capacitors
myself, teflon tape and copper foil, and I can't imagine that they're
very temperature stable.  The further you get from resonance with a
trap, the less its impedance changes with changes in resonant
frequency.  Of course, if you get too far, you don't get enough
impedance to do anything, but I'd bet a trap dipole with in-band traps
is quite a bit more drifty with capacitance changes than one with
out-of-band traps.

>  Trap element lengths? None of the articles I have read provide a
>formula or a rule of thumb for determining the estimated element length(s)
>of the next lower band following the trap.

It really depends on the L/C ratio of the trap... that is, for a given
trap resonant frequency you can pick a *big* inductor or a *small*
inductor and, respectively, a small or big capacitance.  A small
inductor/big capacitor trap provides very little loading on the lower
bands.  A big inductor/small capacitor trap can provide quite a bit of
loading.  So it depends.  The ones I've built, I've just done the
naive thing and cut it to "full size" in total, and then have to trim
a good bit, but it's easy that way.  Again, this is an easy thing to
at least estimate with modeling but hard to give a rule of thumb for
because it just depends on the total loading effects from all the
inboard traps, and even a little on the outboard side .. there's a
little "end loading" because there's not *nothing* beyond the element.

Anyway, I like traps better than parallel dipoles in most cases...
maybe just because I hate untangling stuff ;-)

73
Dan