[Antennas] Trap design, how far from operating freq, should I hit?

[Ed Worst]

Hi John,

Let me offer my meager experience on this subject.  Last year, there was an
article in QEX about the DEP (Dominant Element Principle) Dipole by Al
Buxton, W8NX, that uses traps that are NOT resonant at the antenna operating
frequencies.  The reason you'd like to have the trap resonant somewhere
other than the frequency you're operating on is to minimize the losses in
the trap.  At resonance, the current will be a max, so your I^2*R losses are
higher than off-resonance.  OK, so where should the traps be resonant?  If
you want a two-band antenna, multiply the two operating frequencies
together, then take the square root of the product... F(resonance) = SQRT(F1
* F2).  So if you wanted to operate on 12m and 17m, F(resonance) =
SQRT(24.940 * 18.118) = 21.257MHz.  The traps should be located at
approximately the mid-point of each dipole arm, and the inductance should be
in the 250 - 500 ohm range.  (This will make the antenna about half it's
normal length at the lower frequency.)  Pick an inductance for this
reactance, then find the capacitance that resonates with it at the
calculated F(resonance).  Now fire up EZNEC if you have it, model the
antenna and the traps, and adjust the inductance for the lower frequency of
operation, and the capacitance for the higher frequency of operation.  I did
this for a 20m and 40m dipole, and it works (on EZNEC).  I haven't built the
antenna yet, but it's such an unusual approach that I've gotta see how it
actually works.  The author of the QEX article is convincing.  Note that the
final step of the design is a "guess and check" approach.  The QEX author
has a GWBASIC program to do that, but I couldn't get it to work, and didn't
need it anyway.  As you may have noted, there is a possibility of a third
operating frequency with this particular antenna at 21.257MHz.  W8NX's
software is supposed to help you find trap LC values that will give you a
tri-band dipole with only one set of traps.  I believe it can be done, but I
haven't tried that yet.

I hope this helps, and maybe someone with more experience can jump in and
take our knowledge to the next level.

73 es good luck.
ed - k9ew