[TowerTalk] Trap resonance

[L. B. Cebik]

On Mon, 11 Aug 1997, Marijan Miletic wrote:

> W4RNL gives advice on traped dipole for 160 and 80m and sugests tuning trap 
> for resonance on 3,4 MHz, well bellow 80m band?  I'll assume that 3,75 MHz
> figure would be much better for wide USA 80m band.  With reasonable Q=200
> and 100pF doorknob capacitor we get 88 kOhms of isolation on resonance.

The selection of a frequency below the higher band limit is for a reason:
to equalize the gains of the antennas.  Selecting a frequency for the
trap within or at the low end of the upper band results in unequal gains,
while going too low below the upper band also makes the gains unequal.
something in the 90-95 percent of the lower end of the upper band tends to
yield performance on both bands as listed.

For more details on the affects of using a trap below the end of the upper
band, see the notes on traps at my website.  The models there are for two
bands with a 2:1 frequency range and can be scaled for the 80/160
situation.  Incidentally, the trap frequency will influence both the inner
and outer wires of the antenna.  Also, as long as the traps are the same,
their exact resonant frequemncy is not too critical; hence, the list
suggestion is a ball park number.

-73-

LB, W4RNL



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