Re: Folded Dipoles

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Comments by: GRW1_at_ATS@CTS
Comments:

This is a fairly common problem with parallel dipole modeling. The problem
often turns out to be with the element lengths chosen and not the modeling
method. It turns out that when paralleling dipoles, the lowest frequency
dipole's characteristics are reasonably unchanged near its fundamental
resonant frequency. The feed impedance for the parallel combination of
dipoles at the resonant frequency of the higher frequency elements is
reasonable also. The problem is that the feed impedance at a frequecy
slightly below resonance of the higher frequecy dipoles is outrageous.

In real world paralleled multi-band dipoles, this effect is easily seen with
an SWR frequency sweep. The SWR curve of the near resonance of the higher
band elements rises very much steaper on the low frequency side than the high
frequency side. Estimating element length just a few percent too short for
these higher band elements will often produce the results described below.

An additional practical note: the current distribution in these multi-band
paralleled dipoles is such that there is a twice a much current in the
resonant element as the sum of the currents in the other elements and 180
degrees out of phase. The current in the resonant element is typically twice
the antennal feed current.

Gary Wescom
N6CH

   -------------------------- [Original Message] -------------------------
I tried modeling parallel multi-band dipoles,
where you have a dipole for each band (e.g. amateur 20, 15, and 10 meters)
all ocnnected in parallel at the center feed point, and the wires
running parellel maybe a few centimeters apart.

For two bands, I got reasonable results (a seperate resonance for
each dipole), but for three, I got garbage (e.g. negative resistance).
Increasing the number of segments didn;t seem to make any difference.
(They warn in the manual that it doesn't work well).

Doug Braun
Received on Fri Jul 28 1995 - 20:42:00 EDT