RadioBanter - View Single Post

January 3rd 06, 11:14 PM posted to rec.radio.amateur.antenna

wrote:
Hi de KJ4UO,

I have a MFJ-269 and I measured a value of 65 and j16 for my 6-meter
dipole, the dipole is a little long so I assume that the j16 is
inductive and I need to add a cap of equal reactive value.
Also, The measurement was taken directly at the antenna feedpoint
without any coax.

Once I back calculate the cap value ,should I place this cap across the
antenna input at the feed point or in series with the signal line of
the feed point to cancel out the j16 and leave me with 65 ohm
resistive?

There's no point in adding a capacitor at all. Assuming you're using a
50 ohm feedline, it would only change the SWR from about 1.5 to 1.3. I
can't see where that would be worth the trouble.

But you haven't measured the dipole impedance. One terminal of the
MFJ-269 is connected to one side of the dipole. The other terminal is
connected to the dipole in parallel with the outside of the MFJ-269
itself and, if you were holding it, your body also. So you're measuring
the impedance of an antenna having a wire for one leg, and a combination
of a wire, box, and body for the other.

If you really had measured the dipole impedance and if the reactance was
enough to bother with, the answer to your question would be to put it in
series.

Roy Lewallen, W7EL