Custom Calibration Interpretation

[wa9zvf]

I have an off center fed dipole that is 68 feet long hence close to resonance on 40m. I have a 2Nec model that predicts swr minimums in or very close to 40m, 20m, 15m, and 10m.  It predicts a high swr/Z in the 80m band.

I measured the feed point impedance using a 50.7 foot piece of rg58 and the custom calibration function with a large number of cal points.  The scanned results show the swr minimums at or near 40m, 20, 15 and 10.  The surprise is that there is also a nice swr minimum around 3.4 MHZ.  Is this real or does it have something to do with the transmission line being very close to 1/4 wave length at 3.4 MHZ.  If I run the numbers for 50.7 feet of rg58 with vel factor of .73 I get 3.54 MHZ for a 1/4 wave length.  Given that the vel factor may be off a bit is this coincidence or am I seeing something that is related to the feed line length?  If it is the feed line length then this suggests that there are some things that need to be considered when using custom cal.  Can anyone shed some light on this?

Ray Mikula

WA9ZVF

[g3txq]

Ray,

That's almost certainly a Common-Mode current issue. You are seeing a resonance caused by the outside surface of the coax acting as a "third antenna wire".

You need a *very* good choke at the feedpoint to stop that happening with an OCFD.

Steve G3TXQ