Topband: Help needed: Windom & MFJ-259B measurements

[Sinisa Hristov]

Thomas Herrmann wrote:
 
> I wonder if my MFJ-measurements on TopBand-Windom (FD4, off center fed dipole, ...) are correct. How the results should be while checking both resonance frequency (advanced mode => X=0 ) and impedance (normal mode).
 
> I measured at the feed point both with/without Baluns (1:1; 1:4; 1:6) without getting an clear indication of resonance frequency, harmonic frequencies and inpedanz. How to do it correctly ?


It is very difficult to do it correctly.

Due to highly assymetric construction,
common mode voltage is very high at the feedpoint,
possibly exceeding driving voltage.

A balun with exceptionally high common mode
impedance is required (tens of kOhms),
and it is unrealistic to expect from it
a good choking action and an impedance
transformation simultaneously.

First of all, try to avoid asymmetric antennas.
If you must use them, try to understand what are
you doing, including full understanding of the common
mode voltage at the feedpoint.


73,

Sinisa  YT1NT, VA3TTN