Adjusting the antenna

Tuning the antenna produced something interesting. The plots below were made with an Anritsu Site Master S113C. This device is used a lot all over the world for measuring Antenna circuits at Cell Phone sites etc.

The return loss and corresponding VSWR of the Antenna are very poor according to the Site Master.  I tried several hours improving it using different coupling loop geometry etc. but without success. Also two resonant frequencies, 200 kHz apart, are reported which I could not explain for a while. When adjusting the Capacitor the two dips would move, proving that I was looking at the resonance frequency.

I then borrowed an Antenna Analyzer, ANTAN designed by F6BQU and was in for a surprise. Only one resonance frequency was found, exactly between the two dips on the above plots!  By adjusting the geometry of the coupling loop a VSWR of 1.5:1, equivalent to a return loss of 14 dB, could easily be achieved. (Much) later this was also verified with the Anritsu.

What remains now is figuring out why I can not tune the loop to 7.2 MHz. Loop Inductance and measured minimum Capacity of the Capacitor indicated that this should not be a problem. However this was before I made the housing for the Capacitor. (I forgot to measure the minimum Capacity when the housing was finished.) The housing was made from Laminate normally put on a floor. The top layer of Laminate is very hard as I found out when cutting pieces. Now if that top Layer contains something that conducts the minimum Capacity will have increased. For the moment I have tuned the Antenna to 80m and made a few QSO's, Denmark, UK and local. Nothing spectacular but it proves that the 100W I fed it are not all going up in heath. Also important, VSWR is stable so the Capacitor is holding up and not sparking.