Sorry about my description of the antenna. I still have not yet come to
grips with the terms of reference. I called it end fed because the coax
enters at the bottom but I have been advised it is actually a centre fed
dipole.

The decoupling is via a 1/4 wave sleeve that provides high impedance for
RF returning along outer coax and also as the second 1/2 of the dipole.

The level I noted is the signal strength seen on a spectrum analyzer
(80mm wire at end of 1m cable) - this may be the 30" section that is
producing the 60MHz dips. I also noticed the wavelength of this pickup
wire is close to 1/4 wave at frequency of interest.

Would I be better to put a directional coupler in line with the Signal
Generator output and sniff off to spec analyzer and look at dips in
signal strength as indication of radiation occurring ? (I would like to
see what the radiated power is doing at the same time as the VSWR is all
over the place and hard to see what is happening when looking at that
only). If I can do away with the wire pickup on the spectrum analyzer I
can at least remove the effects of this being super imposed on the results.

The commercial antenna I am comparing against does have a peak radiated
power at min. VSWR at frequency of interest and my antenna does not.

The style of antenna is the same (1/4 wave shunt sleeve and 1/4 wave
radiating element).

Thanks for any more help.

Regards

David



K7ITM wrote:
A half-wave end-fed wire, fed against a ground plane or similar
counterpoise, will show a high impedance at (and near) resonance. If
the antenna is fed through some length of line from the "VSWR
detector," the impedance presented to the detector will be modified by
that length of line; a 1000 ohm antenna might reflect 3 ohms through
the line. But why would you expect a drop in indicated VSWR at
resonance? -- When you say "when the radiated power increases," are
you talking about increased field strength as measured by the spectrum
analyzer, with a constant output setting from the generator? How far
away is the spectrum analyzer antenna, in wavelengths, from the antenna
you are trying to measure?

If, as Cecil suspects, the antenna is really a center-fed half-wave,
what have you done to decouple the feedline from the antenna? If the
feedline comes coaxially out the end of the antenna, it may well be
that the feedline is very poorly decoupled.

Cheers,
Tom