[Antennaware] Hybrid tube/wire 18MHz Moxon

[David Gould]

I would like to build a Moxon antenna for 18MHz.  It will only be 
used as a temporary antenna.

For mechanical reasons I would like to implement the long sides in 
ali tube, and the short sides with wire.  (It will be supported by an 
existing fibreglass X-frame)

I have used one of the Moxon generating tools to give me the required 
dimensions for a wire (2mm) antenna, and when I import into EZNEC it 
models perfectly.

I then used the tool to give me the required dimensions for 1/2inch 
(12.5mm) tube, when modelled in EZNEC it resonated significantly HF, 
but I could bring back to resonance and maintain the feed impedance 
by lengthening the elements.

I then changed the model to keep the long dimension as tube and then 
altered the short dimensions to wire (2mm).  The feed impedance and 
the resonant frequency change significantly.  I have tried a few 
trial-and-error alterations to the model but am not getting anywhere.

Any help would be appreciated.  In particular can anyone advise on 
which characteristics of the antenna contribute the most to 
determining the feed impedance (which dropped to about 40ohms).

Thanks in advance,

73,
Dave, G3UEG