GW3NJW posted:
>I am a newbie here. I have searched the archives for a clue to my problem
>without success. So what is it?
>
>I have made and erected a 15m Moxon using the dims. from W4RNL's site. The
>construction ( a prototype) consists of a small (1x1 metre) X of 20x20mm Al
>angle, supporting bamboo canes to about 9ft ( sorry about the mixed units!),
>and then 1mm wire ( probably about AWG14G?) stretched around. The trial
>height is only about 20ft. The performance is disappointing; that is to say,
>if it were a rotary dipole at 20ft I'd be pleased, but there is no f/b (
>fixed N/W for the trial), and certainly no fwd gain ( cf a simple vertical).
>In fact, I suspect-if anything- it is firing off the back, as the 5Z4 beacon
>is a cracking signal, being well up on the vertical. Checking with the MFJ
>analyser, I was shocked to find it resonant at around 20.25Mhz- this would
>be consistent with the reflector acting as a director, but with very poor
>efficiency. So, we took the whole thing down, and double checked everything,
>finding no errors. Now something even more puzzling. Checking further HF
>with the MFJ, I find sharp "resonances" around 63 and 106Mhz, being roughly
>the third and fifth harmonic of an in band 21Mhz resonance, but which does
>not occur at the fundamental frequency, as described above.
>
>I have some nice fibreglass spreaders for the final job, but there's clearly
>no point in proceeding until I can make the prototype work. Oh yes
>everything checks out FB in EZNEC which indicates that the dims. ARE
>correct, but the &^%$?*** thing just doesn't want to play
Sounds familiar.
I have made several wire-only Moxons for the upper HF bands based
on the dimension calculator on W4RNL's site. The antennas were
mounted say 13m above ground (5m above reinforced concrete
building). They were all reasonant well below the band.
By getting a feel for how much reasonance would shift by cutting
small amounts off the driver & also seeing how much it would shift
after trimming the same amount from the reflector, I was able to get
a reasonable match & some semblance of expected pattern.
Seeing how you may also be experiencing the same thing, perhaps
our real world is somewhat removed what was assumed - if you used
the same starting point for dimensions.
Provided you used uninsulated wire & have the feeder properly
choked off, then I suggest getting it up higher & giving it a
careful trim. Also keep in mind what you see at the end of the
feeder may not what be what is happening at the feedpoint.
73, VR2BrettGraham
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