SOME NOTES ON THE EBAY SPECIAL CHINESE
FLAGPOLE KIT
I purchased a 7.1 mtr “Aussie Flag”
version, the tallest I could find. On arrival, it appeared to be
quite well constructed, consisting of 5 tapered aluminium sections of
approximately 1.8 mtrs each. The sections “lock” in rather the
same fashion as a painters (or pool) pole. Although I did not test
it, I rather suspect you have no choice over how much of it you
deploy - the “twist and lock” internal connectors probably only
function with every section fully extended. The first mild surprise
was that there is no continuity between sections – obviously, the
unseen internal “twist and lock” mechanism is plastic and
insulating. Luckily, I had quite a few worm gear clamps of the
appropriate size, so I made up short jumper wires and clamped them
around each of the pipe transitions. One could do the same thing
with a drill and self-tapping screws, but I wanted, initially at
least, to ensure the I did not damage the unseen “twist and lock”
mechanisms.
As found, the 7.1 mtr flagpole is
resonant around 10 MHz, with VERY reasonable SWR across the 30 mtr
amateur allocation. This is worked against my 10 x 12 mtr steel shed
as a radial farm – really 10 x 14 mtrs, as there is a lean-to off
the back which is almost certainly electrically bonded to the shed,
and the antenna feedpoint at 4.5 mtrs above ground. So, if all you
wanted was a good 30 mtr vertical, you could stop reading right now!
The only comparison antenna left
standing at my shack after recent windstorms is a 55 mtr steel cable
chucked over a tree to roughly the 20 mtr mark, with strong
horizontal and vertical components. It is worked against the same 10
x 12 mtr metal shed radial farm as a sort of “semi-vertical”.
Initial impressions were that the flagpole is slightly better on 30
mtrs, but the 55 mtr random semi-vertical is noticeably better on 40
& 80 mtrs. The flagpole should be a pretty good player on 20
mtrs as well, as 7.1 mtrs must be getting pretty close to 5/8ths
wavelength, though the feed impedance would be nothing like 50 ohms
on 20.
Since the flagpole is conspicuously
short of resonance on 40 mtrs and below, I then replaced my 4
non-conductive guys with conductive guys in electrical contact with
the apex of the flagpole, of a “silly” length, nearly back to
roof level, with just enough “paracord” to ensure that the shed
roof was not part of the antenna. Once done, I could not find
resonance ANYWHERE! Well, not quite true – the system was 1:1 at
19.5 & 20.7 MHz, but I'd expected something substantially lower
than the starting point of 10 MHz. Not to worry – this antenna was
envisioned as a sort of “all-rounder” for days when my fleet of
tree-supported dipoles was grounded due to wind damage, so I knew my
homebrew T-match tuner was always going to be part of the equation.
Impressions AFTER the addition of the
conductive guys: Performance on 40 mtr is now roughly equal to that
of my 55 mtr semi-vertical random wire. Performance on 80 mtrs is
substantially worse. Nobody should be surprised at this – 7.1 mtrs
is a pretty tiny fraction of a wavelength on 80! Performance is not
so bad as to make the flagpole impractical, however.
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