As version 900 something of the lanai antenna struggle continues, I
decided to try the terminated folded dipole. I scrounged a two watt
carbon 390 ohm resistor from the junk box and added a second wire to
my antenna that folds around my L-shaped lanai, 16 feet on one side
and 18 feet on the other side.
In the end, the antenna is a very flat loop of 24 gauge wire fed in
the middle of one run, more or less, with parallel line made of two
runs of coax into an MFJ tuner. At the end pole it is fanned out to
40 inches as it turns around at the ends and heads back towards the
center insulator. The resistor is approximately in the center more or
less opposite the center insulator.
At no point is the wire more than three feet from reinforced concrete
and it is boxed in on three sides by floors, ceilings and walls of the
lanai with a metal railing just below it. Hardly "free space".
Two things were immediately obvious. First it was quieter than the
single wire dipole or other attempts at making a loop structure. It seemed
to have a slight immunity to the RFI roar of the urban environment.
Second it was MUCH broader banded! This was shocking. My MFJ analyzer
sees the creature as self resonant about 14.6 Mhz, but unlike the heavily
structure-influenced single wire dipole which is sharp as a razor and
needs retuning every 5 Khz, this terminated version is happy across the
entire 20 meter band and easily tunes to a 1:1 SWR with a very mellow
and broad dip.
It loads on 40 meters without any trouble, but attempts to transmit on
it have so far failed. There is evidence that with difficulty it could
even be forced to load on 80. If I had the 30 meter module for my TenTec
it probably would load very well on 10 Mhz.
The very first call on 20 meters, however, netted a response at 9:28 p.m.
local time, much later than I have even heard signals on previous versions.
In Japan I was getting a 439 report as the band was failing for me. I
can hardly wait to try this out when the band is usually open to the point
where my previous antennas were managing a few contacts. The fellow on
the Japan end was running a dipole only about 10 meters up an using 100 watts,
no linear or big beam.
The antenna loads beautifully on 17,15 and 10 with no arguments or any
funny hypersharp tuning required, making it a lot more pleasant to work
with. I am very anxious to see if it is going to make contacts on
15 and 17 where I have had my best successes. It is extremely encouraging
that it appears to work on 20 meters. My lanai was suspected of having
some mysterious 14 Mhz feeding gremlin that was soaking up all the 20
meter vibes. There is an excellent chance the metal railing is self
resonant at about 14 Mhz.
Sigh, there is always the chance this is a freak contact it is all due to
that wonderful acne that is starting to recur on the sun's face.
But the very mellow tuning and extremely nice loading behavior of this
T2FD version in such close proximity to reinforced concrete is in itself
a real joy.
My various other designs with loading coils, helical wound elements, and
various wire arrangements have all suffered from critically sharp tuning
and difficulty in loading due to the proximity of all that concrete.
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