> Just for curiosity I once ran a NEC simulation of the "TAK-tenna" on 7 MHz
> at 30 ft
> above an average ground. The input impedance at resonance is 3.4 ohms,
> and
> radiation efficiency just under 9%. Using 50 ft of RG-213 the input
> impedance
> is 5.4 + j 12.4 ohms, and line loss 1.72 dB. Maximum gain, at a 30 degree
> take-off
> angle, shows -7.7 dbi. Nulls off each end are -14 dbi.
> With the above results an input power of 100 W radiates 6 W. I believe
> they claim
> a low input VSWR. Have no idea how they achieve it without a matching
> network.
Running more models, and accounting for the coaxial shield, indicates a
dramatic
improvement in radiation efficiency. Resonances are noted to have a much
better match under these conditions, and are highly dependant on coaxial
cable length and position. The NEC code below shows one version
with an indicated efficiency of 76% and a gain in the region of -1 dbi.
With a modified version of the code, including 29 ft of
vertical transmission line, the input impedance was observed to be
near 25 ohms, and resonant at 7.6 MHz. Probably playing around
with various combinations of antenna taps, transmission line length,
and position would achieve something better. Typically the program
takes 2 minutes to run, so optimizing is very tedious.
It appears that many of these ultra compact antenna designs
rely on radiation from the transmission line.
73,
Frank
CM Tek-Tenna
CE
GH 1 203 6.75 0 5 15 0.03205 0.03205 1
GM 0 0 90 90 0 0 0 0
GM 1 1 0 0 0 0 30 0
GM 100 0 0 0 0 0 0 360
GW 3 30 -6.0428 0 360.704 -6.0428 30 360.704 0.03205
GW 4 1 -300 14 1 -300 15 1 0.03205
GW 5 424 -6.0428 15 360.704 -300 14 1 0.03205
GS 0 0 0.025400
GE 1 -1 0
GN 2 0 0 0 13.0000 0.0050
FR 0 21 0 0 14 0.1
EX 0 4 1 0 1 0
TL 3 15 4 1 50.00 16.317575 ! RG-58C #20 Flex
LD 5 0 0 0 5.8001E7
RP 0 1 361 1000 60 0 1.00000 1.00000
EN
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