On 2009-11-11 2:45 PM, Phil Smeaton VK4BAA / VK4KW wrote:
> An inverted 'L' has been strung up a tree at my QTH, with a 65 foot
> vertical section and the remainder tied-off to another tree. The top
> section slopes downwards to it's fixing point somewhat (at about 50 foot
> or so high) and is parallel to the single radial beneath it at 10 foot
> off the ground - like a big 'C' almost. The height above the ground was
> chosen so that indigenous wildlife (kangaroos!) don't get tangled-up in
> the antenna. The antenna is resonant on 1.845MHz with a good VSWR.
> Bandwidth is about 50kHz or so. Performance during the recent CQWW SSB
> contest was not up to much however.....
>
As Guy, K2AV, noted, this antenna may be somewhat lossy, especially if
the ground conductivity is poorer than average.
One thing that is suspicious is the "good" SWR. My EZNEC model shows a
feed impedance of around 28 ohms, which would give a minimum SWR of
about 1.8:1, which some would say isn't too "good". Antennas like this
that float above ground can have increased losses (and perhaps show
better match) if a feedline choke is not used at the feedpoint. There
can be circulating currents between the floating elevated and earth
ground since the shield of the coax can appear as an on-ground radial if
it is connected directly to the feedpoint. In general, it seems it is
better not to interconnect systems of elevated radials to on-ground
radials and ground rods.
To get a general idea what happens with more radials, I added radials up
to 8 total. The feed Z drops gradually from 28 ohms with one radial to
about 15 ohms with 8 radials. The bandwidth surprisingly remains fairly
constant (35-40 kHz). The average 3D gain rises from -8.3 dBi to -5.4
dBi, with the biggest jump of about 1.8 dB going from 1 to 2 radials.
With 2 or more radials, the maximum gain is away from the open end of
the top wire (opposite that with 1 radial). The gain max occurs at
roughly a 30 degree takeoff, and increases about 2.5 dB going from 1 to
8 radials. Again, the biggest increase (1.5 dB) occurs when adding the
second radial. The F/B ratio at 30 degree elevation is around 3 dB for
1-4 radials and decreases to about 1 dB with 8 radials. The resonant
frequency also declines about 40 kHz as radials are added. Note: since
these numbers are from a NEC2 based model, I wouldn't assume they are
highly accurate, but the general trends are probably valid.
Since inverted Ls typically have a lower feed Z than straight 1/4 wl
verticals, it's probably safe to say that ground systems need to be at
least as good.
73, Terry N6RY
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160 meters is a serious band, it should be treated with respect. - TF4M
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