On Jun 17, 2006, at 1:08 PM, Bill Turner wrote:
> At 05:55 PM 6/16/2006, Bill Coleman wrote:
>
>> Horizontal antennas, like dipoles, make great antennas, but they are
>> affected by proximity to the ground. They have to be at least 1/4
>> wave above ground before the radiation pattern is anything but
>> straight up.
>
> *********** REPLY SEPARATOR ***********
>
> So you are saying that a dipole at less than 1/4 wavelength high
> has *only* straight up radiation?
To make sure I have my facts straight, I've gone and modeled a 20m
long dipole at heights of 10m, 20m and 30m at a frequency of 3.6 MHz
(which is darn close to resonance at low heights).
At 10m high, the main gain lobe is at 90 degrees. At 30 degrees, the
gain is about 3.7 dB down from the peak, and it falls down very
quickly from there.
At 20m high, the main gain lobe moves to 57.4 degrees, although the
90 degree point is only a few tenths down. At 30 degrees, the gain is
down 1.5 dB, and it falls off quickly below 20 degrees.
At 30m high, the main lobe moves to 36.6 degrees, the 90 degree point
is down almost 4 dB from the main lobe. Gain falls off rapidly below
10 degrees.
It's pretty evident from modelling that my statement above is true --
you have to get a horizontal antenna higher that 1/4 wave above
ground before the majority of your signal isn't going straight up.
Did I say that there's no radiation or pattern in other directions?
No, I didn't say that. Even a dummy load has some radiation or
pattern -- it's just not efficient.
Does that mean a low dipole won't work? Heck, no, because EVERYTHING
works. Some antennas work better than others, but if you have enough
patience and put in sufficient effort, you can work DX with any antenna.
Bottom line, if you want to get your radiated signal from a
horizontal antenna below 30 degrees -- the angles at which the DX
signals arrive and depart -- you need to get it 3/8 wave or higher.
On 80m, that's 30m high, or 100 feet. For 160m, double that.
THAT's the reason many hams choose to use verticals for the low
bands, despite all the hassle of putting down radials. They just
don't have the mile-high supports necessary to do a good job with a
horizontal antenna.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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