[TowerTalk] re Radials
n0ath at charter.net
Sun Jun 18 23:50:20 EDT 2006
Is there a Yahoo group or any location anybody knows of that
would help a fellow with any of the antenna modeling programs?
>From a thick headed kid from the country.
Dave / NØATH
----- Original Message -----
From: "Bill Coleman" <aa4lr at arrl.net>
To: "Bill Turner" <dezrat at copper.net>
Cc: <towertalk at contesting.com>
Sent: Sunday, June 18, 2006 5:20 PM
Subject: Re: [TowerTalk] re Radials
> 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 at arrl.net
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
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