Before jumping to the conclusion that miles and miles of copper are needed
under a grounded monopole, here's what I think should be inferred from these
two graphs:
1. Very, very short verticals (as a percentage of wavelength) are a bad, bad
idea. From the knees of the curves on the two graphs shown, 30 degrees'
electrical height is a practical minimum for most situations.
2. Using only two radials (lying on the ground) is not a good idea, either.
(For _elevated_ radials in conjunction with a vertical whose base is similarly
elevated an adequate amount, two radials aren't so bad, although four or eight
are better. "Adequate" is probably a minimum of 1/8-wavelength, which is not
"chump change" in support costs on 160 meters.)
3. Using radials that are longer than a vertical (of reasonable electrical
length) is tall simply wastes a lot of money (and real estate). The graphs
show that for a vertical whose electrical height is about 75 degrees (the
tallest height for which the two graphs can be compared), the difference in
measured field strength at 1 mile between 113 radials that are about
1/4-wavelength long and 113 radials that are 50% longer is 180 vs 190 mV/m.
That's less than 0.5 dB.
4. Using 113 radials instead of, say, 15 radials with a vertical of
_reasonable_ height (let's use 75 degrees again), the difference in measured
field strength for the shorter radials (approximately 1/4-wavelength) is 180
vs. 152 mV/m. That's less than 1.5 dB.
5. Using, say, 60 radials of 0.41 wavelength vs. 60 radials of 0.27 wavelength
with a 75-degree vertical height results in an increase in measured field
strength at one mile from 176 to 181 mV/m, which is less than 0.25 dB
improvement for a 50% increase in radial wire, physical effort, and cost.
Some additional comments:
Note that the field strength scale on the graphs is linear, whereas what counts
when we're operating is logarithmic (dB).
Wires lying on the ground are not resonant anywhere near their free-space
resonant frequency. Better, instead, to think of your radials as long, skinny
capacitors that are important to the operation of a grounded monopole because
they improve your vertical's efficiency by facilitating low-loss passage of
Maxwell's "displacement current" between one pole (the vertical element) of the
antenna and the other pole (ground + radial field) of your antenna.
Given the choice, try to make your vertical as large a fraction of a 1/4
wavelength as you can, given your specific installation (and financial)
circumstances. Top-loading of a tower with HF Yagis is one way to get good
electrical length from a metallic structure that is substantially shorter than
a quarter wavelength. A push-up mast with 3 or 4 top-loading wires of
sufficient length is another. An inverted-L wire is yet a third. (Think of an
inverted L is an asymmetrically top-loaded vertical.)
Once you've put all your psychic energy into making the electrical length of
your vertical as high as you can, _then_ (and only then) put down a dozen or
two radials of whatever lengths "fit" in your space. As others have reported
here and elsewhere, the shorter your radials, the fewer of them you will need
to "max out" your radiated field strength. (Of course, your field strength
will be less with a few short radials than a lot of longer ones. That's the
disadvantage of small spaces that you have to accept. Or use K2AV"s FCP.)
Bottom line: From my two decades of experience DXing on 160, there's far too
much angst about the number and precise length of radials for amateur
installations.
Bud, W2RU
On May 4, 2012, at 6:43 AM, Richard Fry wrote:
> The link below leads to two graphics showing the __accurately measured__
> fields using various numbers of buried radials of 0.274 and 0.412
> wavelengths (radial lengths as measured in free space). These graphics show
> the groundwave fields for linear, unloaded monopoles up to about 95 degrees
> in electrical height. Earth conductivity at that test site in New Jersey
> was not higher than 4 mS/m.
>
> The elevation fields of these monopoles varies approximately as the cosine
> of the elevation angle. Maximum radiated field always occurs in the
> horizontal plane for these electrical heights -- so the greater that field
> in the horizontal plane, the greater the field at angles above the
> horizontal plane. Pattern shapes for monopoles of these electrical heights
> are independent of the operating frequency.
>
> The data show that the system using 113 x 0.412-wave buried radials produces
> the highest field, particularly for shorter monopole heights.
>
> It is a judgment call as to what set of buried radials is needed by the
> user. AM broadcast stations typically use 120 x 1/4-wave (or longer) buried
> radials, but amateur stations may not be able to justify this.
>
> http://i62.photobucket.com/albums/h85/rfry-100/BLandERadials.gif
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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