Buds text here is what I mean - these are all the facts anyone needs and this
information has been out there for a long while.
Sent from my iPhone
On May 4, 2012, at 8:05 AM, W2RU - Bud Hippisley <W2RU@frontiernet.net> wrote:
> 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
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|
