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 <[email protected]> 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
