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installations. Subject 1/4 & 1/2 wave Verticals and Radials > > It has been said: > > > Also, ANY 1/4 wave vertical _absolutely requires_ a > > ground plane to work against!!! 1/2 wave and 5/8ths > > wave verticals are the only exception to the ground > > plane requirement, and even they can benefit > > from the decoupling characteristics of a good GP. > > First, about radials and 1/2 wave vertical antennas: > > Here on Kauai, out at WWVH, they do use Rohn, > 1/2 wave vertical arrays. > > They found an extensive radial system was required to > get their field strength back up, out in the far Western > Pacific areas! The problem occurs because the bottom > end of the 1/2 wave vertical is a "high voltage" point; > and, yes, the current, and hence the radiation is highest > up at the feed point 1/4 wave up the tower. However, > the high voltage point near the ground is a source of > intense E field lines which penetrate the surface of the > ground. > > At WWVH, 1/2 wave vertical, center insulated, > Rohn tower arrays are used on 5, 10 and 15 mHz. These > towers have been up since 1970-71, and now need > to be replaced -- Rohn will not quote them again!! > Evidently they no longer believe the big tower center > insulator is a good idea, hi. > > You should see the 2.5 mHz tower!! A full half wave plus > tall!! But at 2.5 mHz, it is a single 1/2 wave tall tower, not > an array as for 5, 10 and 15. > > Anyway, without a high conductivity radial system on > the ground surface below the lower end of the verticals, > a LOT of current is induced into the ground, which flows > out as the voltage on the bottom end of the 1/2 wave > vertical rises, then flows back toward the tower > as the voltage reverses polarity on the other half > RF cycle. Even near the beach, there was a lot of > field strength loss to the WWVH signals until the > big radial system was installed. > > These radials below the 1/2 wave tall antennas do not > need to be resonant, either. They just need to > be long enough to give the current a low loss > path to flow in as the lower end of the full half > wave vertical oscillates in voltage polarity. > It would have been nice to put the original > Rohn tower arrays out in the ocean; then maybe > none of all these radials would have been needed, hi. > > Second, your post discusses the benefit of using > several "on the ground" radials beneath 1/4 wave > verticals. > > You commented about using the 1/4 wave atop a > fence, or elevated several feet from the ground. > Per the experience of some, if the 1/4 wave vertical is > raised, not ground mounted, but say raised up about > 1/4 wave, then 8 elevated radials will essentially > equal the boost in transmit efficiency about as much > as 60 to 90 ground mounted ones, or will recover about 4 to > 5 dB of what would otherwise be lost to ground loss, "heat", > warming the worms, etc. Four elevated radials saves > maybe half the ground loss, 2 or 3 dB which would otherwise > be available to the worms. `120 or more radials, on the > ground, will supposedly recover "all" the xmit power > which might be lost in the ground, said to approach 6 dB > saving of total power. Or, said another way, a ground > mounted vertical without radials, will deposit about 75% > of the available power from the rig into the ground, for > the purpose of diathermy for the worms! (More than > half the rig power to ground loss, as the vertical antenna > radiation resistance, in ohms, will be less than the > ohmic loss of the coupled ground resistance.) > > You must decide how many radials you wish to > install, versus the saving of transmitter output > power from ground loss and conversion to more > radiated power. Running 100 watts to an elevated > vertical with four elevated, resonant radials should > result in say 30 to 50 watts of total radiated power, or > your signal would be down "only" 3 or 4dB (some say > that would be down about 1/2 S unit) at the DX operators > rcvr across the world somewhere. Add more radials, > up to 40 or so, and your signal strength would increase > out there, maybe 1 to 2 dB; go on up to 90 plus > radials, and for sure your signal would be 2 and maybe > even 3 dB stronger yet, or, if lucky, back up to > the full 100 watts radiated which left the transmitter > system. BTW, all of this info, with only slightly different > power savings estimates is given in the ARRL > Antenna Book, in the two editions I have, not the > very most recent, radial number and power savings > are given in Table 3 within the chapter titled, "The > Effects of the Earth." The data presented by the ARRL > was first presented by John Stanley, K4ERO, in the > December 1976 issue of QST, if you have the issues > or CD covering '76. And most of that data was taken > from "Radio Broadcast Ground Systems", and "Ground > Systems as a Factor in Antenna Efficiency", Proc. IRE, > June 1937, Brown, Lewis, and Epstein. > > For a 1/4 wave elevated vertical, you also want > the ends of the radials way up above ground for > the very same reason as above, that is, there will > be very high RF E field potential at the far ends > of the radials. And, of course, the elevated radials > must also be resonant as they will greatly impact the antenna > impedance/vswr, and you want them symmetrically placed > below the vertical so that the RF field radiated from the > radials will evenly cancel in the "far field" thus maintaining > the desired low radiation angle from the vertical. > > IMO, it is best to ground mount the vertical, and not to > bother trying to resonate the "on-the-ground" radials. And > use at least 16 plus radials so that at least 50% of your > rigs output power is effectively radiated, with the > results as described above. It is not easy to get an > elevated vertical/radial system properly set up, and high > enough to achieve high radiation efficiency. I think it > is easier to accomplish with the ground mounted vertical. > And, if you can, install 60 or more radials, such that > no more than 1 or 2 dB or so of your power is used to > warm up the creepy crawlers and wigglys! And make > your radials as long as possible, at least 1/4 wave, > and as others have written, they do not all have to be > the same length, nor any particular length, beyond at > least 1/4 wave, nor, for that matter real straight! > And a solid sheet of conductor directly below the vertical > element to which the radials would attach is also an > excellent idea, as was written. > > Of course, your thoughts about the 1/2 wave vertical > without a ground radial system are true, and work fine, > if the lower end of this vertical is also elevated 1/4 wave, > or more up. Then, most of the E field lines from the bottom > of the 1/2 wave element will then return to the upper > half of the radiator, rather than terminating down on > the ground where current flow would occur. Otherwise, > to get all the efficiency of radiation possible, be prepared > to put in a good radial field. > > Thought you might find interesting the 1/2 wave vertical > installation at WWVH which does use ground mounted > radials, and why they were installed. > > 73, Jim, KH7M > > > > > -- > FAQ on WWW: http://www.contesting.com/towertalkfaq.html > Submissions: towertalk@contesting.com > Administrative requests: towertalk-REQUEST@contesting.com > Problems: owner-towertalk@contesting.com > Search: http://www.contesting.com/km9p/search.htm > -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. 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