Towertalk
[Top] [All Lists]

## Re: [TowerTalk] Ground radials/elevated radials

 To: "W7TMT" , Re: [TowerTalk] Ground radials/elevated radials "Tom Rauch" Tom Rauch Sat, 23 Oct 2004 08:53:26 -0400
 ```First, absolutely the least reliable model is one used when a wire is parallel with and close to earth. That's pretty well established. Second, most of the work available to us simply involves models. Almost no one actually built an antenna and measured FS with only a radial change. People use a model that is known to not be very reliable (with 5dB or more), and publish results to the tenth of a dB. Third, models like Eznec and so on don't even calculate groundwave. The programs assume the model is over infinite flat earth and FS is taken a few hundred WL (or whatever) from the antenna. The signal really isn't zero at zero degrees at reasonable distances. > 1) In your opinion where is the cutoff between "elevated > radials" and a "ground plane" system. How high off the > ground does the system need to be before it becomes a GP > rather than an "elevated radial" system? It won't be a cutoff, it'll always be a very gradual transition. The wires wouldn't really be divorced from earth below the antenna until the earth is much further away than the area of the groundplane. It's a geometry thing that depends on wavelength and occupied area. The only actual measurements I've seen were at a BC station in Brazil. They showed 8 elevated radials 1/4 wl above ground were just ever so slightly weaker than a conventional system on the ground. GAP actually was handing out a copy of that paper. I measured two systems here where I compared low radials to ground mounted radials on 3.5MHz. The resonant radials were perfectly horizontal. Looking at old data the results were: elevated radials (6-8 feet high) 4 wires -4.3 dB 20.1 mV 8 wires -2.4 dB 25 mV 16 wires -1.2 dB 28.7 mV 60 wires -.1 dB 32.5 mV ground mounted radials 4 wires -5.5 dB 17.5 mV 8 wires -2.8 dB 24 mV 16 wires -1.3 dB 28.4 mV 60 wires (used as 0 dB reference) 33 mV. I repeated the basic test using a Rohn 45 insulated tower ground mounted and sloped the radials up to 10 feet. Results were almost the same, but I saw a bigger difference in the advantage of elevated radials when only 4 wires were used. However, at 16 radials the ground mounted radials were a tiny bit better. WVNJ showed just under 5dB average change when we went from six elevated (~25 ft high) radials to a conventional system. In some directions and distances the change was as little as 2dB or so. > 2) How does the efficiency of 50-60 on-the-ground radials > compare to a reasonable number of radials (insert your > number of radials required here) that are installed under > the feedpoint of a vertical when the feedpoint is let's say > a 1/4 wave off the ground. Never measured that. 60 radials is obviously getting very close to 100% efficiency so efficiency can't be a consideration. I know my 5BTV mounted right above a 20 meter 5 element yagi at 150 feet AGL was almost always less than 10dB weaker than the Yagi. Unfortunately I didn't have one at ground level at the same time to compare. I'm sure results would vary a great deal depending on clutter around the ground mounted antenna and soil conductivity. Living on the ocean or in a salt marsh or open pastures would be different than in a city. Further, if it > was practical to install a vertical either on the ground > with an excellent radial system as you've described or > elevated a 1/4 wave with the appropriate number of redials > which would you choose? I'd use a dipole with the tip of the elevated vertical supporting it! Seriously, the only measurements I've seen show a 1/4 wl high GP with eight radials just ever so slightly weaker than a full system on the ground, and that was on the AM BC band. I'd expect to not see much difference between the two unless some absorber or reflector was cluttering the area around the antenna. I do have an aversion to needless things hanging in the air. How could I cleanly install a dipole with all those GP radials interacting? This whole thing is really mostly just a matter of the specific situation. I won't use an elevated wire system because they are a PITA for multiple reasons. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] Ground radials- the long and short of it, Scott Fike Re: [TowerTalk] Ground radials- the long and short of it, Tom Rauch Re: [TowerTalk] Ground radials/elevated radials, Yo3ctk Re: [TowerTalk] Ground radials/elevated radials, Tom Rauch RE : [TowerTalk] Ground radials/elevated radials, hermans RE: [TowerTalk] Ground radials/elevated radials, Paul Playford Re: [TowerTalk] Ground radials/elevated radials, Tom Rauch RE: [TowerTalk] Ground radials/elevated radials, W7TMT Re: [TowerTalk] Ground radials/elevated radials, Tom Rauch <= Re: [TowerTalk] Ground radials/elevated radials, Jim Lux RE: [TowerTalk] Ground radials/elevated radials, W7TMT Re: [TowerTalk] Ground radials/elevated radials, Peter Sundberg Re: [TowerTalk] Ground radials/elevated radials, Jim Lux Re: [TowerTalk] Ground radials/elevated radials, Peter Sundberg Re: [TowerTalk] Ground radials/elevated radials, Tom Rauch Re: [TowerTalk] Ground radials/elevated radials, Jim Lux RE: [TowerTalk] Ground radials/elevated radials, Paul Playford Re: [TowerTalk] Ground radials/elevated radials, Tom Rauch RE: [TowerTalk] Ground radials/elevated radials, Paul Playford