Ground Radials - Insulated or Uninsulated?

[Ermi Roos]

15 ohms of loading coil resistance is a Q of about 200, which is still not easy to achieve in practice, although it is possible in an unusually good design. It is something to strive for. The .05 % efficiency I said is typical is the total system efficiency, including the final transmitter stage (I was comparing it to 100 mW of radiated power, which is 100 % total system efficiency). I think that .1 %, or even .2 % total system efficiency is possible for a transmitter at ground level, but such results are not typical. The 1.8 meter, or 3 m, radius ground screen, although modest by broadcast standards, is better than what is typically encountered in part 15 AM. Also, any ground screen at all may not be legal under Part 15. In what I reported in posts in hobbybroadcaster.net a couple of years ago, John Reed of the OET said that ground radials are not legal, and also any large metal object below the surface of the soil is not legal to use as a ground. See the "Legal Issues" discussion board in hobbybroadcaster.net. Mr. Reed said that only a standard 8-foot ground rod in the earth may be acceptable. I say "may be," because Reed was not positive about what sort of ground is acceptable.

Reed is now retired, and the people presently at the OET may have different ideas (hopefully). And yes, if only a ground rod is permitted, the ground loss resistance becomes comparable to the loading coil loss resistance. However, there do not seem to be any published enforcement actions relating to ground screens, and so it is not certain what the intent of the FCC is.

 

[rfry]

The .05 % efficiency I said is typical is the total system efficiency, including the final transmitter stage (I was comparing it to 100 mW of radiated power, which is 100 % total system efficiency). I think that .1 %, or even .2 % total system efficiency is possible for a transmitter at ground level, but such results are not typical.

Thanks for your comments. To close the loop then, would you mind posting your values for a 1700 kHz Part 15 AM system using a 3-m, base-fed and base-loaded monopole with the "system efficiency" you believe is typical, including all of the parameters shown in my link at http://i62.photobucket.com/albums/h85/rfry-100/Part_15_AM_Roos_Comments.gif ?

That could provide a valuable input helping us to reconcile these different analyses.
//

 

[rfry]

I remembered that I have a program written by R.J. Edwards (G4FGQ-SK) to calculate the resistance of buried radials.

Using it for the radial length suggested by Ermi Roos in this thread, it would appear that for average soil conditions the loss in a typical buried radial ground system used by Part 15 operators may be higher than stated.

All of the following is a cut/paste from that program.

RESISTANCE TO EARTH OF A SYSTEM OF BURIED RADIAL WIRES

Radials uniformly spaced. Depth > Wire diameter. Length > 5 times Depth.

R. Resistivity of Soil = 100.0 ohm-metres
L. Length of one radial wire = 3.0 metres
D. Diameter of wire = 2.5 millimetres
U. Depth of wire below soil surface = 100 millimetres

Radials 1 2 3 4 5 6 7 8
Ohms 52.3 30.3 23.0 19.3 17.1 15.7 14.6 13.8

Radials 10 20 30 40 50 60 70 80
Ohms 12.7 10.5 9.8 9.4 9.2 9.1 9.0 8.9

For comparison, resistance of a surface disk of same radius = 8.3 ohms

TYPE OF TERRAIN RESISTIVITY Ohm-metres

Freshwater lakes, unpolluted 1000
Sea water, away from river estuaries 0.22
Agicultural plains, streams, richest loam soil 30
Pastoral, low hills, fertile soil 80
Flat, marshy, densely wooded in places 130
Pastoral, medium hills with forestation 170
Pastoral, heavy clay soils, hills 250
Rocky soil, steep forested hills, streams 500
Rocky, sandy, some rainfall, some vegetation 500
Low-rise city suburbs, built-up areas, parks 1,000
High-rise city centres, industrial areas 3,000
Sand deserts, arid, no vegetation > 20,000

An ohm-metre is the resistance between opposite faces of a 1-metre cube of
the material. The above values are averages taken over large distances.

//

 

[Ermi Roos]

I would not rely on a program for calculating ground loss resistance without examining it carefully. The mathematics involved in obtaining the correct results are extremely complex, and the author of the program may have taken shortcuts, or made invalid assumptions. Fortunately, Mr. Fry appears to have access to IEEE papers. I would suggest studying the paper by Trainotti, "Accurate Evaluation of Magnetic and Electric-Field Losses in Ground systems" in IEEE Antennas and Propagation Magazine, Vol. 46, No. 6, December 2007.

 

[rfry]

...I would suggest studying the paper by Trainotti, "Accurate Evaluation of Magnetic and Electric-Field Losses in Ground systems" in IEEE Antennas and Propagation Magazine, Vol. 46, No. 6, December 2007.

For the real-world, measured (not just theoretical) antenna system performance related to various configurations of the buried radial ground systems used with MW monopoles of various heights, I would suggest the study and comprehension of the measured data in the results of the empirical study of Brown, Lewis & Epstein published in the Proceedings of the Institute of Radio Engineers.

That study serves to this day as the FCC basis to define the minimum acceptable performance level permitted for the antenna system radiation efficiencies of the various classes of licensed AM broadcast stations.

Below is a link to that BL&E document (paper 4):

http://rfry.org/Software%20&%20Misc%20Papers.htm

RF

 

[carlvanorden]

As the guy who invented the use of ground radials for part 15 antennas, let me chime in. I haven't for a long time, and I don't want part 15'ers who are not engineers to be confused by this topic.

We are working with 1/10th of a watt. Which means you can plug 40 of these transmitters in and still use as much power as 1 basic night light bulb!!!! So by using ground radials and top hats legally, we can increase the efficiency of the antenna, just by changing the way it is designed and operates.

Top Hats: Yes I build them; 2 kinds. One looks like a mop and I use #12 thhn. The wire circumference is insignificant, but I keep them at 8" long. So, if I figure 24 " for this type of top hat, I reduce the length of the antenna from 108" to 84". In actuallity, after an antenna is tuned up, it probably won't be 108" tall; it will most likely be shorter than that.

Ground radials: I use #12THHN, bare wire. What is the use if you use covered wire?
Make the radials as long as the antenna is high. On a ground based antenna, I use 10 feet for each radial, which includes the idea that there will be a 12 inch insulator. The ground is connected to the mast pipe, the circle of wire which holds the radials connects to the mast pipe. It's not a direct connection, so we are talking wire in the ground. However, besides helping to push 1/10th of a watt through the ground we are also talking lightening protection. And a ground rod helps with wx, but it does nothing to push your signal, even just a little ways.

Any more than 10-20 radials is basically a waste of money and time/work. What we are going for is not really to increase range, but to make the signal cut through the noise of local AM frequencies. So as to make your signal louder, not so much to extend the range. After all, no matter what you do, 1/10th of a watt will travel so far.

I still make the antennas/coils/ground systems for transmitters like the sstran. If interested or if you would like my tip sheet, email me at [email protected], and I'll be glad to send it to you.
Carl