max diameter for the ground radial

[hocine]

Hello

i want to know the maximum diameter of the copper ground radial that we can use for the ground screen of a mw antenna.

thanks

[RGORJANCE]

In the US, the standard wire size/diameter we use is bare, non insulated #10 soft drawn copper. Typical length of the wire is 1/4 wavelength of the operating frequency. Typical number of radials is 120, or one every three degrees. Sometimes, in poor ground conductivity, they will add 50 foot radials in between every long radial. I would not suggest using wire smaller that #12. Copperweld (steel wire covered with copper has been used, but is very poor in high acid soils) is cheaper, but is much stiffer and harder to use.

Some stations use ground a screen about 24 feet square at the base of the tower and then attach the radials to the screen,

I am not aware of any limitations on length of copper wire radials, I refer you back to the first sentence te 1/4 wave statement as a benchmark.

Fossil

[davedybas]

I am not aware of any limitations on length of copper wire radials, I refer you back to the first sentence te 1/4 wave statement as a benchmark.

Fossil

I agree with Fossil...

Years ago tests were done to determine the best number of radials and best length for those radials. It was determined that radials longer than 1/4 wavelength provide very little, if any improvement over those just 1/4 wavelength long.

Dave Dybas

[R. Fry]

Years ago tests were done to determine the best number of radials and best length for those radials.

Below is a clip from a 1937 issue of the Proceedings of the I.R.E. showing the results of this benchmark, experimental work of George Brown, et al of RCA Labs, Princeton. The tests were done on 3 MHz in a location where earth conductivity is about 4 mS/m. The groundwave field was measured 3/10 of a mile from the monopole.

There are differences between using about 120 x ~0.4-wavelength radials and 120 ~0.25-wavelength radials. Whether or not they are important enough to pursue depends on the application and the budget, probably.

(Click on the graphic for a better view)

[KPJL FM]

So, if I was constructing a new 190 degree AM tower, would I still use only 1/4 length radials, because there is not enogh advantage to matching radial length to tower (electrical) length? Or is there a magic formula for figuring the best radial length for a tower length?

Just wondering, haven't ever had to opportunity to design this, but have used towers at 190 to 205 length before, and worked nicely, and the radials were long. These were diplex arrangements, so the tower was constructed for the lower frequency.

[R. Fry]

So, if I was constructing a new 190 degree AM tower, would I still use only 1/4 length radials, because there is not enogh advantage to matching radial length to tower (electrical) length? Or is there a magic formula for figuring the best radial length for a tower length?

A study of the BL&E paper shows that it is important to collect the r-f currents flowing in the earth within about 1/2 wavelength of a monopole, regardless of the electrical height of the monopole. Those earth currents are produced when displacement currents resulting from the capacitance of the monopole with the earth are converted to conduction currents at, and just below the surface of the earth in that area. They need to be collected by the ground plane and returned to the transmit system in order to complete the path needed for r-f current to flow along the monopole.

If those currents are forced to travel more than a short distance through the lossy earth to reach a buried radial, then proportionally more of the transmitter power is dissipated as heat in the ground plane, rather than being radiated as EM energy by the monopole.

If the best length and number of buried radials is related to the highest radiation efficiency for any monopole antenna system (especially short ones with low radiation resistance), then 120 radials of at least 0.4 wavelengths each probably is wise. Economics may dictate otherwise.

I believe that one of the Class I AM stations in Chicago with a single, ~1/2-wave tower uses 360 buried radials, each 1/2-wave long (free space).

[Deep Thought]

From a practical perspective, the minimum antenna height that the FCC will accept for a class B or D non-directional station is 54 degrees (0.15 wavelength), which will give 282 mv/m/km/kw with 120 quarter-wave radials, so the lower height portions of the graph really don't apply to any but class-C stations. If you look at the two graphs there really is not that much difference between the two at or above 54 degrees, certainly not enough to warrant spending almost 50% more on the ground system.

I've added higher frequency stations to lower frequency existing systems and have not seen any significant increase in efficiency in ND radiation between a 90° and 144° (0.4 wavelength) ground system. For example, KFNN and KENO are both on ~142°towers originally designed for their hosts (KKNT and KBAD, respectively) as quarter-wave towers and ground systems and the ND radiation efficiency of all of them is about what you would expect for a tower of the corresponding height on quarter-wave radials. Of course, these systems are multi-tower arrays with far more than just one set of 120 quarter-wave radials out there so the analysis may be tainted by the extensive ground plane connected to the other towers increasing the average length. Still, you'd think there would be some measurable effect.