More on Radials

[druidhillsradio]

Excerpted from the ARRL Antenna Book on the subject of ground radial systems:

Radio Broadcast Ground Systems states, “Experiments show that the ground system consisting of
only 15 radial wires need not be more than 0.1 wavelength long, while the system consisting of 113
radials is still effective out to 0.5 wavelength.” Many graphs in that publication confirm this statement.
This is not to say that these two systems will perform equally well; they most certainly will not. However,
if 0.1 l is as long as the radials can be, there is little point in using more than 15 of them.

The antenna designer should (1) study the cost of various radial configurations versus the gain of
each; (2) compare alternative means of improving transmitted signal and their cost (more power, etc);
(3) consider increasing the physical antenna height (the electrical length) of the vertical radiator, instead
of improving the ground system; and (4) use multielement arrays for directivity and gain, observing
the necessary precautions related to mutual impedances discussed in Chapter 8.

 

[rfry]

However, if 0.1 l is as long as the radials can be, there is little point in using more than 15 of them.

Agreed, and this is supported by the experimental data in the 1937 I.R.E. paper of Brown, Lewis & Epstein (which may be the resource used for the ARRL statement).

(3) consider increasing the physical antenna height (the electrical length) of the vertical radiator, instead of improving the ground system; and (4) use multielement arrays for directivity and gain, observing the necessary precautions related to mutual impedances discussed in Chapter 8.

Just to note that the antenna systems described by the comments in (3) and (4) may raise FCC issues for those wanting to functionally comply with Part 15.219(b).

//

 

[PhilB]

To put this in a little more useful perspective, here are the radial lengths for 0.1 wavelength for the common Part 15 AM frequency range. In actual practice, 61 ft. would be a good radial length choice for a ground mounted 3 meter antenna with 15 radials operating anywhere in the range from 1500 kHz to 1700 kHz.

At 1500 kHz:
1 wavelength = 656 ft.
0.1 wavelength = 65.6 ft.

At 1600 kHz:
1 wavelength = 615 ft.
0.1 wavelength = 61.5 ft.

At 1700 kHz:
1 wavelength = 579 ft.
0.1 wavelength = 57.9ft.

 

[druidhillsradio]

"To put this in a little more useful perspective, here are the radial lengths for 0.1 wavelength for the common Part 15 AM frequency range. In actual practice, 61 ft. would be a good radial length choice for a ground mounted 3 meter antenna with 15 radials operating anywhere in the range from 1500 kHz to 1700 kHz."

I know that conductor size makes little difference, but what about insulated vs uninsulated? At HF frequencies I generally see insulated wire used. Comments?

 

[audioguy]

The ARRL handbook text actually derives from the experimental work of John Stanley, K4ERO, which was originally published in the December 1976 issue of QST. The title of the article was "Optimum Ground Systems for Vertical Antennas".

Based on that article, I built a 16 radial system 0.1 wavelengths long for use with my 40m vertical. It seemed to work fairly well, although I had no means of testing its efficiency. In John's article, I believe he came to the conclusion that this system would only be 3 dB down from a "perfect" ground system, assuming the use of a full sized 1/4 wave radiator. This would not be true for the tiny 3m long antenna used by Part 15 AM systems.

I wish I had enough space for 60 foot radials, but unfortunately I don't. That would be nice!

 

[audioguy]

I forgot to comment on your question about insulation. Most authors I have read have concluded that insulated wire works pretty much the same as uninsulated wire. I prefer to use insulated #12 house wire for my radial systems, just because of the ease of working with it. You can pick up a 500' spool of #12 stranded at Home Depot at a very reasonable price, which great for experimenting.

 

[rfry]

Most authors I have read have concluded that insulated wire works pretty much the same as uninsulated wire.

That is true as far as its r-f performance is concerned. But buried, insulated wire will not provide a very good ground connection for direct current, which means that static charges can build up on the part of the antenna system that is exposed above the earth (unless some other means is provided to prevent that).
//

 

[druidhillsradio]

Thanks Audioguy and Rich. I know that commercial AM'ers use uninsulated wire. I did not take into account bleeding off static charges. Makes perfect sense.

 

[audioguy]

For static/lightning protection, I prefer to rely on a driven ground system. My standard configuration is 3 or 4 8' ground rods at the base of the antenna. I like to put one directly under the antenna, and space the others several feet away. If you use four, you can have one right below the transmitter and the other three in a circle. This seems to have been pretty reliable for me. Again, this is not particularly for RF efficiency, but to protect my property.

 

[druidhillsradio]

I have been using 1/2 inch 10 foot copper tubing for ground rods. I use an old water hose with the end connector lopped off, and using a hose clamp, I attach the hose to one end of the tubing. Then I crank on the water and hose the pipe into the ground. When finished, I remove the hose and attach a clamp on grounding block. I'm good to go. the tubing is cheaper than the ground rods for Home Depot/Lowes.