Using Counterpoises with Part 15 AM Systems

[rfry]

To expand on the subject of using an elevated Part 15 AM transmitter with attached ~3-meter whip and two horizontal radials, on the roof of a building...

An elevated counterpoise does not need more than two horizontal conductors. Using more than 2 conductors is very important if those conductors are buried. But their performance is not the same when they are elevated above the earth. Two elevated 1/4-wave horizontal conductors used as a counterpoise for a vertical monopole can give essentially the same result as using 120 x 1/4-wave buried radial wires. Even two conductors that are much shorter than 1/4-wavelength can perform quite well as a counterpoise for an elevated Part 15 AM system.

A 3-m whip driven against 2 elevated horizontal wires as a counterpoise can produce about the same field strengths and coverage area as when installed at the surface of the earth using a connection to the earth rather than the counterpoise. When using a typical Part 15 AM transmitter operating at 100 mW input power these fields will be much greater than permitted by FCC 15.209.

However an elevated system using a counterpoise has no means of draining off static charges that could build up on the system, and is more susceptible to damage from nearby lightning activity.

The solution for this is to install a "lightning ground" conductor from the transmitter to an electrical ground such as a ground rod buried in the earth. But then we have the issue that using such a conductor length of more than a foot or so likely will be non-compliant with Part15.219(b).

Radiation from that ground lead can be made negligible, and the path could still allow reasonably good lightning protection for the system by installing components that prevent r-f current in the MW band (only) from entering the ground conductor (and the power and audio conductors) where they attach to the transmitter.

Use of ferrite chokes along the lengths of those conductors also might be required, because they could have r-f currents induced on them by radiation from the driven conductors of the antenna system. Unfortunately that also may reduce the performance of that path as a deterrent to lightning damage.

But whether or not the FCC would accept such a system configuration as compliant with 15.219(b) is unknown.

This matter would be good topic to be investigated by the manufacturers of Part 15 AM transmitters, who could develop the necessary hardware, in collaboration with the FCC as to the compliance issues.

Successful resolution of this could produce increased interest in Part 15 AM, and more equipment sales for the manufacturers.

 

[druidhillsradio]

That being said, would the two radials operating against a 3 meter monopole need to be say 146 feet each at 1600KHZ, or just slightly longer than 3 meters?

 

[rfry]

NEC-4.2 free-space data for a 3-meter vertical monopole on 1650 kHz with 15 ohms loss in the loading coil:

2 x 3-m horizontal counterpoise wires (opposite directions)

Feedpoint Z = 15.1 -j 5700 ohms, approx.
System H-plane Gain = -22 dBi

2 x 45-m horizontal counterpoise wires (opposite directions)

Feedpoint Z = 15.7 -j 4400 ohms, approx.
System H-plane Gain = -22 dBi

The version with 3-m wires in the counterpoise would take a loading coil with more inductance to resonate the 3-meter vertical, and would have smaller SWR bandwidth than the other one.

The coil loss in the system using 2 x 3-m counterpoise wires likely would be higher than in the other system, reducing its gain somewhat. For example if the coil loss was 20 ohms, system gain would drop to -23.2 dBi.

 

[audioguy]

I don't know how I missed this thread before, but the subject of elevated ground systems is a topic of high interest to me, especially since I recently came across two new QEX articles by Rudy Severns that are posted on his personal web site; http://www.antennasbyn6lf.com/. There are a lot of good articles on his site, but the latest work is discussed in the third paragraph on the home page. Recommended reading!

There are several questions that are of interest to me. The first is how to choose the optimum elevated ground configuration and the second is whether the FCC will consider this type of installation to be compliant with Part 15.219. As R.Fry has stated, an elevated ground plane does not have to radiate, but according to the studies done by Severns, the radial system needs to be symmetrical in order for that to be true. Thus, you can use just two wires but they need to be the same length, running in opposite directions. They also need to be kept away from other objects, particularly metallic objects. He suggests that a larger number of radials is better than just a few if you are trying to reduce the amount of current induced in the (lossy) ground beneath the antenna. It is interesting to me that his work appears to show that you should not use radials longer than about 1/4 wavelength unless you want to use a large number of them. That's not really an issue for me because I don't have enough property to use even 1/4 wave radials in the AM broadcast band. As R.Fry has stated, you can tune shortened radials to resonance, but then you need to use low loss inductors and they need to be tuned carefully to keep the currents balanced (otherwise they'll radiate). So, I am pondering whether it would better overall to just use two or four longer radials, or a multiplicity of shorter ones, say 10-20 feet long. It would be impossible for me to deploy a large number of long radials on my property.

At such a low frequency, any elevated system is still going to be extremely close to ground in terms of wavelength, whether it is mounted 1 foot about the ground or 50 feet. So, I feel that I might as well put it just a short distance above ground to make it easier to tune and work on, even though this may increase ground losses. Maybe that will make the FCC happy; I don't know (and I'm not going to ask). If I use just 2 or 4 radials, it would potentially be practical to resonate them, but if I use 8 or 16, that would become impractical. I'm leaning toward a small number of tuned radials. Rudy Severns also points out the importance of isolating the feedline using a ferrite choke to prevent it from radiating. This also goes in the direction of making the system FCC compliant. In my case for Part 15 AM operation, that means choking the audio and power lines going to the transmitter box.

In the end, the attraction is the possibility of getting performance that is similar to a ground mounted system with a large number of radials. If I lived on a farm, I could do that, but I don't, so it's out of the question. So what I will probably do is install a test system about a foot above the ground with just 2 or 4 tuned radials and see how that compares to my present ground mounted system. I'll be interested to hear what others think about this.