A Very Good Read

Good Info! Thanks for sharing.

Helpful info, explanations, and formulas ... many thanks ...

Certainly makes the point of having a good ground system 😉

Quote..
Certainly makes the point of having a good ground system 😉

Yea, and it ain't small! I figured ground radial length for 1.62mhz. @ .41 wavelength is around 250' ea. A 500' circle! 180 of them would be around 4500' of wire! WOWZERS!

I guess electric fence wire isn't too expensive.. 😉

What a great read re: transmit ground systems for broadcast stations. However, let me offer this caveat.

Constructing .41 wavelength ground radials under a Part 15 installation is a huge waste. Radio antennas that are very short in relationship to a wave length (e.g. Part 15, 3 meter limit) will produce a "best case" signal with only a couple dozen radials about 15 feet long.

The radiation resistance doesn't change a great deal with many more radials. This means the antenna efficiency doesn't change commensurate with the addition of longer or more radials.

Even though the power is low and the antenna is inefficient, wouldn't a good ground area make a better horizontal pattern instead of a bunch of skywaves? I believe it would be better than just providing enough return current to tickle the xmtr..

I mean, it doesn't matter about the weak power output. Wouldn't the pattern be held closer to the earth where it belongs? One is still dealing with a transmitted frequency which has the same charateristics as the bigger stations.

Just my thought..

Not necessarily. Because the (Part 15) antenna is so short, the antenna pattern is basically in the shape of a ball, or in the case of a ground mounted antenna, half of a ball. The signal coming off the vertical antenna is VERTICALLY polarized, period. The exception would be a reflected signal.

The reactance in the antenna is so high at the end of the radiator there is almost no signal off the end of the antenna. Ground radials, when properly installed (buried), do not produce or add any horizontal component to the signal.

The ground radials act as one half of a capacitor when coupled with the inductance of the vertical antenna which presents a somewhat resonant load to the transmitter. If you were to measure the current in a properly installed ground radial, there will be very little current flowing. Translated, that means the radials don't radiate or add to the signal directly. They do, however, raise the radiation resistance sufficiently to cause the vertical antenna to be a better radiator.

Vertical antennas are used in the AM BCB because they favor vertical polarization. The nighttime radio propagation computer models used by the FCC discourage the use of horizontal antennas at all for permanent licensed use. For AM directional antenna arrays, horizontal wire antennas with horizontal polarization or substantial sky-wave are not allowed. Vertical antenna systems do produce some sky wave due to the very low angle of radiation and the F-layer reflection at night.

And finally, a study of "sectionalized" AM vertical antennas would be in order to understand the use of "gain" vertical antenna systems used in the AM BCB. This antenna scheme forces the signal closer to the horizon.

I hope this sheds some light on the voodoo RF theory that floats around these parts at times.

Marshall wrote:

If you were to measure the current in a properly installed ground radial, there will be very little current flowing. Translated, that means the radials don't radiate or add to the signal directly.

Perhaps low current is true for one of many radials but doesn't the sum of the currents from all the radials equal the current in the vertical radiating element?

The reason ground radials do not radiate is not due to the low current but rather because of the physical layout where each radial in a particular direction from the vertical radiator is paired with another 180 degrees around the circle. The currents in these are equal and 180 degrees spatially out of phase and therefore their radiated fields cancel.

This is my understanding.

Neil

A lamda of about .03 is the maximum distance between radials for "reasonable" efficiency. This is the maximum "gap" between each radial before a reasonable efficiency is lost. I believe this means that Earth currents between the ground radials become a meaningful part of the loss if this number is exceeded.. By using 60 radials you will easily be within this parameter when using 1/4 wave long radials. Once the radials are less than about .025 to .05 wavelength apart at the farthest point (the tips), adding more wire is a wasted effort. This is 13'-26' apart on 160M.

Notice he said between the radials?

Once again, I am not speaking about the distance between the radials, but there length. The threshold for return on investment (copper wire is expensive) is the answer to the question, "Does a longer ground radial contribute to a significant gain in signal improvement as opposed to the increase in investment?".

As an example, please refer to a Canadian company, Valcom Antennas (www.valcom-guelph.com), who recently received FCC approval to be used as an AM BCB antenna system. The 120 radials are only 120 feet long. This system produces an average efficiency of 95 percent when compared to a traditional theoretical quarter wave vertical tower antenna system operated at 1700 KHz. See the study, submitted by du Treil, Lundin & Rackley, Inc., consulting engineers, to the FCC. The lengths of the vertical antenna systems studied are 75 feet and 85 feet. In reality, many "real world" AM antenna systems do not meet or exceed 95 percent efficiency when measured against FCC theoretical standards. Please notice the restrictions the FCC has decided to impose on this antenna system. Been there, done that, own a few of those shirts.

Spacing between the radials does effect the efficiency of the system. However, there is a point of marginal return regarding the length of the radials, which is exactly my point. Ground screens at the base of vertical antenna systems are used to decrease near field losses in the antenna system. The further you get from the antenna base on a short antenna the less effect the ground system has on far field signal improvements.

An additional, concentrated short ground screen is a waste. If 60 radials or more are used, the first 40' or so of the radial field is very dense already - an additional screen will have no worthwhile effect. I was told this screen is rarely used at BC stations - and if used, it's usually a copper flashing material that is used more to keep from tripping over radials and perhaps to protect equipment placed at the base from lightning.

More info from a 160 meter operator...