In the 1990’s, the folded unipole antenna was touted by many to be the savior of AM radio. There were many claims that a folded unipole antenna did not need a complicated ground system, a simple ground rod at the base of the tower would work fine. That turned out to be not exactly the case. Kintronic did a study (.pdf) that basically dispelled that notion, along with several others. The folded unipole antenna performed within a few percentage points of a series fed tower under the same testing conditions.
Folded unipoles do have the advantage of a grounded tower. Grounded towers have a distinct advantage in lightning prone areas, such as central Florida. I can attest through my own experience, a series fed tower is much more likely to induce lightning damage to a transmitter or ATU. Folded unipole tower systems can also be used to co-locate other antennas, such as STL, cellular, PCS, etc. Making some extra rental money on an AM tower is not a bad way to go.
I began fooling around with MANNA-GAL, which is a NEC-2 based program. It is a free ham radio program, so it is a little clunky to use and it took a while to figure out, but once I did, it is fun. I modeled a unipole antenna for medium wave use and the results are pretty interesting. First of all, I drew out X-Y part of the system on graph paper because the program requires all wires (elements) be entered in a coordinate based format. The Z axis is the tower, since there is only one of those, that was easy. I played around with series vs. unipole systems and the results were fairly close to what they are supposed to be. One of the nice things about MANNA-GAL is it allows the user to change the ground conditions. To add a unipole to the tower, I put 3 wires spaced between one to two meters away from the primary Z axis wire, connected them to the top of the tower and changed the drive point to the skirt wires.
The interesting part is when I added an above ground counterpoise instead of a buried radial ground system. I think Ron Nott, of Nott, ltd. did much of this work too. What I found was that with between 5 – 10 above ground radials of 90 degrees or greater, the efficiencies are within about 10 percent of theoretical for a 120 buried radial system. Again, the ground conductivity plays a big roll in this, poor ground conductivity will reduce efficiencies equally for both systems.
As the tower height approaches 110 degrees or so, depending on the spacing from the tower of the skirt wires, the bandwidth really starts to open up. At 110 degrees the base impedance is about 120 ohms with about 80 ohms inductive reactance. Both the impedance and reactance slope slightly upward with frequency but are linear +/- 50 KHz of carrier. This slight asymmetrical sideband distribution can be easily canceled out in the ATU with a few degrees of negative phase shift through the T network.
Again, all of this is theoretical, but I have found that NEC is usually within +/- 10% of real world values. It is difficult to get a handle on ground conductivity unless measurements are taken. Even from season to season, that can change.
The above ground counterpoise requires a partial proof, according to FCC 73.186. If this were a directional station, this would be required anyway. For a non-directional station, it is pretty easy, for six radials, it would probably take about one to two days of driving around with a FIM 41. The other consideration is public exposure to RFR from the radials. This can easily be measured with a NARDA meter. More radials will spread the induced currents out more, for for higher powered stations, 10 above ground radials might be required.
There are several radio stations in the country which are successfully using above ground counterpoises. It seems to be a good system and requires much less material and labor to install than the traditional ground system.
Therefore, if I were designing a new AM station, I’d use a grounded tower between 105 and 110 degrees with a unipole and 6 above ground radials 90 degrees or greater.