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The folded Unipole antenna

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.

three wire folded unipole on a guyed tower

three wire folded unipole on a guyed tower

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.

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3 comments to The folded Unipole antenna

  • J. Aegerter

    Around 1936, the Washington consultants pushed the FCC into rule-making for 120 buried ground radials on vertical radiators in order to obtain an AM station license. Prior to that, the “Flat-Top” antenna supported between two towers was the norm. The 120 number always seemed excessive to me. In 1968, the ground system at AM1340 where I worked was in bad shape, mainly broken and corroded. The land was previously part of a dump called “Pigsville” and was probably responsible for much of the copper that was eaten away. The original ground system was installed with the 450 foot Truscon (Youngstown, OH) self-supporting tower around 1947. I ran field intensity radials before and after the copper replacement and noticed little difference. I had always reasoned that a 30 MHz. ground plane had only four radials, and therefore why was 120 needed at 1.34 MHz.? The FCC has granted a few applications with 4 above ground radials and they appear to work effectively. And some studies have shown that above ground radials perform better than buried ones. I believe installing 4 above ground radials supported on 10-20 foot poles as a counterpoise for a quarter wave or greater tower would be adequate. However, the station grounding for lightning must not be forgotten and several rods driven deep enough to insure a low impedance ground would still be required.

  • Hi,

    this an off topic please bear with me Im not an expert.

    I would like to ask your opinion, and advise , I have a folded unipole real of 72.9 ohms and reactive of 108.6 ohms. The full antenna length is resonated to 871 khz this is a six skirt antenna drop wires. My operating frequency is 963 khz. If I tuned my ATU to 963 khz it will need and output capacitor. I want to removed the capacitor requirement for my ATU tuning . My question is if I removed the 3 drop wires of the antenna do the resonant antenna will increase from 871 to above or it may near to 963 khz? the spacing of 6 dropw ires is 24 inches from the tower the height of the antenna tower is 250 ft.

    Gene

  • Paul Thurst

    Eugene, the short answer is no, removing three of the six drop wires will not effect the impedance and reactance the way you want. Your tower is slightly longer than 1/4 wave length on 963 KHz. At the top of the drop wires or perhaps some ways down from the top, there should be a shorting wire going between the drop wires and the tower. You can attempt to slide this shorting jumper down until you reach a 50 ohm point. Unfortunately, due to the nature of the drop wires in relation to the tower, there will always be some inductive reactance, which will require a capacitor to cancel.

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