ZL Special for 75 Meters
Back in the late 60’s and early 70’s I had a brief infatuation with the “ZL Special” antenna. My station at the time consisted of a Hallicrafters HT-37 transmitter, a Hallicrafters SX-101 receiver, and a variety of antennas; usually a different one every two weeks or so, as I searched for the “ideal” setup. My callsign was W2EUQ at the time, and I lived in Corning, New York.
In 1954, I had built a cubical quad for 15 meters, and enjoyed excellent results with 50 watts AM from my Johnson Viking Ranger, including a surprising S7 report from BV1US on Taiwan. I also kept regular schedules with missionaries in Yucatan and South America. Consistently good signal reports convinced me of the Quad’s excellent performance and small cost.
Later, I discovered the ZL Special—another inexpensive antenna with high performance at small cost. So, I built a 15-meter version using elements made from 300-ohm twin lead taped to well-varnished bamboo fishing poles and turned by a simple TV antenna rotator. Once again, results were very satisfactory on the fifteen-meter band, which by then had become a favorite.
Tried Them All
Years later, after dozens of other experimental antennas had come and gone, including well-known favorites such as end-fed random wires, loops, collinears, folded dipoles, double bazookas, windoms and the like, I had become fascinated with 75-meter AM. I kept regular schedules with friends from Maine to Florida on the East Coast, and to Ohio and Indiana and points “west.”
Many of my QSOs were with stations using kilowatt amplifiers, and they almost always received “S9 – plus” reports from my friends on 75, whereas I usually got “Good copy, S7 – to – S9 OM” reports from the same stations. What to do? I couldn’t afford a big amp at the time, so I began looking into the possibility of building some sort of ‘gain’ antenna to make my 100-watter sound like a “big gun” station.
Then, an idea struck! I reasoned that most stations I worked on 75 and 80 meters were the result of high-angle signal paths to and from my QTH. Hmmm…what if I were able to direct a stronger signal at high angles? What sort of antenna could do that from a small 140 by 75-foot (42.57x 22.86 meters) lot? Then the thought of a ZL special returned, with its 3 – to – 5 dB gain over a dipole. Many sketches later appeared what looked like a broad “arrowhead” pointed at the zenith. Yes, it was the old favorite ZL Special re-configured to shoot a signal straight up and cover a broad angle from the vertical in an east-west direction, with possibly useful radiation in other directions as well—but I didn’t know until I built one and tried it.
So, my forty-foot (12.19m) “zoom-up” mast would support the center of the antenna’s ‘front’ (now top) element and the 300-ohm phasing line would support the ‘rear’ (now bottom) element. Not having two more 40-foot (12.16) masts to support the center and ends, I used the inverted vee idea and relied upon light fifteen-foot (4.56m) aluminum masts to supports the ends—hence the ‘arrowhead’ appearance.
Formula
I used the following formula for the (300-ohm twin lead) elements and phasing line (as best I can remember after nearly 30 years):
Top (shorter) element: 438/3.9 MHz = 112’4″ (34.19m)
Bottom (longer) element: 462/3.9 MHz = 118’6″ (36.03m)
Spacing:121/3.9 MHz = 31’2″ (9.47m)
A ZL Special typically uses 1/8th wavelength (45-degree) spacing between elements; i.e. 123/3.9 MHz, resulting in 135-degree phasing, so that when the phasing line is given a twist of 180-degrees between the elements (180-degrees) – 45 degrees – the 135 degrees phasing is achieved..
A bit of fiddling around with spacing showed that 121/3.9 MHz gave the best results (forward gain and feedpoint impedance) resulting in a difference of 4″ (10.16cm) between the ideal and the actual. Thus, I used the eighth-wave length of twin lead (31’6″) (9.57m) but attached the feedline to the phasing line 4″ (10.16cm) behind the front element instead of directly at the front element. Yes, I know this is ‘fudging’, but it worked!
Since I used a Johnson Viking “Matchbox” at the time, and heavy-duty 72-ohm transmitting twin lead was recommended for the feedline, I used that instead of 75-ohm coaxial cable.
Upon completion of the antenna, attachment of the feeders to the tuning unit, and tuning to resonance at 3.9 MHz, I was pleasantly surprised by the outstanding strength of received signals. It was discovered to have less than 2:1 VSWR bandwidth between 3.8 and 4.0 MHz.
How Did it Work?
Well, all I can say is that my signal now ranked with the Big Guns and kilowatts on 75. I received many reports of “Best signal on the band, OM” and “S9 plus 20 dB, here OM” etc. Very gratifying, indeed, for a cost of under $50 as I recall. That included 50-feet (15.20m) of feedline, 275-feet (83.62m) of twin lead for the elements and phasing line, and assorted insulators and attachment line. Eventually, I laid a length of wire underneath the entire antenna as a parasitic ‘reflector’ which didn’t seem to make any difference to the performance—but, then it didn’t seem to hurt it, either. I left it until lawn-mowing time came around, when I coiled it up and returned it to the ‘junk box.’
You can also use other types of material for the folded dipole elements and phasing line. I would recommend open-wire line if possible, or the ubiquitous 450-ohm ‘ladder’ line.
Why This?
Why use folded dipoles instead of plain wire dipoles? The answer is feedpoint impedance. You will recall that a folded dipole has a nominal feed point impedance of about 300 ohms compared to about 72 ohms of a plain wire dipole. When you bring two dipoles as close together, such as 1/8 wavelength for the ZL Special, the mutual impedances are reduced to very difficult-to-match values. However, by using folded dipoles, the feed point impedance comes very close to the desired 72-75 ohms, with reasonably low values of reactance that can be ‘tuned out’ to make the transmitter happy.
Can you feed the antenna with coaxial cable? Of course, but the pattern will be skewed, and there will be return currents on the shield of the cable. Therefore, you can use a 1:1 voltage balun, or just add a string of ferrite beads to the coax at the feed point to form a current balun.
I’d like to know if you have the same good results with your 75-meter ZL Special.
Originally posted on the AntennaX Online Magazine by Jim Gray, W1XU
Last Updated : 16th May 2024