The CFA Adventures Of KA5NEE

This can’t possibly work!

Like most of us, I was skeptical about the Crossed Field Antenna. Every ham “knows” the physical size of an antenna is inextricably related to its operating frequency. How is a cylinder stacked on a disk, stacked on a rectangle, supposed to radiate a signal on 80 meters, if it’s a small fraction of a wavelength tall? Okay, there’s some sort of “black box” that goes between the antenna and my transceiver that makes it all possible.

Hmm. That must be the expensive part. The antenna can be made with cardboard and aluminum foil, but the black box is stuffed full of big-ticket components, and takes an engineer to put it together. It still doesn’t seem right somehow, but that makes it a little more believable. Surely you can’t get tall-tower performance out of an antenna you could build—and operate—on a card table.

Or can you? The black box, it turns out, holds a very small number of discrete, passive components. Its job is to provide the proper, unbalanced impedance load to the transmitter, while providing the phase-shifted signals that make the three elements of this oddball stack of foil and cardboard radiate a signal. The inventors, and an increasing number of enthusiastic users, claim that this “stack of foil and cardboard” will out-perform a quarter-wave vertical over an extensive ground system, at the same power levels.

With these enthusiastic reports eating away at me, I decided I just had to build one. Given the success stories reported by Dr. Kabbary in Egypt, where several AM broadcast stations are using the antennas with scientifically-verified success, I couldn’t think of a reason to go with a cardboard-and-foil prototype. Dr. Kabbary’s success stories, and the articles and correspondence from Stefano Galastri, whose enthusiasm jumps off every page, convinced me to skip the prototype process, and go with more permanent materials. I settled on aluminum flashing and PVC pipe to get my CFA started.

Building the Antenna

Standing on the shoulders of Stefano, Drs. Kabbary and Stewart, as well as Ted Hart and others (who had already done the heavy lifting), I skipped the cardboard and foil prototype and started with a semi-permanent design that took shape in my garage. I used two rolls of 20-guage aluminum flashing [10 ft. (3m) long by 20 in. (.5m) wide] and several pieces of Schedule 80 1/2-inch PVC pipe.

One CFA article (THE CROSSED-FIELD-ANTENNA PART III, by Maurice Hately, GM3HAT; Dr. Kabbary, Dr. Stewart, MM1DVD & Ted Hart, W5QJR — antenneX, Archive III) includes a diagram of the dipole version of the CFA introduces the idea that the ratio of size of the CFA’s major components to each other, rather than to the operating frequency, is an important concept. I gratuitously applied that concept to the plate-disk-cylinder version, and did some simple calculations to determine the important ratios for various sizes of that design. Some tables showing those calculations appear below.

Cyl (E) Outside Diameter	Cyl (E) Height	D plate Outside Diameter	D plate Inside Diameter	Grid Plate	Space	T Height (in)
Factors	0.8	0.51	3.56	0.20	2	(N/A)
1	1.25	1.98	0.28	5	0.5	2.25
2	2.5	3.95	0.56	10	1	4.50
3	3.75	5.93	0.84	15	1.5	6.75
4	5	7.90	1.13	20	2	9.00
5	6.25	9.88	1.41	25	2.5	11.25
6	7.5	11.85	1.69	30	3	13.50
7	8.75	13.83	1.97	35	3.5	15.75
8	10	15.80	2.25	40	4	18
9	11.25	17.78	2.53	45	4.5	20.25
10	12.5	19.75	2.81	50	5	22.5
11	13.75	21.73	3.09	55	5.5	24.75
12	15	23.70	3.38	60	6	27
13	16.25	25.68	3.66	65	6.5	29.25
14	17.5	27.65	3.94	70	7	31.5
15	18.75	29.63	4.22	75	7.5	33.75
16	20	31.60	4.50	80	8	36

In the “Factors” line you can see the rough numerical ratios I found among the dimensions of the recommended design that is based on an 8-inch diameter cylinder. I simply built some formulas in a Microsoft Excel spreadsheet that yielded the dimensions for cylinders of other sizes, and the dimensions of the other components that would maintain the proportions. All dimensions are in inches.

Building the Matching Unit

The same night the “BALUN kits” I had ordered for the transformer arrived, I wrapped one with the supplied nylon tape and 21 turns of coated, 14-guage wire. I pulled some slack for taps at turns 11, 12 and 13, just to allow myself some leeway in tuning. It wasn’t pretty, but it seemed to satisfy the requirements. I have moved two, old 150 pF variable caps from house to house for over a decade, and I think they will find a home in this project, as C1 and C2. If I need more capacitance to reach a desired operating frequency, I’ll add fixed caps in parallel.

I had no coated wire suitable for coils on hand, so I spent some time searching the catalogs and online sources. A parts house in Indianapolis (about an hour from here, if you’re not driving on Indianapolis 500 and NBA playoff weekend) suggested I check with a local motor-rewinding shop. I went there, waited for half an hour at the counter, and left with 50-plus feet of 14-guage Magnet wire. When he finally brought it out, the counter man waved at my wallet and said, “Just take it and run. Don’t tell nobody where you got it, or I’ll have hams crawling all over me.” He said they usually deal in huge quantities, and 50 feet was just a nuisance—not worth ringing up. I took his advice and left, roll of wire in hand.

The “Care and Feeding” article specified winding the air core inductors on a 1 & 3/8-inch form. I found some 1-inch (ID) PVC pipe with an outside diameter of 1 & 5/16-inch, so I used that. What’s a silly sixteenth (less than 2 mm.) among radioaficionados? I cut off a piece about 18 inches (46cm) long and cut some notches in one end to hold the start end of the wire. I pulled about 8 feet (2.4m) off the reel, and clamped it in a soft-jawed vise at that point. I rolled the wire onto the pipe while holding light tension on it and walking slowly toward the vise. Despite the fact I had never built an air-core inductor from scratch (yeah, I’m an appliance operator, except for making antennas), the coils turned out to be decent, if not pretty. The MFJ 259B says they both have about 11.2 uH of inductance at 3.5 MHz.

The three coils, two capacitors and associated wiring fit reasonably well into an aluminum project box from a project that didn’t pan out. I used combination test jacks (usable either as terminal posts or as banana jacks), primarily because they were isolated from chassis ground, but also because they would allow easy connection and removal. The parts are all mounted on a piece of surplus Plexiglas. I used a small mounting bracket in the center, that was left over from the piece’s last incarnation as a display stand, to anchor the toroid transformer with some cable ties. The tap points are oriented so I can reach them with an alligator clip lead. Either side of the transformer, I stuck the two variable caps to the Plexiglas with silicone glue. The air core inductors are supported by their own leads from their attachment points.

Tuning Up (or trying to)

As everybody trying to make one of these antennas will say, “The tuning is the hard part.” Is it ever—I spent several hours over a long weekend and several workday evenings working only with the antenna system and an MFJ 259B. The system “acted” like a tuned circuit right away, at least alleviating my fears that I had assembled the tuning unit incorrectly. Right away, however, I noticed that the value of C1 was too low—about 150 pF maximum. I had to substitute another scrounged part, with a ganged maximum of 400 pF. This gave me the range I needed for the 75M band, but the tuning action was so fast that the shaft backlash would send me from the 2:1 range into infinity just by letting go of the plastic knob fastened to the shaft. I also noticed a very high “proximity effect” when reaching for C1. Just touching the knob could change the SWR from near 1:1 to 7:1.

Tuning with as close to a safecracker’s touch as I could muster, I managed to reach an SWR of close to 1:1. I gathered up the courage to connect my HF transceiver (an Icom IC745) to the tuner and apply some low-power RF. I tuned to a dead spot in the band (which was dead everywhere I tuned, except for distant lightning crashes, in late morning, local time). The relative field strength meter (a $20 wonder from Radio Shack) came to life, and I set the sensitivity at about mid-scale. I made several small adjustments to get a low SWR on the 745’s internal meter. As I coaxed the SWR down, and raised the 745’s RF output to the SWR calibration point, the SWR began to rise on its own, without any changes to the settings. Over a key-down period of about 15 seconds, the SWR rose smoothly from nearly perfect to about 2.5:1. I let up on the key, waited about a minute, and tried again. Within seconds of putting carrier (and a fast CW ID) to the system, the SWR began to rise again. I let up on the key and felt the components. Only L2, the air-core inductor across C1, was slightly warm; the other components were at ambient temperature. I still suspected the toroid of saturating, even though it was represented as being appropriate for HF use at 1KW, but it was as cold to touch after the SWR rose as before. I ran out of time and gave up for the night, grumbling about the touchy tuning, strong proximity effect, and—possibly—a saturating toroid.

The Shadow Under the Groundplane

While I was in the shower the next morning, I remembered that most of the CFA “success stories” seemed to include a mention of the tuning unit being placed UNDER the ground plate. For the tests, I had the unit sitting on a plastic milk crate out a few inches from the edge of, and beneath the plane of the ground plate. That evening, I re-arranged the test area with the tuning unit several inches under the ground plate, but close enough to the edge that I could reach the capacitors conveniently. The change was remarkable. The tuning of C1 was still somewhat sharp, but much less sensitive, and some of the proximity effect was gone. Tuning for 1:1 became much easier. When I applied some RF, the saturation phenomenon (if that’s what it was) was absent until I applied about 75 watts for about 30 seconds. At the SWR calibration level of about 20 watts, it didn’t happen at all. Progress! It was only slightly less mysterious than sacrificing a goat and praying to the radio gods, but it WAS evident that the ground plate was casting a “shadow” of some sort on the tuning unit. I made several calls on 75M, but nobody answered. I heard many complaints of poor band conditions, and there was a lot of lightning QRM. It was getting late, and I had to get to bed. Despite not making an on-air contact, I felt as if I were making some progress.

Minor Tweaks

After some correspondence with Jim, N7CXA, I removed the original L2 and replaced it, per Jim’s suggestion, with a coil of about the same dimensions made from insulated 12-guage hookup wire, of the type found in AC household wiring in the US. I also replaced the tuning knob on the shaft of C1 with an 8 in. (about 20.5 cm) length of small-diameter nylon tubing. While I didn’t spend a lot of time playing with this arrangement, I did note that the awkwardness of tuning with this arrangement was more than made up for by the lack of detuning due to body coupling. I was able to achieve nearly 1:1 SWR across a large portion of the 75M band. Still no contacts, however, and the field strength meter is minimally excited, despite my efforts.

At a point when I needed to get away from the tune-up process for a few minutes, I remembered a Korean War vintage military antenna tuner given to me by an Elmer/friend, Bob, W5JVX, over a decade ago. I pulled the tuner off a shelf and took it out of its case. It is very ruggedly built, with two large, tapped inductors and two variable capacitors with vernier drives. I vaguely recall Bob telling me this device was used to tune a trailing wire antenna used in aircraft. The table under a glass window on the front of the case shows switch and capacitor setting for a range of frequencies just under 7 MHz. Most of the major components terminate in a row of screw terminals along the top of the chassis, probably making them configurable to meet my needs. Hmm. There are some possibilities here….

Building in some flexibility

Realizing that any excursion from the 75-meter design frequency for the system would require reducing the inductance of L2 and L3, I decided to wind two new inductors with multiple taps. I used the same form, and wound two, slightly longer coils with taps every five or six turns. (See picture, below.) They show an inductance of about 4-17 uH on the MFJ259B. I soldered clip leads onto the “high” end of each coil to give me some coarse inductance variation. With the clips on the second-to-highest taps on both coils, I was able to tune for low SWR on 40M. I wonder what inductances and capacitances are required to tune this system up on 6M?

Is it an antenna, or a dummy load?

Following several evenings and weekends spent trying to make this odd stack of aluminum and plastic act like an antenna, I began to wonder if it would ever happen. After receiving several helpful and encouraging suggestions from other CFA eXperimenters (Stefano, Jean-Marie, Jacque, Anselmo, et al.) I moved the antenna from its garage birthplace out into the yard, and recalibrated it for 40 meters SSB (about 7250 KHz). Thinking I would save some switching time, I ran the coax through a window into my shack and connected it to one of the SO-239 ports on my small MFJ tuner. I figured I would select the CFA (untuned) or one of my conventional wire antennas (tuned) with the rotary switch on the back of the tuner. Hah! I figured wrong. Another mysterious trait of the CFA manifested itself. Passing through the tuner, even in “coax bypass” mode, changed the SWR from 1.2:1 to 3:1, and made it nearly impossible to use. Taking the tuner out of the circuit restored the tuning job I had done outside, but left me with no fast way to “A-B switch” between antennas for comparison. Too bad.

It’s Alive!!!

One or two more nights went by while I tried in vain to make a contact. I kept “forgetting” to bring in the field strength meter into the shack from the garage—perhaps because I was half-afraid the system wasn’t radiating at all, and I didn’t really want to know. A hint from Jacques led me to try a bit of a departure from the normal “modus operandi.” He suggested I disconnect the ground plate between the matching network and the antenna and try tuning up. Stefano had already suggested I take L1, the toroid transformer, out of the circuit. Following Jacques’ suggestion also seemed to satisfy Stefano’s suggestion, because disconnecting the ground plate lead effectively took the transformer out of the circuit. Moving the clip lead from one turn to another had no effect on the SWR.

There was one big change, right off. The tuning of C1 was as touchy and cantankerous as ever, but one the network was tuned, it was amazingly broad. I had less than 2:1 SWR from 7000 to 7300 KHz! Several other eXperimenters have commented on the increased operating bandwidth with increase in operating frequency, but I was still astonished to see so much improvement for myself.

Another new feature was the complete lack of the detuning effect that I attributed to the core of the toroid transformer saturating. Even 30 seconds of key-down power (as much as I wanted to inflict on anyone “lucky” enough to hear me) didn’t change the SWR a bit. The evidence is still circumstantial, but I think the toroid has been convicted of saturation. After a quick CW ID, I almost wanted to hear a ham grumble about “some idiot tuning up on me,” but there were no complaints. Darn.

About 0115 UTC, after making several fruitless calls, I decided to respond to a very strong CQ on 7227.0 KHz (17976 bytes) from Mike, N8MUP, who sounded like he was local—maybe 60 miles from here. Imagine my surprise to hear him tell another ham he was in West Virginia—almost DX! Well, sort of. Weston, WV, Mike’s QTH, is about 300 miles (about 500 Km) from here. The contact was short, Mike’s report of my signal was “4 and 5,” and he lost me in a 7 S-unit noise level in less than a minute after the initial contact, but, HE HEARD ME!

Wrapping it up

This is the end of the journal on this particular eXperiment, but it’s by no means the end of the eXperiment. While the CFA I built (with a LOT of help from other CFA eXperimenters) was anything but a raging success, it is an antenna, and not just an odd pile of aluminum and plastic. The suggestion to disconnect one of the elements is just one example of the opportunities there are for improvisation and eXperimentation in this thoroughly unconventional antenna design. After years as a passive member of the amateur radio fraternity, it’s exhilarating to find myself among eXperimenters out on the ragged edge, learning about new technology by—of all things—playing with it in my own garage! Thanks for reading, and keep eXperimenting!

The Further Adventures of KA5NEE

The KA5NEE CFA Becomes an Organ Donor

Once again, the KA5NEE Institute for Research and Development is alive with the sounds of antenna building. One recent evening, the engineering department (known locally as “the garage”) was the scene of a reincarnation. The CFA project (See Archive III of antenneX) was in the process of being reborn as a DDA. In my desire to re-use both labor and materials (known locally as “being stingy and lazy”), I decided to retain the cylinder and upper disk, and I cut the second disk out of the CFA’s ground plane.

Readers of the August article will recall that my CFA was built from aluminum flashing and PVC pipe. By coincidence, the proportions of the disk and cylinder for that antenna, and the spacing between them, fit in rather well with the list of proportions for the DDA in Rich Morrow’s introductory article (now in Archive 3), reproduced below:

78/F = Diameter of cylinder
1.25(diameter of cylinder) = length of cylinder.
2(diameter of cylinder) = diameter of disks
.25(Diameter of disk) = Spacing between cylinder and disk

In the diagram below are the dimensions for the KA5NEE DDA that was re-incarnated from the CFA:

Working back from Rich’s formula for the operating frequency of the cylinder, 79 divided by the diameter of the cylinder should equal frequency, assuming the units of measure are inches and Megahertz. That works out to an operating frequency of 8.667 MHz for this cylinder, which doesn’t correlate with anything I found resembling a “natural” operating frequency for this system using the MFJ 259B analyzer. Of course, recent discussion of the DDA indicates spacing of the disks from the cylinder and the presence and size of holes in the centers of the disks are also factors in determining the operating frequency.

The DDA Network Evolves

The DDA network for this project is also salvaged from the parts of its CFA ancestor. I rewired the components according to Rich’s drawing in his October, 1999 follow-up article, “The New DDA and Network Test Results,” with some exceptions. The “lazy and stingy” force made me dispense with Rich’s input transformer, substituting a W2DU-style ferrite bead balun at the input. This type of balun keeps RF off the outside of the coax shield by using a number of close-fitting ferrite beads to raise a high-impedance barrier to HF currents on this path. I also re-used the variable caps from the CFA design, although they were not nearly high enough in value, at 150-160 pF each. Rich’s work recommends values in the 400-500 pF range. All components in the new network are mounted on clear acrylic plastic in the aluminum case from the CFA project.

Here’s a picture of the new network:

New DDA Network

I removed the toroid autotransformer, and wired from the stator side of C2 (referring to Rich’s October article, again) to the cylinder, and from the rotor side of C1 to the plates. Everything else is wired according to Rich’s schematic, except for the missing input transformer. And one other exception, which I discovered during the testing phase.

Testing

The first testing I did was with the MFJ 259B, making a “sweep” of the operating range of that device, with the 259 feeding the network through the bead balun. I also had the Radio Shack relative field strength meter nearby, with a clip lead connected to the whip antenna and draped loosely around the base of the DDA. The first sign of life I saw was around 3980 kHz. This was the first place the SWR dipped into the readable range — around 4:1.

The field strength meter also came to life, showing a reading at the same point. I connected my MFJ 941C antenna tuner between the balun and the 259, and easily reduced the SWR to 1:1. That’s when I decided to connect the system to my Icom 745 and make some test transmissions.

I found pretty decent received signals in the 3900 kHz area, and made two contacts (at about 100 miles and 150 miles), with 5/9 reports from both stations. These contacts occurred around 2300Z. I felt pretty smug. Later, I noticed I hadn’t finished re-wiring the network. C1 wasn’t connected to ground, which pretty much explained the lack of any effect on the SWR to be had by adjusting it. I’m only admitting to such a bush-league mistake because it’s interesting that the antenna did produce contacts on 75M, even if I had to use a tuner to get on the air..

Once I added the wire from C1 to ground, I had a different system! The first promising activity (without the tuner, this time) on the 259 was around 2,150 kHz, with readable deflection on the FSM, and SWR just below 2:1 on the 259. I tried using the caps to coax the network up into the low end of 80M, but it wasn’t interested. The next hot spot was around 14,300 kHz — 20M! That one tuned nicely to 1:1, so I decided to set up on 14.275 MHz and see what the 2:1 bandwidth was from there. The following table shows my observations.

Frequency (MHz)	SWR (__:1)	R (per MFJ-259)	X (per MFJ-259)
14.100	2.0	31	20
14.125	1.8	33	17
14.150	1.6	35	17
14.175	1.4	38	11
14.200	1.2	41	8
14.225	1.1	44	6
14.250	1.0	47	2
14.275 *	1.1	55	3
14.300	1.1	55	6
14.325	1.3	59	10
14.350	1.4	64	15
14.375	1.5	70	19
14.400	1.8	78	22
14.425	1.9	85	25

*Center Frequency

During on-the-air activity on the portions of this 325 KHz band I am licensed for, I was able to transmit 5 or 10 seconds of key-down CW with no heating of network components. I haven’t had time (as of 1900Z, October 24, 1999) to hunt for contacts, but I have done considerable listening. Apart from the annoying computer hash — which I could only prevent by shutting down the PC on which I am writing this article — received signals are at least decent, and I haven’t found any fading — I’m as curious about that as are the other DDA users. (Editor’s Note: The lack of QSB remains one of the big mysteries about the DDA yet to be uncovered thru more eXperimentation, or when one of our mathmatical minds figures it out! There IS an reason and an answer! It is, however, one of the advantages of the DDA.)

This particular member of the cylinder/disk family (versus wire) antennas does seem to have some promise. I still lack some simple dipole antennas and the means to switch between them and a test antenna rapidly, but that will come. Meanwhile, I will be attempting to implement an idea from brother Jim, KA9PBO: using threaded nylon rod stock to allow systematic adjustment of the cylinder-disk spacing. This characteristic has been described by other experimenters as being important to determining the operating frequency.

What Next ?

Clearly, there’s a lot of open ground to explore in the use and refinement of the DDA. As mentioned above, I plan to make the cylinder-plate adjustment convenient enough to lend itself to systematic variation and measurement with the 259B. What are the chances that we could “tune” a DDA with some DC motors rotating those threaded nylon rods?

Rich mentioned trying to make the design work in the VLF 160 – 190 KHz band. Imagine the burst of interest in the “LOWFERS'” realm if a DDA can be made to radiate efficiently down there without having to be as big as a city water tower! That would probably get the FCC’s attention, which is not necessarily a good thing. The power limitations of those eXperimental VLF stations is based on the premise that the antenna system is horrendously inefficient.

Over the next few weekends, I hope to make some more intensive efforts to gather off-air reports on 20M, which I will pass along to the other eXperimenters. I look forward to the reports from the more serious, methodical researchers (Rich Morrow comes to mind, among others…) on different sizes, element spacing and network circuits. For instance, Harold Allen, W4MMC has proposed some interesting matching network circuits which Rich is also conducting eXperiments. The possibilities are truly endless!

Imagine! You don’t have to be a physicist or RF engineer to get your hands on leading-edge antenna technology and make a contribution to the body of knowledge. Just read antenneX, and get to work! 73, from Tom Cox, KA5NEE

Originally posted on the AntennaX Online Magazine by Tom Cox, KA5NEE

Last Updated : 16th May 2024

The CFA Adventures of KA5NEE