Crossed Field Antenna
Update August 28, 2008: Back when I first built this antenna and published the results, I could not believe the hate-mail I received. There were all sorts of folks who were so convinced that this antenna was an incredible invention and that they could not believe anyone would say anything bad about the design. I did and got QRM'd for it big time! Well, since then this antenna design has been shown to be what it is, smoke and mirrors, mainly hype and little delivery. It's now a design that's been exhaustively evaluated by none other than Trainotti of the IEEE and who is someone who really understands antenna theory and the results were published in QEX Quarterly. If you want to build one of these go ahead - it's a great April 1 project but don't expect stellar results. Some hams speak ill of the CB crowd who, as some would suggest, will believe anything when it comes to signal propagation. Well I invite anyone to send me a picture of a single trucker anywhere who has one of these antennas and reports good results. If the CB crowd isn't sold then you can be sure the antenna is a joke. I built this design and published my results 6 years ago and others have since confirmed it's poor performance . ( Lest anyone ever think I was sold on this design.)
Below is my original article:
There seem to be several versions ( and names ) of this controversial item, but they all have one thing in common, they are electrically short, and I mean short, for the frequency they are tuned to. Call them E-H, CFA or Short Fat Diploes, they are short antennas for the frequency they are tuned to operate in.
Here's a model I built from what I've been able to glean from the internet. It claims to create an E and H field so that the vector relationship between the two fields is the same as a traditional wire-based antenna. I wasn't at all impressed with what I built but I offer my test results here for others.
I built a couple of these with the second one tuning up exactly as promised with a 1.01 : 1 SWR. It does receive pretty well and is quieter than a similar standard monopole type ( there's a good reason why ). The first one was too short and I didn't get the lower coil right. The second one was good and I tested it out eventually at 100 watts. Read on though... at one point during my tests it started smoking and caught fire !!!
The lower capacitor resonates the upper cylinder and there's a huge voltage that appears across the plates. After repairing the smoke and fire damage (!) ended up with another set of caps inside the tube and there was a spacing of over 1/2 inch of insulation from anything to anything else and it was this unit I tested today. It ran without arcing at 100 watts CW but hot spots still developed. At 500 watts though it arced over.
April 14 - 2002 ... in the parking lot of California State University Hayward, I chatted with three stations on 20M and below are the results. The fourth station heard the tests and sent me an email.
The comparison antenna was a short mast ( 36" ) top loaded, top fed antenna using a commercial Hustler resonator. It's an antenna whose characteristics are well known to me and is documented elsewhere on this site.
From a good location in the parking lot... car pointing west - a lovely sunny day around 12 noon... first using the 'CFA type' I tried to call Ham 3 at 100 watts and he could not hear me. I had to go to 500 watts where he said he could barely hear me. We stayed in touch on 147.240, a local repeater. All of the rest of the tests below were done at 500 watts.
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Participating Station |
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Ham 1 - Danville , CA |
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Ham 2 - Livermore , CA |
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Ham 3 - San Luis Obispo , CA |
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Ham 4 - Vancouver , WA |
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From the results above, and several verifications with Ham 1, there was a difference between the two antennas of about 6 S-units. Taking into account errors in S meter readings, say 4 S units for sure.
The best thing I can say is it's not a stellar performer and in my opinion way too complex for the performance it gives on 20M and does not really function as an antenna except as a by-product of use. I think this antenna may be a re-invention of the plasma chamber technology I am familiar with.
In the semiconductor industry we use gas plasmas created by RF injection into cylindrical chambers where the plasma etches away impurities, etc. from masks and wafers. The chamber is a closed system with a faraday cage around it to prevent radiating the signal which is often in the 20 or 30 MHz industrial range. If you take the faraday cage away you radiate the signal all right but it's not a great antenna.
It's my opinion that the concept of a phased antenna of this type that purports to create an orthogonal E and H field ... or some other such complex vector relationship, is really more like a gas plasma chamber assembly. A possible proof is the behavior of the device during this test.
Several areas of the copper tapes used to form the upper and lower cylinders, and the capacitors, were heavily blued in spots. This means there were E-fields that induced a fair amount of current into the foil ... and holding a fluorescent light near this device under other circumstances sure proved this theory. But as one antenna expert said, while there may have been a huge E-field, it could have been reactive.
Also, after a while, a key area the lower capacitor arced over and carbonized the white CPVC of the antenna assembly. This was one of the high voltage points in a parallel resonant circuit part of the assembly. I'm familiar with corona's ( see my 160 M TX experiments ) so I was not surprised to see the results here at 500 watts, but considering the abysmal comparative performance of this antenna, I have to ask where the energy went.
Again, in my opinion, the antenna created a huge E field ( I don't doubt that ) and in doing so did it's best to ionize trace gases in the air, and at one point formed a local corona that carbonized the CPVC.
To me it makes sense since plasma etchers typical run anywhere from 250 watts to 5 kilowatts and the energy, which is turned into heat, is removed constantly by the evacuation of old contaminated gases and introduction of fresh gas mixtures. In the open air the antenna did it's best to form a plasma, I think, and created a fair amount of heat doing so.... but little RF energy of the type that we consider suitable for communications.
So folks, there's the results of my tests which may create some added controversy on this already controversial antenna. There may be those who say I didn't build it right... but lacking access to any commercial plans that may well be, but I was able to build such a gadget in a couple of evenings and the unit performed according to the information available to me via the internet. So my guess is what I built is pretty close to what the inventors) claim works. If not, I'll gladly buy a commercial unit and test it also when one becomes available.
Until that day, however, for me, the controversy is settled. Call it what you will... this antenna really isn't an antenna in the classical sense at all and may well be useful for something else. Or, it's possible that it's secret is revealed only under very narrowly defined or strict conditions... and if that's so, that's OK with me because I probably won't be using one in that type of setting.
So I thank the hams who helped in this test ( you know who you are ) and I've had a lot of fun as well.
When I set out to test this antenna I said there were only three possible outcomes - better, the same, or worse than what I already have... and the table of results above says it all.
73...
Wil - AB9U
Edited and updated July 4, 2002 - Happy Fourth of July !
