Alternate CFA Matching
It has been noted over the past several months that many have endeavored to construct a CFA using the information and drawings set out in the U.S. patent. The reports have shown little success and usually it’s because of the matching network. With this in mind, I decided to examine the original U.S. patent #5,155,495 to see what helpful information I could reap from the information contained within its numerous pages. You can see the patent for yourself at this web site server using the patent search feature: <www.patents.ibm.com>
Bear in mind that the CFA version in the patent is not exactly like the construction article describing the ham band 75/80-meter version presented in the last issue. That one utilized wound toroids—this one does not. Also, it further differs as the patent describes a “hatted” version. Therefore, this article is only meant to provide supplemental assistance toward finding different ways to tune a CFA. As far as an application to the ham use, this is in addition to another article by CFA series co-author, Ted Hart, W5QJR based on his own construction of the 75/80 meter version with a newly-described matching network. That article is more directly related to the reader projects over the past month.
Now, for those who have never dug through a patent, let me tell you that they are about as baffling as the instructions for filling out income tax forms—at least for me. But, I felt it needed to be done. So the information here has been filtered to get to the real nuggets hidden in the patent language. This information also summarizes what the inventors did to arrive at their decision to apply for a patent. It is hoped this will be of some further help to all wanting to experiment with this antenna.
CAVEAT: There are a few things that need to be stated about what was in, or not in, the patent. Such can cause confusion and questions to arise about what is being presented here. First, no data is shown as to what antenna element impedance is present at each terminal of the matching networks presented here. There was no mention of the terminal impedance presented by the antenna elements connected to the network in the patent. The signal phase reference in the patent refers to the voltage fed to the elements, yet in another place, current is mentioned as having a 90 degree displacement to the voltages that are exciting another element of the device. So, all I had to go on was that the different elements has to be fed by two signals that causes the elements to be 90 degrees apart. There is no mention of what the proper voltage/current ratio between the elements should be or voltages present on the elements. The coaxial delay lines are used to generate a phase delay and this is explained in the patent. I am mentioning these things so any experimenter will be aware of this information or lack thereof. Do not be discouraged, but understand this is an indication of the way information is presented in the patent. This is typical of a patent, to be vague so as to perhaps hinder anyone from designing around the patent. It will also give an insight as to what was intended to be the requirements for correctly feeding and phasing/matching the CFA.
Not Resonant Dependent
First, the operation of the CFA is not dependent on being resonant, so with that in mind, we will move on. No, I am not going to even try to explain how it works. That is for the mathematicians in our midst, of which I am not one. Having said that, let’s proceed to Figure 1. This is the basic matching network and the purpose of this network is to provide two signals at output A and output B that are +45 degrees at output A and -45 degrees at output B. These signals are referenced to the input signal. These two signals are fed into the device with a total phase difference of 90 degrees. This is what causes the CFA to radiate. Remember, a coil will cause any signal fed through it to become a signal that will lag the input signal by X degrees. The X degrees are dependent on the circuit constants and signal frequency, all of which combine to create a lagging signal on the output side of the coil.
A capacitor will cause the signal to lead by X degrees for the same reason that the coil causes the signal to lag the input circuit constants, etc. I am not going to get any deeper into this part of the network as the purpose here is to give more information and insight into the purpose of the phasing network and to aid in solving any problems that have occurred in getting the CFA to work properly. This is the sole purpose of this network, and the matching network should be considered as a separate part of the system, although it plays an important part in this project. Also the purpose of switch SW-1 is to allow the phase shift between the outputs to be interchanged as in certain cases, the + and – phase shift between the elements that make up the device may need to be switched for proper operation.
Rotary Switches
One of the methods mentioned in the patent was the use of rotary switches to select coaxial phasing lines to get the proper phase delay on the inductance side of the network. This is shown in Figure 2a. In this network, calibrated coaxial delay lines are switched in and out to allow the proper delay line to be inserted into the circuit, and the variable capacitor is tuned to get the proper phase shift between the two outputs. Switch SW-3 is part of an auto-transformer-matching network, which should allow matching to be achieved.
Figure 2a has a small change I introduced to the circuit to allow for even finer phase tuning. L-1 has been added to the input to the inductive side of the network to allow small adjustments to be added to the phase shift that will occur through the delay lines. This item was not in the patent and was something I believed would improve the ease of getting a good stable phase-shift tuning method. This may not be necessary, but coaxial delay lines age and small changes will take place in the phase shift through each line. True, the variable capacitor should be able to tune out the changes, but this may not always work out, particularly if the variable capacitor is small in value.
DPDT Switches
Another method to obtain the same result is shown in Figure 3a and uses DPDT switches to accomplish the same thing. As illustrated, when all the switches are in the up position, all the delay lines are switched out of the circuit. By switching in different lengths of coax, different delay amounts can be switched in and out and the capacitor can be tuned to get the correct phase difference. This seems to be a much more versatile system than having set lengths of delay lines. This is because you will be able to switch in delay lines for more than the amateur frequencies. The rest of the circuit functions as in the previous diagrams. The patent calls for coax lengths of 1/8, 1/4, 1/2, 1,2,4,8,16 and 32 meters. It seems for amateur bands, a 1/8 wavelength of coax would suffice with a small variable inductor in series with the coax for fine-tuning. The longer lengths of coax may be coiled up and stored under the table supporting the matching unit.
Figure 3b is the same as the previous diagram, with the small variable inductor added for fine phase tuning. Again, this may not be required.
Field Strength Meter Needed
I realize those last two networks can be expensive and more complicated with the use of coaxial cable delay lines, but felt that they were worth mentioning. Besides, someone may find out this will best suit their purposes. It also will allow the antenna to be used as a mono-band. It also will be possible to remote tune the last network with the use of DPDT relays and some sort of drive to tune the capacitor slowly through its range as you watch the field strength meter. YOU MUST HAVE A FIELD STRENGTH METER TO TUNE THIS ANTENNA ACCURATELY! Otherwise you will never get this antenna tuned up correctly and I am speaking from experience.
By tuning for minimum VSWR, you will not get the desired results except by accident. There are several points where you can get minimum VSWR, but very little radiation. All of these networks were designed to be located at the operating site and connected to the antenna by cables. However, this does not mean you can’t build one at the base of the antenna for mono-band operation or remote tuning. If you build one of these, be sure the cables to the antenna are the exact same lengths to prevent more phase shift difference between the two cables that can be handled by the network.
Those Toroids
Now some more tidbits. If you are using the toroids, be sure to wind the coil by twisting the wires together making up the coil. It makes winding the coil a lot easier, plus, it maximizes magnetic coupling and minimizes the effect of distributed capacity. The windings should be evenly distributed around the coil and the link coupling should be located in the middle of the windings.
It would further help to use a simple, dual input phase monitor to allow even more accurate tuning of the elements. All it would have to do is to compare the two outputs to the input signal and let you know when the outputs were +45 and -45 degrees with respect to the input.
Persist!
About 100 years ago, a young inventor went to his government and asked for some money to help him perfect his invention. He was told his invention was almost useless and there was no future in it at all. History has proven the government official’s discouraging advice to the young inventor was rather incorrect. As we all know since, the lad went down a lot of dead-end experimental paths, but he persisted. Due to this persistence, we have the radio communications systems that came from Marconi in his relentless pursuit of the answers to the unknown.
The same thing applied to all of the other wireless experimenters from around the world. The new wireless invention caught attention and others were drawn to explore the potential this new invention represented. The determined ones kept after it and their collective efforts created the many things that make up the myriad radio systems we now have. Those that didn’t persist and learn from their mistakes fell by the wayside and became naysayers. In this light, my belief is we are at the same place Marconi was 100 years or so ago, as far as this CFA is concerned. I could be wrong, but that is the way I feel. There is a huge amount of information and new data to be discovered and old information to be re-examined in a new light. The idea of the radiating fields originating from a very small non-resonant antenna with significant field strengths is totally new. Particularly when the radiating source may less than 1/200 wavelength long at the operating frequency. The implications of such an invention are enormous. Only time and more persistence will tell.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 30th April 2024