Introducing the Bi-Plane
After two years or more of CFA rage with very heated debates on the forums we have seen a gradual CFA fatigue as the promises of that compact antenna were not fulfilled, or at least very difficult to achieve for most experimenters. We were then introduced to different new concepts like the CTHA or counterwound helical antenna, the DDA or duo-disk antenna, the DLA or double loop antenna and, more recently, the EHA or biconical antenna. During all these discussions the name of the Isotron antenna also kept coming back. antenneX Science Editor, Joel Hungerford, KB1EGI came up with the class name of ” those capacitor antennas” which I think describes very well what some of these antennas really are.
It All Started as a Pizza Parlor
Fellow GARDS’ member, Good Old George Sharp (KC5MU) decided to put up a “pizza parlor” in his back yard starting with a “pizza pie” antenna and began to tinker with it in the “Isotron mode” with quite good results. In the Isotron mode we use a large capacitor plate with a tuning coil and tune the whole thing against a second capacitor plate or against the coax cable and/or the tower and ground (see Figure 1). These antennas are easy to make and definitely are a possibility if you have antenna restrictions in your area. To their neighbors, some people refer to them as “bird feeders” as they look so unusual. The general belief is that in contrast to magnetic antennas the primary field generated by these antennas is electrical in nature and that the coil really does not contribute much to the radiation.
George’s Original “Pizza Pie” Antenna
And Now George’s New Brain Wave
During the same period around June 2000 that Old George was putting up his Pizza Parlor (see the November 2000 Ham WorkShop ) there was also a lot of talk about the DLA antenna causing George to concoct one of the weirdest antennas I have ever seen. It was thought to be considered as a crossbreed of a DLA and an Isotron antenna. I am not sure how he came up with the idea but it was so weird that he first called it “The Thing”. Much to his own surprise it outperformed everything he had been brewing so far and right out of the box! The Thing, which later was anointed with the name “Bi-Plane”, actually flew during a wind storm and almost behaved like a kite on the coax feeder hence its name for which I would love to get the credit for.
One way of looking at the Bi-Plane antenna is to compare it to the basic DLA (see Figure 2). In essence, you expand the physical dimensions of the capacitors and coil up the loops.
The Bi-Plane antenna was described by George in a previous Ham WorkShop article “From The Thing to a Butterfly”. It actually consists of four aluminum plates that form two sets of fixed capacitors which are then resonated with two small coils. In between the larger capacitors there are two trimmer capacitors to fine tune the antenna and to bring the SWR to 1:0.
The Basic Dimensions for Building Your Own Bi-Plane
The basic dimensions for figuring the bi-plane are quite simple, for example: Use capacitors (square or rectangular) of about 100 square inch for the 20 meter band. For the forty meter band you double this value, for the 10 meter band you cut it in half. The distance between the plates can be anywhere from 1/2 inch to 1 or even 2 inches. This distance will determine the value of the capacitor which can be calculated with the following formula (dimensions are centimeters, F=Frequency, C=Capacity):
C = 0,0885 x surface area / distance
The value of the corresponding coils: L = 25330 / FxFxC
Table in Dimensions
Band | Plates | Coils (# Turns on 1.125in Form) |
---|---|---|
10M | 50 sq. in. (7″ x 7″) | 4.5 |
20M | 100 sq. in. (10″ x 10″) | 9 |
40M | 200 sq. in. (14″ x 14″) | 18 |
80M | 400 sq. in. (20″ x 20″) | 36 |
160M | 800 sq. in. (28″ x 28″) | 72 |
George was so exited about his “thing” that I decided to build one myself. After another trip to Home Depot, I ended up with two “door kick panels” 8 inches wide and 34 inches long. A little bit on the expensive side but they look great. I cut both panels in half and had all I needed to build my two large fixed capacitors of 8″ by 17″.
My Version of the 20 Meter Bi-Plane
Now time had come to put the thing together in a somewhat stable and rigid way, In my garage I found a polyethylene yard stick of some 36 inches long and 3/4 inch thick. It had a square cross cut section which would enable me to make stable capacitors as the slightest movement of the plates would alter the value of the capacitors and detune the antenna.George uses PVC pipe but I seem to prefer a square cross cut section and by now have found Nylon square bars of different diameters on the internet. Nylon is virtually indestructible and easy to work. The four plates were then screwed onto the plastic bar as can be seen from the pictures. I measured both capacitors with a precision capacity meter and found both to be 52 pf.
The coils theoretically needed to have an inductance of 2,41 microhenries. I did use plastic wine corks for the coil formers as they do tend to keep the coils under tension so the turns do not move. 12 turns of “gauge 12” enameled copper wire gave me a measured inductance of 2,3 microhenries. Close enough to put the thing together and find out for myself why George got all that excited. Please look closely at the pictures to see how I have put up the coils. I made a small bracket of aluminum strip for the antenna connector and used a combination of ferrite clamps as well as a coax coil to avoid that the coax would be part of the radiating system.
George Adds a New Word to the English Dictionary: Titervating
“Titervating” is a new verb (and sometimes adjective) enriching the English language. Innovator George came up with this new word when he described the way in which he fine-tunes the antenna by simultaneously turning two capacitors with both hands looking for the hot spot.
It took me quite a while to resonate the antenna. At first I wanted to do this without adding trimmers to the main capacitors at all. This was a brave but not a wise decision. I finally gave up this idea and installed a small 30 pF trimmer between one of the wings and then the thing sprung to life. My bi-plane had finally taken off! (note: trimmer should be on that wing where the bottom plate is connected to the center of the coax ). If you have trouble finding a good SWR just add another trimmer on the other wing and start “titervating”.
I nearly fell off my chair when I scanned the 20-meter band. Low noise and very strong signals from all over the world and this with the antenna sitting just behind me on a small support at 5 feet from the ground. The band conditions must have been very good that evening, but still, to be able to follow a QSO between two South Africans via the long path is not something I have the pleasure of doing every day. The 2 to 1 bandwidth covers the entire twenty-meter band.
Over the next two weeks I did a lot of A/B tests with my MFJ magnetic loop and a homebrewed DLA, both sitting some 25 feet higher in the air than the Bi-Plane’s low position. I found that some stations are nearly as strong on my Bi-Plane as on the other antennas-some are weaker and others even stronger depending on the distance. The bi-plane works very well from inside the house and I did find some directivity but not a lot. Using some ladder line and a tuner, I was also able to get good signals on receive on all of the other amateur bands.
Meanwhile I have developed a ” one coil ” version which according to my ” kitchen ” experiments seems to give stronger signals. It is easier to build than the ” two coil ” version and shows more bandwidth. I found 1,5 MHz on the 10-meter band. It is easy to get a low SWR with only one trimmer and another advantage is that this antenna can be tuned over several MHz only by using the trimmer. It is important to know that you can also make a one coil version by connecting the two bottom plates with the coil. The feeding system is the same as shown in the drawing and remains unchanged. Strangely, the two top plates are totally unconnected. Another advantage is that the coil can have a lower inductance value.
Conclusion at this Stage
After some initial problems tuning the antenna, my Bi-Plane seems to perform at least as well as any other antenna that I have ever made. A lot more experimenting needs to be done to understand more about this antenna, its radiation pattern and ways to further improve its performance. But, I am convinced that there is scope and I hereby congratulate George with his new concept and would like to invite others to share his and my experience of excitement so that this little beauty will not disappear under the dust of time.
George Continues to Bake Ideas and Test
George from his side has reported similar results and finds S values close to or as strong as for his MFJ vertical dipole. A two-meter version of the Bi-Plane, affectionately called the “Butterfly ” came to within 2-3 dB of the readings of his J-Pole antenna. These comparisons were done at the same height as the reference antennas.
More optimization is possible in the way we construct the antenna (hardware) and with the manner of tuning with or without trimmers or variable coils. More can be done to find the real optimum size of the capacitor plates and the optimum L/C ratio. We can look at the advantages of making a remotely controlled variable coil or capacitor tuning system. George’s findings after trying just some of the different articulating orientations of his capacitors are shown in the photos and sketches below.
George provided some sketches of various positions of the capacitor plates and the resulting relative field strength as originally measured, however, George wants us to know that he believed the coax feed line was not isolated well enough for the first readings to be accurate. So, George took more readings with the coax isolated and found the readings to be not quite as strong, but still showed the increases and signal directions as indicated by the sketches. More “tinkering” needs to be done in this area to make the most use of the articulation for directivity and gains.
In that regard, George’s subsequent “tinkering” came up with this: “…I remembered that the capacity of two plates is controlled by the area of the smallest. So I put my standard biplane on the test stand. The FS read about 80. I attached a plate of +/- equal area to the bottom of one wing with clothespins. The reading went up to +/-90. Then I attached another equal area to the bottom of the other wing. It pegged the needle…a little over 100! This afternoon I will make a biplane with the bottom wings double the area of the top, put it on the roof and see how it does versus the MFJ vertical….” Stayed tuned folks as more results will follow in future issues of antenneX! Now, let’s look at those sketches of George’s:
The circular arrows above and below the vertical antenna indicate that the antenna sketched just above was turned 360 degrees with a constant reading that’s inside the circle. The readings in this configuration were the weakest however.
One could also make higher value capacitors and hence smaller coils by using some dielectricum between the capacitor plates. Capacitors are supposed to be low loss elements so increasing their value should lower the loss by using smaller coils etc., etc.
Finally it would be of great interest if someone skilled in the art of antenna modeling would take up the challenge of analyzing this type of antenna.
An interesting aspect of this design is that each “petal” capacitor plate is able to articulate therefore changing the dynamics for vertical and horizontal polarization and perhaps the directivity. George is getting higher FS readings already with even more changes! Come on experimenters. Join in on the fun. 1000 heads are better than one or two!
Originally posted on the AntennaX Online Magazine by George F. Sharp, KC5MU
Last Updated : 23rd May 2024