Chicken Wire Delta Loop
3:1 SWR Bandwidth: Nowhere on the chart.
The measurements taken below were made with my MFJ-269 Antenna Analyzer. All measurements were taken at the end of the coax connected to the transmatch (which was located immediately beside the transceiver). The opposite end of the coax was terminated at the strange antenna being measured. Just before being connected to the antenna, 10-turns of coax were snuggly wrapped around a plastic jar to create a coax choke.
In each of the following charts, the little gray boxes adjacent to the 5:1 SWR line indicate the approximate location of the amateur radio bands (160- to 6-meters). This provides an impression of each antenna's performance from 1-MHz through 54-MHz.
Each chart displays the measured SWR curve. Below the chart the measured series resistance (Rs) and series reactance (Xs) are noted. Sometimes I also recorded the specific frequency at which these measurements were derived. If there is no such note you may assume the frequency was that printed above the notation.
Resistance (Rs)
The transceiver used requires a 50-Ohm non-reactive load to be seen at the transceiver's antenna output (SO-239) for it to transmit at full power, therefore an ideal antenna would present 50-Ohms Rs, and 0-Ohms Xs (reactance). Most antennas (even "real" ones) do not present this ideal load to the transceiver, so a transmatch is commonly placed between the antenna and transceiver. The transmatch (also called an antenna tuner, or just a tuner) removes any reactance, and adjusts the pure resistance presented to the transceiver in such a manner as to provide a nearly perfect 50-Ohm non-reactive load. (We used a CLC transmatch with a roller inductor - either the MFJ-969 or the MFJ-962D.)
Reactance (Xs)
Reactance is real energy, there is nothing imaginary about it. However, it is energy caught up in a loop and unable to perform any useful work (radiating a RF signal). Energy caught up in an electrostatic loop is capacitive reactance (XC), and energy caught up in an magnetic loop is inductive reactance (XL). The numerical value associated with it identifies its magnitude, measured in Ohms. Smaller magnitudes of reactance are desirable. When zero reactance is present, the antenna system is resonant. This does not always occur at the frequency of lowest SWR.
One of the best references to learn about this topic is "Reflections II" by Maxwell W2DU (available from World Radio). It is however, difficult reading if you are new to the subject. I consider this to be requited reading, although not for someone new to the topic. If you need an introduction to the subject, or would like to read more about Strange Antennas, you may prefer to start with my "K0S Field Manual" (PDF, about 5 MB).
In the charts below, reactance is noted after the resistance. For example, "69, 14" means Rs=69-Ohms and Xs=14-Ohms.
When I look at these charts I of course glance at the SWR curve. However, low SWR does not always a good antenna make, so if the data is available, always look at the resistance and reactance. Any series resistance between 25- and 75-Ohms ought to work very well, provided there is not a large amount of reactance present.
In other words, if Rs=65-Ohms and Xs=18-Ohms, this should be a pretty easy antenna to tune up and it should work pretty well. Much of the energy you deliver to the antenna should radiate as RF. On the other hand if you have the same 65-Ohms resistance, but with 200-Ohms reactance present quite a bit of your energy will be caught up in reactance ("loop") and therefore not available to radiate RF.
Is a low SWR always good?
Also be aware of ground losses. A standard 1/4 wave vertical is an example of this. If you only have a few ground radials you will see a very low SWR. If you increase your number of ground radials to 30-40 you will see your SWR rise. What is going on and which antenna will actually work better?
The low SWR is present with the smaller ground plane system in place because the earth is sucking up so much of your RF. Something like a giant dummy load. Dummy loads display a great SWR value, but they generally do not radiate very well as antennas. In fact, they are designed NOT to radiate RF, but to instead suck it all up while displaying a perfect match to your transceiver (and if successful a perfect 1:1 SWR).
As you add more and more ground radials your RF is being reflected better and better off the planet, and as a result your SWR measurement increases in value. The result is a higher SWR and a greater amount of RF being radiated from your antenna system.
Please keep this in mind as you look over the charts below or when you construct your own strange antenna (which is really a lot of fun).
3:1 SWR Bandwidth: Nowhere on the chart.
States Worked: AZ, CA, LA, MI, TX, WA
Number of QSOs / Contacts: 7
I tried to tune this antenna with the MFJ-969 on 20-meters but it would not tune up. When I switched to the MFJ-962D is easily tuned up. The roller inductors in each of these transmatches are the same but the 962D has much larger capacitors, and while the maximum capacitance is not extremely greater than found in the 969, the distance between the plates is much greater (lowering the chances of arcing) and the area of the plates is much greater. From an operator's standpoint, this made all the difference in the world!
Looking at the resistance we can see only the areas around 80-meters, and the high side of 10-meters offers a decent amount of resistance, and then the reactance is over 100-Ohms. There really is not anywhere in the amateur band where the chicken wire delta loop offers a very good match.
3:1 SWR Bandwidth: Nowhere in the amateur bands (39.866 ~ 48.000 MHz).
States Worked: MO, TX
Number of QSOs / Contacts: 2
This was the worse antenna used in May 2004.
We can see at 17-meters the SWR rockets upwards and become erratic. At a couple points it is greater than 31:1 SWR (beyond which my equipment will not measure). Note 14-MHz: Rs is a whopping 464-Ohms, and reactance is through the roof at 752-Ohms. This is going to be difficult to tune up! In fact, my MFJ-969 was unable to do so. The MFJ-962D was able to do so, but it was very difficult and very, very touchy (just tap a tuner knob and watch the meters go nuts).
With so much reactance present even if we can achieve a tune we can not expect great efficiency from the system. Losses in the coax will be very high, and since we are running 100-feet of coax this would present a significant loss of radiated power.
At 15-meters this antenna should operate pretty well. The other frequencies above this up to 6-meters there is either rather low resistance or fairly high reactance (although nothing like 750-Ohms).
We operated this loadlock dipole at 40-meters. There was not very much reactance here, but there was also very little resistance. We did not bring a field strength meter, but it would have been interesting to note the relative RF output at various frequencies.
3:1 SWR Bandwidth: 17-M through 6-M.
States Worked: Ontario (Canada), NM, NV
Number of QSOs / Contacts: 3
This has the best looking SWR curve of the strange antennas used May 2004 (the next closest in terms of SWR is the Chicken Wire Semi-Vertical), and it was very broad-banded. Once we were tuned up on 20-meters we were able to roam the entire band with ease and we never had to re-tune. Noting the series resistance values find 15-meters looks very good indeed! 52-Ohms resistance is perfect, and there is only 22-Ohms reactance present, which is certainly manageable (1.5 SWR at 21.300 MHz).
We didn't take these measurements until after using the truck dipole. We had been operating on 20-meters, which still is not too bad: 3.7 SWR, with 14-Ohms series resistance and 11-Ohms reactance, at 14.300 MHz. (Our power output was put to 62-watts due to a depleted battery.)
By the time we reach 40-meters the SWR is still looking good but the series resistance has fallen all the way down to 4-Ohms. At these longer wavelengths it is likely the ground loss (absorption by planet earth) is sucking up a lot of our RF energy.
Based upon the number seen above, I would expect the best performance to take place between 20- and the low-end of 10-meters, with 15-meters providing the very best opportunities to have QSO's. This is because these bands all provide a resistive match close to 50-Ohms, while displaying very little reactance.
3:1 SWR Bandwidth: 20-M through 6-M.
States Worked: CO, MI, NC, VA
Number of QSOs / Contacts: 5
This is a pretty good antenna, for a "strange" antenna. You can see it should have worked well on 6-meters (we called CQ here a number of times but never caught a band opening). 10-meters would have also been a good band for this antenna, but it too never opened. For both these bands you can see the series resistance runs between 37- and 46-Ohms. There is some reactance present, but not enough to cause any severe matching problems.
Reactance, in fact, is never really terribly high: 57-Ohms is the highest magnitude I measured. This loadlock vertical appears to be resonant very close to 17-meters where it displayed a 2:1 SWR.
Below 20-meters the resistance falls off quickly and obtaining a good signal should get more difficult. 20- through 6-meters this antenna should work pretty well. This is a very broad-banded antenna (for its 3:1 bandwidth).
3:1 SWR Bandwidth: 40-M, and 15-M through 6-M (30-17M just above 3:1).
States Worked: AL, AZ, CA, CO, FL, IL, IN, KS, KY, LA, MI, NC, NEB, NM, NY, TX, OR, PA, UT, VA, WA, WI, WY
Number of QSOs / Contacts: 46 (counting KJ4RB a second time after he switched to the shop light dipole)
The chicken wire semi-vertical shows a rather tame SWR graph. It is only the lower bands 80- and 160-meters where it begins to head upwards, and even then it is less than 10:1. There are quite a few bands where this displays very workable vales of resistance and reactance: 6-meters, through 17-meters, and it is especially good on 40-meters. However, on either side of 40 it either displays high reactance or low resistance.
I would expect to be able to easily tune this up all the way from 6-meters down to 40-meters, and expect to have it work pretty well throughout this entire range.
The chicken wire "tube" from which the semi-vertical element was fashioned was 25-feet long and very broad-banded due to its roughly 1-foot cross-section. Once it was tuned on a band we were able to roam the band quite easily. This is certainly a plus.
Another factor in this antennas favor was its ground plane, the neighborhood fence. Both this antenna and the loadlock vertical used this fence as their ground plane and I believe it provided a very good foundation from which the radiating element could work.
For comparison the following two charts show the measurements taken for a commercial grade vertical antenna designed for 11-meters and converted to a 10-meter antenna.
3:1 SWR Bandwidth: 15-M, 12-M, and 10-M.
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73 de Erik, nØew
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