Welcome to the Great Antenna Shootout!
Have you ever wondered how well your antenna works? Have you wondered if you have the best antenna? Well, welcome to the K9EQ/K0ZE antenna shootout. Presented here are the results of two antenna construction projects. As it turns out, both antennas work quite well with plenty of on-the-air experience to back up the numerical models.
In general, the antennas were designed to match the following criteria:
Ken chose to build an 80 meter horizontal loop antenna. The loop antenna is particularly well suited to operation with an antenna tuner since the feed point impedance is typically low and the feed point is balanced. (Note that this loop is completely closed.) It was shown using modeling that a loop of almost any size would work with a tuner like the AH-4. Ken decided to take the project a bit farther and design the loop to work well on most bands without a tuner, thus allowing the use of high power. The disadvantage of this approach is that the antenna gets a bit large - it does use 80 meters of wire! However, for those without so much space, a smaller antenna can be built and fed with the AH-4 with good performance.
Putting up an 80 meter loop is too much work! Chris picked out two trees and strung wire between them. The antenna is symmetrical about the feed point. 450 ohm ladder line connects the antenna to the AH-4. While Chris must use the AH-4 (or similar tuner), he has the advantage of needing only two mounting points - a great portable antenna. The length is not critical. A camping special was built that was 56 feet long and it worked quite well.
A note on using baluns. Folks have noticed that the feedline of my antenna is directly connect to the AH-4. Yes, it works! I have used the antenna with and without a balun. With the balun I cannot operate on 6 and 160 meters. This is a limitation of the balun's design. Other than that, I have not observed any difference in the operation of the antenna. Note that the AH-4 is not grounded locally - the ground is on the far end of 100 feet of coax. In effect I let the AH-4 "float" at the feed point of the antenna. This works because the antenna is balanced.
If you use a balun, be sure to use the right one. Some "current mode" baluns I have examined are built using 50 ohm coax - not a good idea if we want to match a wide range of impedances. I suggest winding your own just so you know how it is built. I hope to have some balun construction information in the future.
So there you go. Take a look at the two reports by clicking on the links below. You'll need Acrobat Reader to view them. Note the SWR curves at the end of the K0ZE loop report. At the end of K9EQ's report, you'll find some comparisons with other types of antennas. Enjoy, and let us know what you think.
K0ZE 80 Meter Loop
K9EQ Flat TopWhat is a "Flat Top"?
It is simply an antenna that is flat on top - like a dipole. In this case I use a center fed dipole and 450 ohm ladder line to connect the dipole to the tuner.
I have two configurations of the flat top that I have used. My "camping special" is 17 meters (56 feet) long connected to an arbitrary length of 450 ohm ladder line. 17 meters was chosen such that the SWR on the ladder line (450 ohms) would never exceed 10:1 on any amateur frequency between 80 and 6 meters. The antenna can be made smaller, if necessary. The nice thing about the AH-4, is that you can select an antenna of just about any convenient length. Avoid lengths where then antenna becomes one wavelength long an any frequency you intend to use.
The other configuration, is one I plan to write more about later. This antenna will tune several amateur bands without the AH-4, and all bands between 80-6 meters with it. It will, in fact, tune parts of 160 meters with the AH-4!
This antenna is like the flat top described above, except that the ladder line terminates into 50 ohm coax. While the ladder line on the camping special can be any length, the ladder line for this antenna must be a specific length since it is being used to transform the impedance of the antenna down to 50 ohms.
Theory says the antenna should be 28.9 meters (94 feet) long and the ladder line should be 12.9 meters (42 feet) long. My actual antenna works out a bit better at 26.7 meters (87 feet) and 12.6 meters (41 feet) respectively. This antenna tunes 3.5 mHz (SWR about 2.5:1), 7.1, 14.1, 18.2, 21.1, 25.0, and 28.6 with SWRs below 1.5 to 1.
I found it best to tune the dipole part of the antenna first. Adjust it so that your SWR is a minimum around 7.0-7.1 mHz. Then slowly shorten the ladder line until the SWR reaches a minimum around 28.5 mHz. Both adjustments interact with each other, but in general changes to the ladder line have a much greater impact as the frequency increases. I have not used a balun.
How it works
Since the impedance of the antenna is never 450 ohms, standing waves will exist on the ladder line. We don't worry too much about this as the ladder line, unlike coax, has low loss under these conditions. Since there are standing waves on the ladder line, the impedance of the signal will vary depending upon where you are on the line. We just happen to cut the line so that the impedance is 50 ohms at the end where we attach the coax.
The impedance of the dipole is different on the different bands. The ladder line is also a different electrical length on these bands. By carefully selecting the length of the dipole and the length of the ladder line, it is theoretically possible to get the transformation to produce a 50 ohm impedance on 4 amateur bands. Real-life use shows that you may be able to use the antenna on even more bands.
If you happen to build one of these antennas, let me know how it goes. If you have any hints for others, I can pass them along.