Performance Characteristics of Trap Antennas
I have been curious to know what frequency the traps in a commercial antenna such as the Hy-Gain AVQ and Hustler BTV antennas are tuned to. I have a spare Hy-Gain 12AVQ on hand at the moment that is not in service, so I disconnected the traps and checked them with a grid dip meter. The 10M trap was tuned to 27 MHz and the 15M trap was at about 20 MHz. This was good news. According to W8JI, traps should always be tuned outside the band of operation to achieve minimum loss. Apparently they can be tuned either above or below the band of interest. He claims that even traps with modest Q exhibit low loss when they are tuned outside the band you're operating on. When you do this of course, the antenna elements need to be adjusted accordingly.
Apparently, advertising claims about "lossy traps" are largely hype. If they are tuned correctly, traps aren't all that lossy. Traps made out of coaxial cable are an exception, and this is because they have much lower Q than lumped element traps. But they are popular with hams because they are inexpensive and easy to make. They will work satisfactorily if they are tuned outside the band in use.
Even if the traps don't add a lot of loss when they're performing their isolation duties, there are still caveats to be aware of. Trap antennas are physically shorter than their non-trap brethren, because traps act like loading coils above their resonant frequencies. For example, in the 12AVQ vertical, the 10M trap adds loading on 15 and 20M. The 15M trap only adds loading on 20M. It's not so bad because those "loading coils" are in the middle and upper part of the antenna, which is the most efficient place for inductive loading. Because of the traps, the antenna only stands about 13' tall, whereas a full sized one (without traps) would be 16-17' tall.
The loading provided by the traps does introduce a small amount of loss, and of course it shortens the antenna. A secondary effect is a reduction in bandwidth, but this is usually not a problem on the higher HF bands because the antenna is still relatively close to being a physical quarter wavelength. Where we see a much larger effect is on the antennas that use a 40M trap or a loading coil at the top to achieve resonance on 75/80M. These antennas typically have a bandwidth of only 30-100 kHz, and outside this range performance drops very rapidly.
I have one actual data point to compare a full sized 10M quarter wave to the 12AVQ using our club's beacon as the signal source. It isn't super accurate, but the 12AVQ appeared to be down about 1/2 S-unit in comparison to the full sized antenna. I would like to re-do this test in the spring, but I will have to take down all of the elevated quarter wave radials to run this test. That's why I'm not going to do it until warmer weather has returned. That test was done with both antennas at ground level over the same ground system, one at a time. Obviously, the comparison wasn't made in real time. I'd like to redo the test with both antennas using elevated radials. I am very curious to know how they will compare. I suspect that part of the reason the 12AVQ was down slightly compared to the quarter wave was due to a combination of ground clutter and ground system losses. With both antennas elevated for the test, I would expect these losses to diminish and perhaps both antennas will perform very similarly. When ground losses are more or less eliminated, any losses from the antenna itself are also reduced. I remember some excellent articles by Jerry Sevick many years ago on this very topic.
I'm always interested in these matters because I think often we are misled by advertising claims that are made to promote a certain type of antenna. Trap verticals certainly will perform poorly if they are installed over a lossy ground system, but I think if they are elevated and ground losses are reduced to the point where they are not significant, trap antennas can work very well. They are very convenient and useful in many situations.
Apparently, advertising claims about "lossy traps" are largely hype. If they are tuned correctly, traps aren't all that lossy. Traps made out of coaxial cable are an exception, and this is because they have much lower Q than lumped element traps. But they are popular with hams because they are inexpensive and easy to make. They will work satisfactorily if they are tuned outside the band in use.
Even if the traps don't add a lot of loss when they're performing their isolation duties, there are still caveats to be aware of. Trap antennas are physically shorter than their non-trap brethren, because traps act like loading coils above their resonant frequencies. For example, in the 12AVQ vertical, the 10M trap adds loading on 15 and 20M. The 15M trap only adds loading on 20M. It's not so bad because those "loading coils" are in the middle and upper part of the antenna, which is the most efficient place for inductive loading. Because of the traps, the antenna only stands about 13' tall, whereas a full sized one (without traps) would be 16-17' tall.
The loading provided by the traps does introduce a small amount of loss, and of course it shortens the antenna. A secondary effect is a reduction in bandwidth, but this is usually not a problem on the higher HF bands because the antenna is still relatively close to being a physical quarter wavelength. Where we see a much larger effect is on the antennas that use a 40M trap or a loading coil at the top to achieve resonance on 75/80M. These antennas typically have a bandwidth of only 30-100 kHz, and outside this range performance drops very rapidly.
I have one actual data point to compare a full sized 10M quarter wave to the 12AVQ using our club's beacon as the signal source. It isn't super accurate, but the 12AVQ appeared to be down about 1/2 S-unit in comparison to the full sized antenna. I would like to re-do this test in the spring, but I will have to take down all of the elevated quarter wave radials to run this test. That's why I'm not going to do it until warmer weather has returned. That test was done with both antennas at ground level over the same ground system, one at a time. Obviously, the comparison wasn't made in real time. I'd like to redo the test with both antennas using elevated radials. I am very curious to know how they will compare. I suspect that part of the reason the 12AVQ was down slightly compared to the quarter wave was due to a combination of ground clutter and ground system losses. With both antennas elevated for the test, I would expect these losses to diminish and perhaps both antennas will perform very similarly. When ground losses are more or less eliminated, any losses from the antenna itself are also reduced. I remember some excellent articles by Jerry Sevick many years ago on this very topic.
I'm always interested in these matters because I think often we are misled by advertising claims that are made to promote a certain type of antenna. Trap verticals certainly will perform poorly if they are installed over a lossy ground system, but I think if they are elevated and ground losses are reduced to the point where they are not significant, trap antennas can work very well. They are very convenient and useful in many situations.