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November 8th 14, 07:03 PM
posted to rec.radio.amateur.antenna
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A short 160M antenna - folded elements
John S wrote:
On 11/8/2014 10:45 AM, wrote: John S wrote: On 11/7/2014 12:58 PM, wrote: wrote: snip The only downside to this antenna is that it is extremely narrow banded, only about a kHz or so. snip I realized I should expand on that. With all 5 inductors the same value the 5:1 bandwidth is about 500 Hz. By staggering the values of the inductors in the four legs the bandwidth can be improved by a little bit. The best I could accomplish was about 1 Khz by making the leg values .96, .98, 1.02, and 1.04 times the central leg value. Going beyond a step factor of .02 made little difference in the bandwidth and the resonant frequency SWR started to increase. Okay, but the starting target was to be able to feed a short antenna with good efficiency and I think you hit that target. I know you want to keep it as practical as possible, but I am impressed with your results. Thanks. The whole point of the exercise was to show there are way to overcome the generally low impedance of short antennas. Exactly! It was a challenge which has been shown to be surmountable by design. Feed losses become less of a burden this way. Do you think your results could be practical? Could ground resistance be used to widen the BW? I know, there are losses. But, maybe worth it? What do you think? I think if the goal is a practical antenna, the starting point should be how high can you practically go keeping in mind that a 1/4 wave 160M is on the order of 130 feet and in general the higher the greater the bandwidth and the less you have to be concerned with minimizing losses. In my urban lot, anything over about 30 feet becomes a problem. I do have a 33 foot tall vertical in the back yard with an autotuner at the base. It started out as just a 40M vertical. With the addition of the autotuner, it will tune and load 160 through 6 M. The performance on 6M is horrible as it is a cloud warmer at that frequency, but most of the other bands are OK or better. The 160 and 80 performance was poor, which I attibuted to losses in the tuner, so I put in a relay controlled high Q tapped coil to take some of the burden off of the autotuner on those bands. That helped quite a bit. I have been thinking about using the folded monopole technique to further improve things. That would require some more relays to switch the folded parts into the main radiator, essentially making it a fat radiator on other bands. The biggest issue is mechanical so until I figure out that part, I have left that project on the back burner for now. -- Jim Pennino |
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