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A short 160M antenna - folded elements
In the last installment we got the input impdance up to the order of
2 Ohms with loading and a top hat. 2 Ohms will still lead to lots of loss in the matching device, so let's see if the input impedance can be further increased. Those familiar with the folded dipole are aware that the input impedance of a folded dipole is higher than a dipole, so let's try a folded monopole and see what happens. I started with a single folded element spaced at 2 feet from the central radiator that drops down and connects to a radial. That brought the impedance up to about 4 Ohms. If a little is good, more must be better so I added a second folded element and got about 12 Ohms. Going for the gusto, I then added 2 more folded elements for a total of 4 as well as adding a loading inductor into all elements starting at the 50% level and let the optimizer have at it to optimize the inductance and inductor height for SWR and gain. When it finished, here is what I got: Impedance: 41.32 -9.5 Ohms SWR: 1.3:1 gain: 1.1 dBi @ 25 degrees Inductors: 787 uH So here we have a 0.029 wavelength antenna that is only about 2 dB down from a full 0.25 antenna and can be fed directly from a 50 Ohm transmitter without any matching device losses. As an aside, the differences between zero resistance and copper were insignificant as one would expect with this input impedance. The only downside to this antenna is that it is extremely narrow banded, only about a kHz or so. Next I added the 7.5' top hat radials and let the optimizer go at it once more achieving: Impedance: 58.83 -6.1 Ohms SWR: 1.5:1 gain: 1.5 dBi @ 25 degrees Inductors: 577 uH Not a significant decrease in the inductance as with a single element and the change in bandwidth was trivial. In this case I don't think the top hat is worth the bother. The practical issues with this antenna is getting 5 adjustable inductors so the antenna is usable over a broader range and as the current in the inductors is a bit high, they would have to be good adjustable inductors. At this point I think the notion that "short antennas are inefficient" can be regarded as nonsense and the choice boils down to bandwith/size, pick one. Next up, a 160M rubber ducky, but only after attending to some roof leaks revealed by recent rain. -- Jim Pennino |
A short 160M antenna - folded elements
wrote in message ... In the last installment we got the input impdance up to the order of 2 Ohms with loading and a top hat. 2 Ohms will still lead to lots of loss in the matching device, so let's see if the input impedance can be further increased. Those familiar with the folded dipole are aware that the input impedance of a folded dipole is higher than a dipole, so let's try a folded monopole and see what happens. I started with a single folded element spaced at 2 feet from the central radiator that drops down and connects to a radial. That brought the impedance up to about 4 Ohms. If a little is good, more must be better so I added a second folded element and got about 12 Ohms. Going for the gusto, I then added 2 more folded elements for a total of 4 as well as adding a loading inductor into all elements starting at the 50% level and let the optimizer have at it to optimize the inductance and inductor height for SWR and gain. When it finished, here is what I got: Impedance: 41.32 -9.5 Ohms SWR: 1.3:1 gain: 1.1 dBi @ 25 degrees Inductors: 787 uH So here we have a 0.029 wavelength antenna that is only about 2 dB down from a full 0.25 antenna and can be fed directly from a 50 Ohm transmitter without any matching device losses. As an aside, the differences between zero resistance and copper were insignificant as one would expect with this input impedance. The only downside to this antenna is that it is extremely narrow banded, only about a kHz or so. Next I added the 7.5' top hat radials and let the optimizer go at it once more achieving: Impedance: 58.83 -6.1 Ohms SWR: 1.5:1 gain: 1.5 dBi @ 25 degrees Inductors: 577 uH Not a significant decrease in the inductance as with a single element and the change in bandwidth was trivial. In this case I don't think the top hat is worth the bother. The practical issues with this antenna is getting 5 adjustable inductors so the antenna is usable over a broader range and as the current in the inductors is a bit high, they would have to be good adjustable inductors. At this point I think the notion that "short antennas are inefficient" can be regarded as nonsense and the choice boils down to bandwith/size, pick one. Next up, a 160M rubber ducky, but only after attending to some roof leaks revealed by recent rain. -- Jim Pennino %%%%%%%%%%%% Interesting. I think I follow the design. Could you increase the bandwidth by tuning the various paralleled parts to slightly different frequencies? |
A short 160M antenna - folded elements
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A short 160M antenna - folded elements
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A short 160M antenna - folded elements
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A short 160M antenna - folded elements
Wayne wrote:
"John S" wrote in message ... snip I'll comment, just to see if I'm envisioning his configuration properly. If a folded dipole is constructed, the feedpoint impedance can be increased by adding another parallel wire, forming a 3 wire folded dipole. A 5 wire paralleled folded dipole would have an even higher feedpoint impedance. So if I envision the configuration correctly, half of a 5 wire folded dipole is used to construct a 5 wire folded vertical monopole. The antenna is then shortened (but resonated) by putting a loading coil in each of the 5 wires. Yep, that's it. Total coil losses are lower because only 1/5 of the current flows through each, giving each a resistance loss of (1/5*I)^2*R. Is that it? I would have to go back to the model to see the relation of the central current to the current in the four legs. -- Jim Pennino |
A short 160M antenna - folded elements
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. |
A short 160M antenna - folded elements
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. I did notice the resident gas bag has nothing to say about posts containing numbers. It appears numbers are his kyptonite. -- Jim Pennino |
A short 160M antenna - folded elements
On Fri, 7 Nov 2014 01:08:33 -0000, wrote:
In the last installment we got the input impdance up to the order of 2 Ohms with loading and a top hat. 2 Ohms will still lead to lots of loss in the matching device, so let's see if the input impedance can be further increased. Idea: There are times when matching to 50 ohms isn't the best approach. I had that problem once and decided that the solution was to move the power amplifier stage to the antenna. With an emitter follower power output stage, I was able to get source impedances of less than 1 ohms fairly easily. The base of the emitter follower ended up about 100 ohms, which was easily matched to 50 ohms with a transformer. The big problem was keeping the emitter followers stable. The usual ferrite bead on the emitter leads helped, but did not provide unconditional stability throughout the range of antenna tuning. I never did stabilize the amplifier mostly because this was new to me at the time and I didn't have decent models of the components involved. Another problem was the high currents required that everything be big, silver brazed, silver soldered, and expensive. Using silver plated tubing seems to work best. I was playing with milliohms and spent much time with a Kelvin bridge measuring tiny resistances. The worst problem was that the antenna also had to handle the high currents, which required heavy construction, thick plating, and lots of silver. Most of the current flows in roughly the first 3 * skin depth. At 1.8MHz, that would be about 0.15 mm. I didn't think the world was ready for silver plated antennas, but it was worth trying. Ever see a lock washer glow red? That was fun. Fortunately, it was a symmetrical antenna so I didn't need to deal with high RF currents through a counterpoise (i.e. grounding system). Using cut-n-try and copious overtime, I managed to deliver a prototype that mostly worked. The fatal flaw was that the customer wasn't too thrilled with having a fairly complex system split between two locations (transmitter building and tower shack) and supply high currents via garden hose size cables between sites. When lightning hit nearby and blew up the output stage and most of the attached monitoring equipment, it was decided to use a more conventional design. I was not disappointed as I was seriously worried how I was going to make it work reliably. Fortunately, the split site might not be a major problem for ham use. For 160 meter operation, I think this can be a useful method if the power levels were kept low enough to prevent meltdown. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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