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#11
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Horizontal loop antenna
I'm missing something here. I don't see anywhere in the response which
explains how "radiation is a function of loop area" and why increasing the loop circumference would be advantageous. Richard Harrison wrote: Roy Lewallen, W7EL wrote: "Can you please explain that a little more? (Radiation is a function of loop area.)" Not being a typist, I`ll refer you to "TV And Other Receiving Antennas" by Arnold Bailey. On pages 407 and 408 Bailey gives two formulas for computing the antenna resistance for a loop antenna. On page 408, Bailey has Fig. 8-14 which plots radiation resistance (the stuff we build antennas for) versus the loop perimeter in wavelengths. For a square closed loop of one wavelength perimeter, the graph indicates about 50 ohms. Perhaps you build antennas for radiation resistance, but I think most people build them to maximize radiation in some direction. In any case, please exclude me from the "we" in your statement about what "we" build antennas for. You said "radiation is a function of loop area" and recommended a larger circumference. Equations are useless unless you understand how to apply them. So can you use those equations to show us how much more a loop of 800 foot circumference will radiate than one with a 400 foot circumference? You can assume 100 watts applied to each, and either zero wire resistance or any reasonable value. Bill Orr, W6SAI in "All About Cubical Quad Antennas" gives the full-wave vertical loop antenna an impedance of 125 ohms on page 15. On page 14, Orr writes: "For purposes of illustration, the two wire folded dipole may be "pulled open" to a diamond-shaped loop fed at the bottom point. If this distortion of the loop is continued the antenna will become a shorted transmission line." A perfect circle is the geometric shape enclosing the most area for a given perimeter. The more corners a closed figure has, the more closely it usually approximates a circle. That is why I commented on an octagon versus a square. Which brings me back to the question I asked, Is the radiation pattern the same for a long skinny loop as for a round one, as long as the enclosed area is the same? Roy Lewallen, W7EL |
#12
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Horizontal loop antenna
Roy Lewallen wrote:
I'm missing something here. I don't see anywhere in the response which explains how "radiation is a function of loop area" and why increasing the loop circumference would be advantageous. What you seem to be missing is that Richard H. said absolutely nothing about radiated *power*. The radiation *pattern* is certainly a function of loop area. -- 73, Cecil http://www.w5dxp.com |
#13
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Horizontal loop antenna
On Mon, 18 Feb 2008 19:31:00 -0800, Roy Lewallen
wrote: Putting the balun the input has the disadvantage of making the tuner chassis hot. Roy, Thanks for ANOTHER lesson. I have a SGC-230 tuner in the RV that I use to tune a wire when we go camping. I think the metal mounting brackets are isolated from the floating tuner chassis and that you have to use a grounding lug on the bottom. Never verified that since I have always used the ground lug anyway. I was thinking of adding ferrites to the input side of the tuner and then using it to tune a dipole fed with 450 ohm window line. Maybe I'll just stick with the longwire. I noticed the guy who wrote that article mounted his tuner on an el cheapo cutting board, probably just for that reason. S.T.W. |
#14
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Horizontal loop antenna
Sum Ting Wong wrote:
I have a SGC-230 tuner in the RV ... I was thinking of adding ferrites to the input side of the tuner and then using it to tune a dipole fed with 450 ohm window line. My SG-230 tuner manual says that is a no-no. It says: "The coupler must be located *at the antenna*. Never use a feed line or coaxial cable at the output of the antenna coupler." -- 73, Cecil http://www.w5dxp.com |
#15
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Horizontal loop antenna
On Tue, 19 Feb 2008 23:18:14 GMT, Cecil Moore
wrote: Sum Ting Wong wrote: I have a SGC-230 tuner in the RV ... I was thinking of adding ferrites to the input side of the tuner and then using it to tune a dipole fed with 450 ohm window line. My SG-230 tuner manual says that is a no-no. It says: "The coupler must be located *at the antenna*. Never use a feed line or coaxial cable at the output of the antenna coupler." Cecil, Saw that and wondered about isolating the coax and DC power with the ferrite and maybe changing the rules about doing that. Since you're a whole lot sharper than I am on this stuff, what do you figure the reason is for not tuning a balanced feeder with it? That would make things sooooooooo much easier if it worked. Thanks. S.T.W. |
#16
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Horizontal loop antenna
I have a SGC-230 tuner in the RV ... I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a dipole fed with 450 ohm window line. My SG-230 tuner manual says that is a no-no. It says: "The coupler must be located *at the antenna*. Never use a feed line or coaxial cable at the output of the antenna coupler." On the other hand, page 16 of the SGC-230 manual PDF specifically shows a balanced dipole, being fed through "balanced line feeders, 300-600 ohms, up to 40 feet". The ladder line is shown as being connected to the antenna "hot" terminal, and to the case of the coupler. The manual is quite clear about not using a coax feedline above the antenna, or having a feedline out to a longwire antenna which has more than a very small amount of capacitance to ground. The information shown on page 16 suggests that you may be able to bend this restriction if you're using a balanced feedline, although I suspect that you'll still want to make sure that you've got low capacitance between ground and the hot side of the line (keep the feedline well away from metal, and use high-quality feedthrough insulators). I don't know about using a balun at the base of the feedline... this will (almost of necessity) require the use of a length of low-impedance feedline of some sort, and this could be exactly the sort of thing that prevents the SGC from matching the line properly. Using a choke on the coax-and-control side might work better, although this too seems to be something that SGC discourages. I haven't been terribly happy with the SGC tuner I picked up (admittedly for cheap, at a ham swap-meet). It's an early-model SGC 230, and its tuning algorithm seems very finicky. In particular, it won't tune into any but the easiest loads when fed by my Kenwood TS-2000. I suspect that the reason is that the TS-2000's high-SWR foldback circuit is rather aggressive, and chops the power down to well under 10 watts anytime the rig sees an SWR of more than 2:1 or so. The fluctuating transmitter power seems to confuse the coupler's matching algorithm... no big surprise there. Seems to work better when fed from a simple TenTec Scout, which has a different high-SWR- protection mechanism that doesn't cause the power to flop around as much. Anyhow... my impression is that the SGC tuners are best suited for their original application - feeding a whip or longwire, whilst being securely bolted and multiply-grounded to a Big Metal Vehicle Chassis. They also seem to do OK feeding verticals over a big bed of radials. I just don't think they're all that well suited to feeding balanced antennas. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#17
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Horizontal loop antenna
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#18
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Horizontal loop antenna
Sum Ting Wong wrote:
On Mon, 18 Feb 2008 19:31:00 -0800, Roy Lewallen wrote: Putting the balun the input has the disadvantage of making the tuner chassis hot. Roy, Thanks for ANOTHER lesson. I have a SGC-230 tuner in the RV that I use to tune a wire when we go camping. I think the metal mounting brackets are isolated from the floating tuner chassis and that you have to use a grounding lug on the bottom. Never verified that since I have always used the ground lug anyway. I was thinking of adding ferrites to the input side of the tuner and then using it to tune a dipole fed with 450 ohm window line. Maybe I'll just stick with the longwire. I noticed the guy who wrote that article mounted his tuner on an el cheapo cutting board, probably just for that reason. S.T.W. And you can also thank Tom Rauch, W8JI who questioned my incorrect belief, causing me to re-think it. Roy Lewallen, W7EL |
#19
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Horizontal loop antenna - Dual Wire
I seem to remember an article about someone taking a full-wave
horizontal loop antenna and making two loops (or was it a half-wave loop?). The second loop caused the antenna to match 50 ohms so it could be fed with coax and (I think) it operated multiple bands. Anyone remember this article and know where it might be? Thanks Buck N4PGW -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
#20
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Horizontal loop antenna
Sum Ting Wong wrote:
Saw that and wondered about isolating the coax and DC power with the ferrite and maybe changing the rules about doing that. Since you're a whole lot sharper than I am on this stuff, what do you figure the reason is for not tuning a balanced feeder with it? I was so sharp, I called SGC. :-) They said if the transmission line accidentally opened up, the SG-230 could develop 30,000 volts at its output. If it was mounted in an attic, for instance, it could burn the house down. I think they are worried about being sued. Moral: If you decide to use transmission line on the output, put it in a location that will not matter if it catches on fire. -- 73, Cecil http://www.w5dxp.com |
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