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#1
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3/2 wave dipole
Probably my first HF antenna as a novice was a 40/15 meter dipole
where 40 meters was 1/2 wave and 15 was 3/2 wave. I have never had a problem dropping SWR on 15 meters to 1:1.1 with this antenna center fed. However, as I understand antenna radiation on a wire, the optimum point should be somewhere else along the wire to match the impedance properly. I suspect it should be close to 1/3 of the way from one end. Any thoughts about this? -- 73 for now Buck N4PGW |
#2
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Buck wrote:
Probably my first HF antenna as a novice was a 40/15 meter dipole where 40 meters was 1/2 wave and 15 was 3/2 wave. I have never had a problem dropping SWR on 15 meters to 1:1.1 with this antenna center fed. However, as I understand antenna radiation on a wire, the optimum point should be somewhere else along the wire to match the impedance properly. I suspect it should be close to 1/3 of the way from one end. Any thoughts about this? Think about it. There would be three current maximum points on a 3/2WL wire. These three current maximum points correspond to the low impedance feedpoints on the antenna. They would be at 1/6, 1/2, & 5/6 from the ends. 1/3 from the end of a 3/2WL would be a high impedance point, i.e. not a good feedpoint. -- 73, Cecil, W5DXP ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#3
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Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire." Yes, and I believe they occur 1/4-weave back from the open-circuit ends of the antenna and at the very center of the antenna. Best regards, Richard Harrison, KB5WZI |
#4
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"Richard Harrison" wrote in message ... Cecil, W5DXP wrote: "There would be three current maximum points on a 3/2WL wire." Yes, and I believe they occur 1/4-weave back from the open-circuit ends of the antenna and at the very center of the antenna. Best regards, Richard Harrison, KB5WZI That's how my math works too Dale W4OP |
#5
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Dale Parfitt wrote: "Richard Harrison" wrote in message ... Cecil, W5DXP wrote: "There would be three current maximum points on a 3/2WL wire." Yes, and I believe they occur 1/4-weave back from the open-circuit ends of the antenna and at the very center of the antenna. Best regards, Richard Harrison, KB5WZI That's how my math works too Even less generally, one could think of the feedpoint in terms of 1/12-wave, 1/4-wave, or 1/2-wave from the end, depending on frequency. But I'd be inclined to use 1/6 or 1/2 of the total length for fear of brain fartage. ac6xg |
#6
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Dale Parfitt wrote:
"Richard Harrison" wrote: Cecil, W5DXP wrote: "There would be three current maximum points on a 3/2WL wire." Yes, and I believe they occur 1/4-weave back from the open-circuit ends of the antenna and at the very center of the antenna. That's how my math works too Remember I said the current maximums would occur at 1/6, 1/2 (3/6), and 5/6 points? In a 3/2WL antenna, 1/6 is 1/4WL from the end. -- 73, Cecil, W5DXP ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#7
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#8
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Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and building. While they look attractive on a simulation program (where the feedline wasn't also modeled), he found that in practice it was very hard to keep the feedline common mode current down to a low value. The consequence of common mode current is that the feedline becomes part of the antenna, and that makes it very difficult to duplicate or to realize the idealized performance you might expect if you hadn't taken feedline radiation into account. Multiple current baluns should be able to reduce the common mode current to a reasonable value, but they'd have to be used unless you want a lot of feedline radiation and performance that's difficult to predict. Roy Lewallen, W7EL Buck wrote: On Wed, 20 Apr 2005 23:21:29 -0500, (Richard Harrison) wrote: Cecil, W5DXP wrote: "There would be three current maximum points on a 3/2WL wire." Yes, and I believe they occur 1/4-weave back from the open-circuit ends of the antenna and at the very center of the antenna. Best regards, Richard Harrison, KB5WZI Thanks, That's what I was looking for. so I can use the antenna with 1/4 - 3/4 or center loaded. Would the off-center setting create another resonant frequency for the antenna? Assuming the antenna were cut for 1/2 wave at 7 MHz, it would operate on 21 MHz, but if off-center fed, would it also resonate on 20 MHz with the 1/4 wave element? Thanks. |
#9
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A bit more on the baluns Roy mentioned...
If the feedpoint is a high impedance (an even number of half-waves back from both ends of the antenna), it's difficult to get enough choke impedance to do much good if you put a single balun right at the feedpoint. It may end up being overkill, but if you don't have a good way to measure your line for antenna currents, you'll do well to put chokes about a quarter wave apart at the highest operating frequency. You want to avoid having a length of line between two chokes being a resonant half-wave long without anything in the middle to break up the resonance. So for operation on multiple bands covering a wide range, you should think about putting in three or more chokes. Of course, if you put it up without any chokes and it does what you want, that's fine...just don't expect it will match the model of an isolated wire. Cheers, Tom |
#10
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On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote: Some years ago, John Belrose VE2CV published an article in QST about off-center fed antennas, based on a combination of modeling and building. While they look attractive on a simulation program (where the feedline wasn't also modeled), he found that in practice it was very hard to keep the feedline common mode current down to a low value. The consequence of common mode current is that the feedline becomes part of the antenna, and that makes it very difficult to duplicate or to realize the idealized performance you might expect if you hadn't taken feedline radiation into account. Multiple current baluns should be able to reduce the common mode current to a reasonable value, but they'd have to be used unless you want a lot of feedline radiation and performance that's difficult to predict. Roy Lewallen, W7EL A few years back I put up an OCFD. Feed it with open line. Seemed to work pretty well and behaved itself running less than about a 100-watts. However, when I increased my power to above 500-watts I was able to work every electrical and electronic appliance in the house! The common mode on the transmission can be a killer! Danny, K6MHE |
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