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300 ohm folded dipole from ARRL Handbook, early 1990's
On Wed, 15 Feb 2006 22:44:00 GMT, Cecil Moore wrote:
It might be interesting to some to explain how/why this works. .... Some thoughts on why it might not work to plan, though it should be close to plan and capable of fine adjustment if needed. Worth noting that the characteristic impedance of some lines is quite different to their market labelling, especially 300 ohm and 450 ohm lines. In this part of the world, nominal 300 ohm TV ribbon is more like 360. Wes measured a range of nominal 450 ohm lines to be from 360 to 400 ohms. Although the calculated answer might be a shunt capacitor of 147pF, in practice a close value would be used and both the dipole length and feedline length could be juggled for better match if needed. Alternatively, a shunt stub could be used and tuned along with the series line length. The feed point impedance of the dipole will depend on its environment to some extent, and may be reactive if formulas are used to cut it to length. Of course, it is a one-band matching arrangment. Owen -- |
#12
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300 ohm folded dipole from ARRL Handbook, early 1990's
"Geoffrey S. Mendelson" wrote in message ... Hi, the early '90s ARRL handbooks had construction information for a simple folded dipole made of 300 oHm twinlead. The article was not in the '96 edition or later. The antenna was made of 300 ohm twinlead. It was fed in the center by a 300 ohm twinlead feeder to a matching capacitor which made it aproximate 50 ohms. The capacitor could be either a regular capacitor (I used silver mica WWII surplus) or a "stub" of more twinlead. At the end of the feed line, which was part of the antenna, you could attach it directly to a 50 omh unbalanced transciever or a 50 ohm coax of any length. The length of the antenna was one half wavelength without the velocity factor and then shorted at the wavelength points compensating for the velocity factor. For example, if it was 10 meters long for a 20 meter dipole, the shorts were about 4 meters from the center for twinlead with a velocity factor of 0.8. If anyone has a copy of the article and could scan it for me, I'd appreicate it, or even better yet, a pointer to an online version of it. Thanks in advance and 73, BTW, if you are also in Israel and asking yourself "where does he get the twinlead"? I don't. I brought it with me when I moved here in '96. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM IL Voice: (07)-7424-1667 IL Fax: 972-2-648-1443 U.S. Voice: 1-215-821-1838 Visit my 'blog at http://geoffstechno.livejournal.com/ I played a lot with a folded dipole for 14 MHz, both with twin lead and with two parallel wires about 20 cm apart. To feed it, I used a simple 75 ohms to 300 ohms Neosid transformer currently/formerly intended to connect a 75 ohms VHF television set to an antenna with twin lead. Depending on the country, such a bal/un is called a "symetriser", a "bazooka", a "pig's nose", etc... This small pig's nose can cope with 100 watts output on 14 MHz and higher. I never tried it on lower frequencies. I expect it would get rather warm on 80 m. |
#13
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300 ohm folded dipole from ARRL Handbook, early 1990's
Have you experience running 100w into one of these transformers
yourself? I kinda doubt they would take any watts. If I understand you correctly you are talking about the little unit that connects the cable to the old 300ohm input of 1970s type tv. These units have very small toroids, only maybe a quarter inch in diameter, that are designed for microvolts of excitation at the bottom end of TV band - which is somewhere near say 50Mhz. They will not handle any magnitizing flux and I would bet a watt at 20m would overheat it. regards, Bob N9NEO |
#14
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300 ohm folded dipole from ARRL Handbook, early 1990's
wrote in message
oups.com... Have you experience running 100w into one of these transformers yourself? I kinda doubt they would take any watts. If I understand you correctly you are talking about the little unit that connects the cable to the old 300ohm input of 1970s type tv. These units have very small toroids, only maybe a quarter inch in diameter, that are designed for microvolts of excitation at the bottom end of TV band - which is somewhere near say 50Mhz. They will not handle any magnitizing flux and I would bet a watt at 20m would overheat it. regards, Bob N9NEO Those VHF 75/300 ohms transformers where designed for the 48 to 300 MHz VHF TV-segment. The neosid core is not that small: it just covers my thumbnail. It indeed can handle 100 watts on 14 MHz. I tested it thoroughly: a friend had taken his transceiver to Costa Rica where he was visiting family and I wanted to raise an antenna very quickly to stay in touch with him. I suspended a folded dipole under the roof of my house and used the described VHF bal/un to feed it. We had (almost) daily skeds on 14 MHz. The "almost" was not due to propagation but had more to do with the fact that the electricity plant was only up 3 or 4 days a week in Costa Rica at that time. Later I used the same bal/un with a folded dipole for the 20 m band made with twin lead. I never used it on lower frequencies. I am sure that it would get pretty hot with 100 watts on 80 m. Kind regards, Phil |
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