300 ohm folded dipole
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| Feb 16th 2014, 20:15 | |
KD8VLYTotal Topics: 0 Total Posts: 0 |
I'm lacking practical knowledge building an attic folded dipole out of 300 ohm TV twin lead wire. I'm shooting for 14.175mhz center frequency so it is about 33' long with a 33' twin lead feed line. I am lucky enough to have a friend borrow me a MFJ 269 analyzer but I cannot get the swr under infinity anywhere near the design frequency. At first I trimmed and trimmed some more until I got something close to 2:1 swr. At that point the antenna was about 28' long. Being completely baffeled by this I patched the antenna back to 32' 9" and am starting over. I guess my questions how much faith should I place in my instruments? Is there another way to verify its readings? How much of a change should I get in the swr by trimming each end 3" or so? A sweep from 12 to 28 MHz gives me dips at: 12.151mhz almost infinity SWR. 300ohm 12.814mhz 3+ SWR. 200ohm 17.650mhz. 2.2 SWR. 28ohm 21.610mhz. almost infinity SWR. 380ohm 25mhz. 3+ 18ohm 73's to all |
| Feb 17th 2014, 14:07 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
You can expect an impedance around 300 ohms when a folded dipole is properly tuned--this is a 6:1 SWR. You may want to use a 4:1 balun to get the impedance down to 75 ohms for the antenna analyzer. As the impedance gets high, the analyzer loses accuracy. Zack Lau W1VT ARRL Senior Lab Engineer |
| Feb 24th 2014, 02:05 | |
KD8VLYTotal Topics: 0 Total Posts: 0 |
as it turned out was the i was reading the mfj269 meters not the LCD display. After advise on another forum I programmed the analyzer for a Z of 300 and things started making sense. The meters were still not displaying anything close but the display was telling the truth. Since I was using a tuner 2 I was not worried about taking the antenna impeadence down to 50 ohms. THANKS anyway. |
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