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300 ohm folded dipole

Feb 16th, 20:15

KD8VLY

Joined: Sep 1st 2013, 01:24
Total 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, 14:07

W1VT

Joined: Apr 4th 1998, 00:00
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, 02:05

KD8VLY

Joined: Sep 1st 2013, 01:24
Total 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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