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ARRL Antenna Book OCF dipole & 4nec2

Apr 10th, 17:08

gordyburdo

Joined: Nov 29th 2013, 18:14
Total Topics: 0
Total Posts: 0
I entered the parameters of the OCF dipole listed in the latest Antenna Book and ran it on 4nec2 modeling software, using specified lengths (45.3' & 84.7'), a height of 66', sloping to 33' and 14 gauge antenna wire. At an impedance of 200 ohms it gives high swrs on all the bands. Any suggestions?
Thanks, Gordy - WA2RUP
Apr 10th, 18:30

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Using basic trigonometry, I calculate the following coordinates
(x, y, z)
(0, 0, 33ft) end one
(0, 125.7ft, 66ft) end two

3.7MHz 105.9+j38.66
7.5MHz 138.1+j30.08
15MHz 176.9-j26.62

#14 wire, 51 segments, fed 33% from end 1.

Zack Lau W1VT
ARRL Senior Lab Engineer
Apr 10th, 19:06

gordyburdo

Joined: Nov 29th 2013, 18:14
Total Topics: 0
Total Posts: 0
Thanks, Zack. I put in the numbers you gave me and the lowest SWR I could find was 2.5 at 3.9, 6.0 at 7.3, and 6.0 at 14.3.
I've tried all sorts of numbers without much luck and haven't seen any coordinates that give a good swr at 200 or 300 ohms.
Gordy WA2RUP
Apr 10th, 19:42

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

You might try extending the length of the antenna to 136 feet.
EZNEC gives the following SWRs
3.5MHz 2.06
7.1MHz 1.48
14.3MHz 1.3

In practice, the SWR of an OCFD is highly dependent on feedline orientation--unlike a center fed dipole, short of eliminating the feedline altogether (tiny QRP rig and a battery), there is no practical way of orienting it so that it doesn't affect the SWR by coupling more to one side of the dipole than the other.

Zack Lau W1VT
ARRL Senior Lab Engineer
Apr 15th, 17:16

gordyburdo

Joined: Nov 29th 2013, 18:14
Total Topics: 0
Total Posts: 0
Zack,
That got me close. When I set the length to 137' and changed from inverted V to horiz. dipole at 60' I got great results. Again, thanks for your help.
Gordy - WA2RUP

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