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Author Topic: OCF80 with 600 ohm ladder line  (Read 3470 times)
VA3KBC
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« on: June 20, 2012, 01:43:40 PM »

Hidee ho good neighbours!!
Well I've looked through my ARRL antenna book and can't find anything on this subject, so here goes.

I want to put up an OCF80 meter dipole with 600 ohm ladder line coming down to a Array Solutions 4 : 1 balun and continues with 50ohm coax to my DX Engineering antenna switcher.
122' for 3.83mhz.
One length 40.66' and the longer length is 81.32'

Is there any reason other than the impedance may be an issue?
I could just get a 4-1 balun at the dipole, but I have the Balun that is for 600 ohm connection to do this project.

Thanks
73
VA3KBC
DJ/Don
« Last Edit: June 20, 2012, 01:52:41 PM by VA3KBC » Logged
KB3FFH
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« Reply #1 on: June 20, 2012, 04:21:45 PM »

Hi Don,
           From what I have learned the balun ratio depends on the antenna height. 4 to 1 balun when antenna is mounted up to 50 feet. Higher than that use a 6 to 1 both are current baluns. Look at this site---www.packetradio.com/windom.htm   you will learn alot. Bill kb3ffh
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WB6BYU
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« Reply #2 on: June 20, 2012, 05:31:10 PM »

I'd connect the balun right at the antenna, unless you want the feedline to be radiating
along with the antenna.  If you really want to use some 600 ohm line, put a few inches
of it between the antenna and the balun.

Although, even with coax feed, it may still be, depending on the type of balun you use.

Generally the reason for using an OCFD in the first place is to get reasonably low SWR
on multiple bands using 50 ohm coax.  Any significant length of 600 ohm line will likely
throw that off on at least some bands.

And then there is the problem that the feedline is balanced but the antenna isn't (because
the wires are different lengths).  So you would really want a balun between the antenna
and the 600 ohm line to keep the feedline balanced.


There actually used to be such an antenna described in the ARRL Handbook, using 300 ohm
twinlead.  And that was my first ham antenna strung on the roof of my house for my
novice transmitter (which was designed for a balanced output.)   I managed to work 4
stations in the first 4 months of operation, in spite of whole pages of unanswered calls
in my logbook:  I might have been running 20 watts input, but probably less than a watt
output.  Ham radio certainly didn't seem very fun!  So I'm rather hesitant to recommend
such an antenna to anyone unless they have a good understanding of how it works and
why they are choosing to use the 600 ohm line.
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VA3KBC
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« Reply #3 on: June 20, 2012, 06:34:51 PM »

I think I know what I'm going to do.
I'm going to put it up as a regular 75 meter dipole cut for 3.80 around 123'.
I can run about 55' down to the balun.
The reason I wanted to make it an OCF was because of the existing length of the coax I have. Well DAH!!
If I use ladder line I will have enough coax to run to the antenna switcher.
Anyways...I've got it figured out now!! hi hi..

73
VA3KBC
DJ/Don
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KH6AQ
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« Reply #4 on: June 21, 2012, 11:20:15 AM »

Let's say the dipole presents an input impedance of 50 ohms. The 55' of 600 ohm line is just about 90 degrees and will transform this down to 4 ohms. That's not what you want to feed into a 4:1 balun.

It would be better to run 50 ohm coax to a 1:1 balun. The 55' run of coax will have some common-mode current and will radiate but that's not so bad.
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WB6BYU
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« Reply #5 on: June 21, 2012, 11:52:06 AM »

Quote from: WX7G

Let's say the dipole presents an input impedance of 50 ohms. The 55' of 600 ohm line is just about 90 degrees and will transform this down to 4 ohms...



I think you divided instead of multiplied.  Or vice versa...

If the SWR is 12 : 1 on the 600 ohm line (as it would be with a 50 ohm load), the
impedance at the end of a quarter wave section would be 12 * 600 = 7200 ohms
rather than 50 / 12 = 4 ohms.

Which would indeed be a better match using a 4 : 1 balun, but still a rather high
SWR for coax feed.


If you are trying to solve the problem of how to feed a 132' wire when your coax
isn't long enough to reach and you have to use a quarter wave of 600 ohm line,
then you could try a Zepp feed or a Delta Match (which can match 600 ohms, and
a 4 : 1 balun at the base would give a 3 : 1 SWR on the coax.)  Both approaches
would still have a high SWR on the coax on most other HF bands.
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KH6AQ
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« Reply #6 on: June 22, 2012, 06:56:38 AM »

BYU, you're right! In the middle of night I woke up and realized my mistake.

To make this work the ladder line would have to around 100 ohms. Then it will transform the impedance to around 200 ohms. Balanced line made of two RG-58 coaxial cables would fit the bill. Or twisted pair line would work.
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W5DXP
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« Reply #7 on: June 22, 2012, 11:36:25 AM »

BYU, you're right! In the middle of night I woke up and realized my mistake.

The original mistake was assuming a 50 ohm feedpoint impedance for an OCF dipole. Smiley
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