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Author Topic: Shortened OCFD fed with ladder line feed and balun possible?  (Read 445 times)
KW4CQ
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Posts: 264




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« on: December 03, 2017, 05:52:52 AM »

I have been successfully using my 133 foot OCFD at 60 feet on the 10m-80m bands.  I would like to modify it adding a 160m band capability.  I dont have the room for 270 foot dipole and just barely enough for the 133 foot OCFD.  I believe I read somewhere that one can build a 117 foot OCFD (LOA) and feed it with ladder line and a balun to get the added 160m band capability. Does anyone have construction experience or knowledge of an actual design along these lines you can share with me? 

Mega-thanks and 73,
Bob KW4CQ
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WB6BYU
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Posts: 17192




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« Reply #1 on: December 03, 2017, 08:06:33 AM »

My first transmitting antenna was an OCFD fed with twinlead, as shown in the ARRL Handbook.
I could never get it to work properly, so I cringed when I read your question.

Now, I have to give that antenna credit - it got me started learning about antennas, to the point
where my Handbook broke apart at the antenna chapter by the time I upgraded from Novice.
And I'm sure with better equipment (and a better understanding of impedance matching, etc.)
that I might have different results today.  But, needless to say, I remain biased.

I'd probably try feeding the current OCFD plus the feedline as a vertical on 160m.  For example,
at the point where the current coax feedline drops to the ground, you can add a switch (or a
temporary fixture for testing) to either feed it in the normal manner, or using the shield as an
inverted L.  How it works will depend on the characteristics of your balun, and the quality of
your ground system will also make a difference.

It may be possible to use ladder line to a balun at ground level, but I'm skeptical that the line
would actually be balanced.

A half-sized loaded dipole works fairly well on 160m.  You might be able to build a 160m element
to connect in parallel with the current OCFD, but that's not trivial to do without messing up the
impedances on the original antenna.
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KU3X
Member

Posts: 435




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« Reply #2 on: December 03, 2017, 08:26:30 AM »

Here we go again.
Never feed an OCF antenna with ladder line. It's an unbalanced antenna and the balanced ladder line WILL RADIATE. There is no advantage of feeding it with ladder line. Feed it in the center.  Now you can use it on numerous bands.

Barry
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PA1ZP
Member

Posts: 611




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« Reply #3 on: December 03, 2017, 10:44:05 AM »

Hi

I go with Barry.
An OCFD is unbalanced , do not try to feed it witha a balanced line, it will give troubles.
Feed it in the center as a balanced doublet and tune it with a balanced tuner.

Or feed it at one end and make a zepp antenna out of it , but that would be a zepp only on 80 mtrs maybe usefull on 40 as a full wave zepp.

But an OCFD can not be fed with a balanced line very easy, as the antenna is very unbalanced by its design.
I know there are ways it can be done, but that would be rather inefficient and expensive to do it.

73 Jos
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WB6BYU
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Posts: 17192




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« Reply #4 on: December 03, 2017, 07:55:13 PM »

Here is an antenna description from G8JNJ that combines a doublet, OCFD, and "L" or "T" antenna for various bands.
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NO9E
Member

Posts: 720




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« Reply #5 on: December 04, 2017, 09:45:13 AM »

I fed a 80m OCFD on 80m via a tuner. It worked very well, until the balun shorted.
Lost of different antenna designs, nearly all aiming at low SWR. But efficiency is way important.

Recently I compared 3 antennas on 160m using WSPR. These were:
1. A vertical to a 100ft tree with extra 30ft sloping down. 8 70 ft elevated radials.
2. W9INN dipole at 40ft with coils.
3. 80m doublet at 60 ft fed by ladderline.

The vertical was best. W9INN was about -10db down. The doublet was mostly only 1-2 db down, including DX!

Perhaps your best bet is having a doublet fed by a laddderline with a tuner. MFJ-998 is pretty good matching at high power.

Ignacy, NO9E

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