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Author Topic: HF antenna ideas using 300 ohm TV twinlead  (Read 2449 times)
KB1RDL
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Posts: 47




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« on: June 22, 2009, 10:50:13 AM »

I recently got a whole bunch (several hundred ft) of TV 300 ohm twinlead for almost nothing.

Wondering, what kind of use it may have on the HF bands, 40-10 m?  I'm itching to put it to good use.

I'm thinking about terminated antennas, like T2FD or folded dipole.  Something that could fit within ~66 ft of hanging space (currently have a half wave end fed wire antenna hung as a sloper).

Or can something end-fed be done, like a ZEPP?

Don't care what the feed impedance is too much  - winding baluns/or matching directly with 300 ohm antenna tuner input is no problem for me.

Looking for "tribal knowledge" here.

73 de KB1RDL
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WB2WIK
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Posts: 20536




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« Reply #1 on: June 22, 2009, 10:57:36 AM »

You could make a 40m folded dipole; or an EFZ, which works on its primary resonant frequency and also odd harmonics thereof.  Higher-Z line like 450-600 Ohms is better, but 300 Ohms will work.

Is it high quality stuff?  Some TV twin lead is very good (especially if it's really "old" stuff) but a lot of the newer stuff is so frail it will hardly withstand outdoor use.  What's your stuff like?
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VE3XDB
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Posts: 139




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« Reply #2 on: June 22, 2009, 11:02:06 AM »

How about a fan dipole?  Try building a dipole, tuned up for the desired frequency on 40 meters.  Cut it long to start, and trim to resonance.  Then, figure out the ratio of the length to frequency.  Using that ratio, cut the bottom radiator at the appropriate spot, leaving a 1" gap along the length of the twin lead.  

If you cut it for 40 and 20 meters, you should also be able to work 15 meters.  

Have fun!

Best regards,

Doug VE3XDB
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KB1RDL
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Posts: 47




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« Reply #3 on: June 22, 2009, 11:06:30 AM »

Yeah it appears to be pretty sturdy.  The plastic is brown non-translucent kind, no foam, unlike the cheapo folded FM dipoles are made out of.
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KZ1X
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Posts: 3227




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« Reply #4 on: June 22, 2009, 12:57:17 PM »

I have a folded dipole as you describe, made from the parts kit DX Engineering sells for this very purpose.

<http://www.dxengineering.com/Parts.asp?ID=1853&PLID=158&SecID=77&DeptID=40&PartNo=DXE-UWA-KIT>
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WA3SKN
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Posts: 5419




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« Reply #5 on: June 22, 2009, 01:25:19 PM »

I am not a big fan of twinlead, but I do believe in using what you already have and in experimenting, so go ahead and string something up!
You can parallel the elements and make a "broadband dipole", you can make a folded dipole, you can make an end fed Zepp, and don't forget the fan dipole multibands!  With 66 feet you can do 40 meters and up without much trouble.
Start building!
73s.

-Mike.
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KB1RDL
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Posts: 47




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« Reply #6 on: June 22, 2009, 01:59:58 PM »

Fan dipole sounds like the best idea so far.

Thanks!

KB1RDL
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WB6BYU
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Posts: 12973




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« Reply #7 on: June 22, 2009, 04:47:13 PM »

I've build HF J-poles using twinlead as well as using it
as a feeder for doublets, loops, Windoms and end-fed Zepps.  
As long as you provide sufficient mechanical relief so the
wires don't break, it works well.  (Sometimes I've been
in a position where that was the only feedline available
to me.)

For a single band antenna a folded dipole made from, and
fed with, the twinlead can be matched to 50 ohms by
the addition of a shunt capacitor across the twinlead
a specified distance above the transmitter end.  I'd have
to go look up the numbers.

But except for my 15m J-pole I generally wouldn't choose
twinlead for the actual antenna wire, unless you didn't
have other wire available.  The T2FD is relatively poor
in regards to efficiency, and regular folded dipoles
are not a good choice for multi-band use, particularly
on the second harmonic.

I've often used parallel dipoles on a common feedpoint,
but except in specific circumstances I prefer to use
separate wires tied off to different support points as
the wires are much easier to tune due to reduced
interaction between them.

But you could use twinlead to feed a TRUE fan dipole:
cut 10 wires each 40' long and connect 5 of them to
each side of the feedline.  Spread the wires on each side
out into a fan over an angle of about 20 to 30 degrees.
You should have a reasonably low SWR on the twinlead
from 7 to 30 MHz continuously.  (Commercial antennas
of this type claim a 2.5 : 1 SWR from 3 to 30 MHz.)
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