300 ohm twinlead antenna.

[Geoffrey S. Mendelson]

Take a length of 300 ohm twinlead. Short the far end, End feed one side of
the close end, and leave the other side disconnected.

What do I have?

I know if I short both ends, I have a wide(er) bandwidth end fed random wire.

Is it still an end fed random wire with an electrical length of twice its
physical length?

Something completely different?

If for example, if I have 10 meters of space, and hang it, will I have
a 20 meter long random wire, which would sort of be a 40 meter band
1/2 wave end fed antenna? Or would I have an odd 10 meter long end
fed antenna (for 20m?)

Can it be switched between the two by adding a shorting switch at the close
end? Open it would be for 40m, and closed for 20?

TIA,

Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
It's Spring here in Jerusalem!!!

[W5DXP]

On Thursday, June 6, 2013 8:34:04 AM UTC-5, Geoffrey S. Mendelson wrote:
> What do I have?

http://en.wikipedia.org/wiki/Folded_unipole_antenna
--
73, Cecil, w5dxp.com

[Channel Jumper]

Get yourself a ARRL antenna book




--
Channel Jumper

[Ian Jackson]

In message <cf2bd581-e2b3-4872...@googlegroups.com>,
W5DXP <w5...@hotmail.com> writes

I think what Geoff is describing is not the usual folded unipole (twin
feeder shorted at the top, and at the bottom, one side fed from the TX,
and the other side grounded and/or connected to radials), but rather
twin feeder where the top end is shorted, and at the bottom, one side is
fed from the TX, and the other side is open circuit.
--
Ian

[W5DXP]

On Friday, June 7, 2013 4:42:13 PM UTC-5, Ian Jackson wrote:
> ... one side is fed from the TX, and the other side is open circuit.

The 1/4WL folded monopole is resonant with a feedpoint impedance around three times a 1/4WL single-wire monopole. Those two antennas have approximately the same gain.

If one disconnects the folded element's ground connection leaving an open-circuit, the feedpoint reactance becomes extremely high and the antenna loses about 6dB of gain compared to the standard 1/4WL monopole. It looks more like a 1/4WL shorted stub than an antenna.

[Ian Jackson]

In message <aedbf7fe-1f06-4283...@googlegroups.com>,
W5DXP <w5...@hotmail.com> writes

I thought something like that would happen. However, because of the
close coupling between them, you might instinctively think that the two
conductors would still simply act as a single conductor, and the feed
impedance would be low (more like a non-folded monopole).

Can't you make a trapped dipole (or whatever) using 300 twin for each
leg, but with sections of one conductor cut out so you form a succession
of 1/4WL shorted stubs (of various lengths) in series, acting as traps
on the various HF bands?
--
Ian

[Irv Finkleman]

You are thinking about the Lattin Antenna -- there's
lots of info available on the net for it. Instead of a dipole,
you could use one half the dipole as a vertical

Irv VE6BP


"Ian Jackson" <ianREMOVET...@g3ohx.demon.co.uk> wrote in message
news:K237GIDp...@g3ohx.demon.co.uk...

[Ian Jackson]

In message <JLIst.20260$6_4....@newsfe31.iad>, Irv Finkleman
<fin...@shaw.ca> writes

Yup. That's the one. Thanks.
--
Ian

[Irv Finkleman]

If the twin-lead portion is lambda/4 x Velocity factor,
and you add sufficient wire to the end to extend the
total length to lambda/4 you will find it gives a
reasonably good match to 52ohm coax. I built one
once. It went halfway up my tower and was then folded off at an angle and
the top end tied to to a fence.
It was cut for 75M and was a remarkable performer.

A chum of mine who DXes the low bands put one
up and really liked it. The antenna is described in Bill
Orr's book "All About Vertical Antennas".

Irv VE6BP

"Ian Jackson" <ianREMOVET...@g3ohx.demon.co.uk> wrote in message

news:eiw+zxT1...@g3ohx.demon.co.uk...

[tom]

I have several different revisions of it but have never run across what
Geoffrey is asking about. Could you please point to an example in the
ARRL Antenna Book? And please give the revision.

Thanks.

tom
K0TAR

[Wayne]

"tom" wrote in message news:E_mdnYdHWtasnEfM...@iphouse.net...

# I have several different revisions of it but have never run across what
# Geoffrey is asking about. Could you please point to an example in the
# ARRL Antenna Book? And please give the revision.

# Thanks.

# tom
# K0TAR

If it were a single wire, there would be a voltage node at the end. If it
is instead a length of twin lead with one wire fed and the far end of the
twin lead shorted, I can't envision what would change. One possibility is
no change, another is that the voltage node would be back at the starting
point, and of course, something else.

My initial thoughts were that the voltage node would be at the far end of
the twin lead, with the return wire having close coupling to the fed wire.

After a little head scratching, and having some spare time, I plugged an
experiment into EZNEC.

I characterized a 16.5 foot whip over real ground and got a resonance at
14.4 MHZ at 36 ohms.

With the 300 ohm line simulated with the second wire separated by 0.05 foot,
there were resonances at 10.7 MHZ (4.7 ohms) and 19.6 MHZ (63 ohms).

My conclusion: I didn't model it correctly :)

Wayne W5GIE

[Wayne]

"Wayne" wrote in message news:kremmu$hb5$1...@dont-email.me...

"tom" wrote in message news:E_mdnYdHWtasnEfM...@iphouse.net...

On 6/6/2013 4:18 PM, Channel Jumper wrote:
> Geoffrey S. Mendelson;805681 Wrote:
>> Take a length of 300 ohm twinlead. Short the far end, End feed one side
>> of

<snip>

OOOOOPS.....I've got a big error in the model.
Nevermind...I'll get back.

[Wayne]

"Wayne" wrote in message news:kren1l$j7p$1...@dont-email.me...

"Wayne" wrote in message news:kremmu$hb5$1...@dont-email.me...



"tom" wrote in message news:E_mdnYdHWtasnEfM...@iphouse.net...

On 6/6/2013 4:18 PM, Channel Jumper wrote:
> Geoffrey S. Mendelson;805681 Wrote:
>> Take a length of 300 ohm twinlead. Short the far end, End feed one side
>> of
<snip>

# OOOOOPS.....I've got a big error in the model.
# Nevermind...I'll get back.

OK, after correcting the error in my EZNEC model parameters, the results
were too weird to have significance.

[W5DXP]

On Monday, July 8, 2013 11:41:47 AM UTC-5, Wayne wrote:
> OK, after correcting the error in my EZNEC model parameters, the results
> were too weird to have significance.

Here's a zipped EZNEC file model of an 80m folded dipole.

http://www.w5dxp.com/80mFD.zip

[Geoffrey S. Mendelson]

Thanks, but my whole point was that it was not a folded dipole. It was an
end fed random (or 1/4 wavelength) wire connected to another 1/4 wavelength
wire at the far end, making it a 1/2 wavelength wire.

BUT since it is twinlead the 1/4 wavelength wires would be very tightly
capacitively coupled, which may between the capacitance and the poximity
make it not a 1/2 wavelength wire.

It was a thought to get another band out of a stretch of twinlead. Short the
far end. At the frequency it was 1/2 wavelength, use it as an end fed
wire with the near end shorted, and at half of that, use it as a 1/2
wavelength wire with the near end opened.

It does not seem that it will work that way.

[Wayne]

"W5DXP" wrote in message
news:bf5963e5-e203-4a18...@googlegroups.com...

On Monday, July 8, 2013 11:41:47 AM UTC-5, Wayne wrote:
> OK, after correcting the error in my EZNEC model parameters, the results
> were too weird to have significance.

# Here's a zipped EZNEC file model of an 80m folded dipole.

# http://www.w5dxp.com/80mFD.zip

Thanks. I'm filing that away for future reference.