On 30/10/14 20:44, Ian Jackson wrote:
In message , Wayne
writes


"Ian Jackson" wrote in message
...

In message , Wayne
writes


"Frank Turner-Smith G3VKI" wrote in message
...

On 30/10/14 14:04, Ian Jackson wrote:
In message , Frank Turner-Smith G3VKI
writes
On 30/10/14 08:47, Ian Jackson wrote:
In message , Frank Turner-Smith
G3VKI
writes
In a full wave dipole the voltage at both ends will always be in
phase,

Are you sure? Think on't!

so I would expect to see a very high impedance at the feed point.

Correct.

As you point out, matching the full-wave could be difficult and very
lossy.

Double zepp?

OK, what did I miss? In a full wave dipole, at the instant the voltage
at one end is peak positive, the voltage at the other end will also be
peak positive. Similarly, at the feed point, both legs would be at
peak negative and no current would flow in the feeder, hence the high
impedance. There would be a current flowing in each leg of the dipole,
but the currents would be in anti-phase. Where have I got it wrong? Do
I need another drink?

Maybe I need a drink too. However, all dipoles/doublets have to fed
'push-pull', so when one leg goes +ve, the other leg goes -ve. The
voltage at all points along the antenna that are equidistant from the
feedpoint will be in antiphase, so if the feedpoint is in the centre,
the voltages at the ends will be in antiphase. [Or is my thinking
seriously muddled?]

# Looks like I owe you a pint. You've described the situation where a TX
# is feeding the dipole. I was trying to visualise the RX conditions,
but
# it reciprocates. One of us has to be wrong, and I strongly suspect
it's
# me. Time for a drink.

With drinking involved, I must throw in my 2 cents.

I'd go with Frank....for full wave assume positive peak at one end,
negative peak in the middle, and positive peak at the other end. (or
vice versa)

But, I suppose I should think about it a little more.....Laphroig
would help

# See:
# http://tinyurl.com/q8nxqep
# ten rows of images down, second from left:

# This shows the amplitude and the polarity of the voltage and current
for
# a halfwave dipole. [Lots of diagrams only show the amplitude.] You will
# see that the polarities on each leg are +ve and -ve. For a fullwave,
# just imagine it continuing on for another halfwave each side.

# -- # Ian

Isn't that figure for a full wave?... lambda

Maybe you're looking at the wrong one. I've had another look, and it's
now 9 down, far left. It's the one with the thick black dipole, entitled
"Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig 1):
http://www.digikey.com/en/articles/t...standing-anten
na-specifications-and-operation

The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it
doesn't seem very likely.

--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

In message , Frank Turner-Smith G3VKI
writes
On 30/10/14 20:44, Ian Jackson wrote:
In message , Wayne
writes


"Ian Jackson" wrote in message
...

In message , Wayne
writes


"Frank Turner-Smith G3VKI" wrote in message
...

On 30/10/14 14:04, Ian Jackson wrote:
In message , Frank Turner-Smith G3VKI
writes
On 30/10/14 08:47, Ian Jackson wrote:
In message , Frank Turner-Smith
G3VKI
writes
In a full wave dipole the voltage at both ends will always be in
phase,

Are you sure? Think on't!

so I would expect to see a very high impedance at the feed point.

Correct.

As you point out, matching the full-wave could be difficult and very
lossy.

Double zepp?

OK, what did I miss? In a full wave dipole, at the instant the voltage
at one end is peak positive, the voltage at the other end will also be
peak positive. Similarly, at the feed point, both legs would be at
peak negative and no current would flow in the feeder, hence the high
impedance. There would be a current flowing in each leg of the dipole,
but the currents would be in anti-phase. Where have I got it wrong? Do
I need another drink?

Maybe I need a drink too. However, all dipoles/doublets have to fed
'push-pull', so when one leg goes +ve, the other leg goes -ve. The
voltage at all points along the antenna that are equidistant from the
feedpoint will be in antiphase, so if the feedpoint is in the centre,
the voltages at the ends will be in antiphase. [Or is my thinking
seriously muddled?]

# Looks like I owe you a pint. You've described the situation where a TX
# is feeding the dipole. I was trying to visualise the RX conditions,
but
# it reciprocates. One of us has to be wrong, and I strongly suspect
it's
# me. Time for a drink.

With drinking involved, I must throw in my 2 cents.

I'd go with Frank....for full wave assume positive peak at one end,
negative peak in the middle, and positive peak at the other end. (or
vice versa)

But, I suppose I should think about it a little more.....Laphroig
would help

# See:
# http://tinyurl.com/q8nxqep
# ten rows of images down, second from left:

# This shows the amplitude and the polarity of the voltage and current
for
# a halfwave dipole. [Lots of diagrams only show the amplitude.] You will
# see that the polarities on each leg are +ve and -ve. For a fullwave,
# just imagine it continuing on for another halfwave each side.

# -- # Ian

Isn't that figure for a full wave?... lambda

Maybe you're looking at the wrong one. I've had another look, and it's
now 9 down, far left. It's the one with the thick black dipole, entitled
"Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig 1):
http://www.digikey.com/en/articles/t...standing-anten
na-specifications-and-operation

The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it
doesn't seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.



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In message , Ralph
Mowery writes

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

On that point, you'll have to ask the experts!


--
Ian

On 30/10/14 22:26, Ian Jackson wrote:
In message , Ralph
Mowery writes

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it
doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

On that point, you'll have to ask the experts!


The gain would be at the cost of a very narrow front lobe. You'd need a
big rotator.

--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

In message , Frank Turner-Smith G3VKI
writes
On 30/10/14 22:26, Ian Jackson wrote:
In message , Ralph
Mowery writes

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it
doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

On that point, you'll have to ask the experts!


The gain would be at the cost of a very narrow front lobe. You'd need a
big rotator.

In the UK, would you be wanting to rotate it for TV? Don't forget that
one type of antenna used in the very early days of BBC TV (Channel 1,
vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole,
with the short leg being a quarterwave, and attached as high as possible
(maybe to a chimney or a gutter). The other leg was an odd number of
quarterwaves, and attached much lower down. As a result, the antenna had
one of its major lobes sort-of off the end (say 30 degrees off the
wire), in a more-or-less horizontal direction, and responding well to
vertically polarized signals.
--
Ian

"Ian Jackson" wrote in message In
the UK, would you be wanting to rotate it for TV? Don't forget that
one type of antenna used in the very early days of BBC TV (Channel 1,
vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole,
with the short leg being a quarterwave, and attached as high as possible
(maybe to a chimney or a gutter). The other leg was an odd number of
quarterwaves, and attached much lower down. As a result, the antenna had
one of its major lobes sort-of off the end (say 30 degrees off the wire),
in a more-or-less horizontal direction, and responding well to vertically
polarized signals.
--
Ian

I have not kept up with TV signals for a long time. In the US they started
off as all horizontal. I think that some may have gone to circular, but not
sure. It might be the FM stations I am thinking about. Not sure what they
are using now on the digital signals.

What are they using in other countries ? Horizontal, vertical ?



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Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

EZNEC handles it just fine.

I modeled a 120' (I had forgotten the exact number posted) long wire
at 6' over real ground at 600 Mhz:

Impedance: 55-j308
Max vertical gain: 21 dBi at 4 degrees
Horizontal gain: two 21 dBi lobes at +/- 4 degrees
Front/back: 9.5 dB

LOTS of little lobes...



--
Jim Pennino

On 31/10/14 01:17, wrote:
Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

EZNEC handles it just fine.

I modeled a 120' (I had forgotten the exact number posted) long wire
at 6' over real ground at 600 Mhz:

Impedance: 55-j308
Max vertical gain: 21 dBi at 4 degrees
Horizontal gain: two 21 dBi lobes at +/- 4 degrees
Front/back: 9.5 dB

LOTS of little lobes...

Interesting, and I assume the -j308 is due to the capacitance between
the wire and ground. Since we are discussing a 50cm wavelength I would
imagine a change of only a cm or so in the overall length of the wire
would cause a significant change in impedance.
Another problem could be finding a big enough plot of land facing in the
right direction.


--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

Frank Turner-Smith G3VKI wrote:
On 31/10/14 01:17, wrote:
Ralph Mowery wrote:

"Ian Jackson" wrote in message
...
The original proposal in this thread was that long antennas performed
better than short ones. If that was true you'd get a good 600MHz UHF TV
picture using a 132ft end fed longwire. I've not tried it, but it doesn't
seem very likely.

A 132' endfed will have one hell of a gain on 600MHz - but it will be
almost straight off the ends.
--
Ian
I know the gain will be off the end of the wire, but still wonder if an
antenna that long (in wavelengths) will actually work or will it be too
long and the gain does not meet the expectations or if programs like NEC
will predict it or fall apart.

EZNEC handles it just fine.

I modeled a 120' (I had forgotten the exact number posted) long wire
at 6' over real ground at 600 Mhz:

Impedance: 55-j308
Max vertical gain: 21 dBi at 4 degrees
Horizontal gain: two 21 dBi lobes at +/- 4 degrees
Front/back: 9.5 dB

LOTS of little lobes...

Interesting, and I assume the -j308 is due to the capacitance between
the wire and ground. Since we are discussing a 50cm wavelength I would
imagine a change of only a cm or so in the overall length of the wire
would cause a significant change in impedance.
Another problem could be finding a big enough plot of land facing in the
right direction.

I did a little playinng around...

Changing the length from 120' to 132' has little effect on the pattern
as this is on the order of 80 wavelenths.

As the wavelength is so short, the impedance varies greatly with small
changes in length and the reactive part is heavily influenced by the
height above ground and the quality of the ground.

So in addition to finding enough land, you would also have to keep it
always wet or always dry otherwise you would be constantly retuning
as the ground moisture changed.

However, if one lived right on a beach and set up a series of floats
across the water...

Then your only problem is waves causing changes in height.


--
Jim Pennino