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.

On 31/10/14 00:44, Ian Jackson wrote:
In message , Ralph
Mowery writes

"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 ?

Historically, both. The very first UK TV transmitter (45MHz), launched
in 1936, was vertical. This closed down swiftly on 1 September, 1939,
and WW2 started two days later.

After WW2, TV resumed in 1946. Four more channel frequencies (all BBC
only in those days) were added throughout the country in the Low Band
(Band 1, as we call it), and the split of vertical and horizontal was
around 50/50. In 1954, the independent network (ITV) arrived, all in the
High Band (Band 3) - again with around a 50/50 split.

In 1963 (?) UHF was launched - initially carrying only a second BBC
channel, on 625-lines. Eventually, this expanded to four - and in some
places - five channels, all transmissions being co-sited - or almost
co-sited (so that only one, fixed antenna was required). Without
exception, all high(er) power analogue transmitters were horizontal, and
almost without exception, all the low(er)-power fill-in relay stations
were vertical. Although these days it's all digital, same pertains -
except for (I think) one new fairly high power directional vertical
transmitter.

BTW, the old 405-line VHF network was totally closed down in the 1980s.
Although VHF is no longer used for TV, part of the Band 3 allocation is
now digital radio - all vertical. FM was originally all horizontal, but
gradually the benefits of circular - and the simpler mixed -
polarizations became implemented. Very few are now purely horizontal.

Other European countries have had a somewhat different history, but I'm
pretty sure that very few TV transmitters were/are vertical - except for
local fill-in and low power. For FM, the Irish Republic has always
(sensibly) used vertical.

VHF FM radio was indeed horizontally polarised at first, and was aimed
at fixed receivers with rooftop aerials. I believe horizontal
polarisation (HP) was first chosen in an attempt to provide greater
coverage than vertical polarisation (VP). AIUI, over a distance from the
TX, a VP transmission twists toward HP as it "grazes" the curvature of
the Earth. In doing so energy gets absorbed and the signal is
attenuated. The arrival of FM car radios meant a growing number of
listeners were using vertical antennas so a change to slant or circular
polarisation was introduced.

The convention for UHF TV stations in the UK was for high powered "main
stations" to use HP and low power relay stations to use VP. This cross
polarisation provided about 26dB protection against co-channel interference.

--
;-)
..
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 in
:

This cross
polarisation provided about 26dB protection against co-channel
interference.


That's a useful figure. I asked a few weeks ago about the prosects of wiring
an external vertical dipole for FM VFH broadcasts, via a MAR6 based
amplifier boosting by maybe 20dB, to an internal horizontal dipole to
overcome local digital hash from nearby flats that gets in to degrade the
signal from a portable radio with a telescopic whip. As it is the SNR rather
than the raw strength which is an issue, an ideal situation would be to allow
thwe whip to be in its resting horizontal, cotracted position, while still
allowing clear use of radios carried around the flat while I work.

Various possible problems have been discussed, and I haven't pushed for this
with a trial, but if 26 or more dB are cut in the difference between antenna
based on polarisation, and the amp boosts only by 20dB, it seems that
feedback can be avoided, AND also the risk of interference to other FM VHF
radios in other flats. (Which might even benefit, if my own would.)

This is the first time anyone's mentioned a figure for isaolation (for want
of a better word) between similar dipoles based on 90° difference in
orientation, so I'm taking this moment to reopen the subject in passing...

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

On 31/10/14 18:19, Frank Turner-Smith G3VKI wrote:
VHF FM radio was indeed horizontally polarised at first, and was aimed
at fixed receivers with rooftop aerials. I believe horizontal
polarisation (HP) was first chosen in an attempt to provide greater
coverage than vertical polarisation (VP). AIUI, over a distance from the
TX, a VP transmission twists toward HP as it "grazes" the curvature of
the Earth. In doing so energy gets absorbed and the signal is
attenuated. The arrival of FM car radios meant a growing number of
listeners were using vertical antennas so a change to slant or circular
polarisation was introduced.

http://www.bbc.co.uk/rd/publications/rdreport_1960_16

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

On 31/10/14 18:40, Lostgallifreyan wrote:
Frank Turner-Smith G3VKI wrote in
:

This cross
polarisation provided about 26dB protection against co-channel
interference.


That's a useful figure. I asked a few weeks ago about the prospects of wiring
an external vertical dipole for FM VHF broadcasts, via a MAR6 based
amplifier boosting by maybe 20dB, to an internal horizontal dipole to
overcome local digital hash from nearby flats that gets in to degrade the
signal from a portable radio with a telescopic whip. As it is the SNR rather
than the raw strength which is an issue, an ideal situation would be to allow
the whip to be in its resting horizontal, contracted position, while still
allowing clear use of radios carried around the flat while I work.

Various possible problems have been discussed, and I haven't pushed for this
with a trial, but if 26 or more dB are cut in the difference between antenna
based on polarisation, and the amp boosts only by 20dB, it seems that
feedback can be avoided, AND also the risk of interference to other FM VHF
radios in other flats. (Which might even benefit, if my own would.)

This is the first time anyone's mentioned a figure for isolation (for want
of a better word) between similar dipoles based on 90° difference in
orientation, so I'm taking this moment to reopen the subject in passing...

26dB cross-polarisation protection was the "rule of thumb" figure in use
by the BBC service Planning Section in the 1970s when planning TV relay
stations. I know, I was there.
"Active Deflectors", relaying TV signals down into valleys without
changing channels, have been doing exactly what you describe for many
years. For safety's sake, keep the TX and RX aerials as far apart as you
can.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

In message ,
Lostgallifreyan writes
Frank Turner-Smith G3VKI wrote in
:

This cross
polarisation provided about 26dB protection against co-channel
interference.


That's a useful figure. I asked a few weeks ago about the prosects of wiring
an external vertical dipole for FM VFH broadcasts, via a MAR6 based
amplifier boosting by maybe 20dB, to an internal horizontal dipole to
overcome local digital hash from nearby flats that gets in to degrade the
signal from a portable radio with a telescopic whip. As it is the SNR rather
than the raw strength which is an issue, an ideal situation would be to allow
thwe whip to be in its resting horizontal, cotracted position, while still
allowing clear use of radios carried around the flat while I work.

Various possible problems have been discussed, and I haven't pushed for this
with a trial, but if 26 or more dB are cut in the difference between antenna
based on polarisation, and the amp boosts only by 20dB, it seems that
feedback can be avoided, AND also the risk of interference to other FM VHF
radios in other flats. (Which might even benefit, if my own would.)

This is the first time anyone's mentioned a figure for isaolation (for want
of a better word) between similar dipoles based on 90° difference in
orientation, so I'm taking this moment to reopen the subject in passing...

I've often heard this '26dB protection' quoted, but I'm sure that it's
just a 'wet finger in the air' figure. Even if it is sort-of a 'typical
average', at any location it could equally turn out to be almost
anything between 'not a lot', and a lot more than 26dB - mainly
depending on reflections.
--
Ian

Frank Turner-Smith G3VKI wrote in
:

26dB cross-polarisation protection was the "rule of thumb" figure in use
by the BBC service Planning Section in the 1970s when planning TV relay
stations. I know, I was there.
"Active Deflectors", relaying TV signals down into valleys without
changing channels, have been doing exactly what you describe for many
years. For safety's sake, keep the TX and RX aerials as far apart as you
can.


Nice. That looks like bearing out my hopes for the scheme after all. The
distance isn't huge, but I've already decided to aim for the internal
horizontal dipole to be central to the flat, so I can use the smallest boost
that will do the deed.

Small extra question.. Would I get away with a simple low loss coax to dipole
with ferrites on the coax to prevent leaky signals running outside on the
screen, or should the internal dipole get a balun or other appropriate
treatment for a transmitting antenna, despits the tiny signals and extreme
locality involved?

Ian Jackson wrote in
:

at any location it could equally turn out to be almost
anything between 'not a lot', and a lot more than 26dB - mainly
depending on reflections.

Good point. In optics reflections wreak merry hell with polarisation (often
intentionally for good purpose) so I assume that it's the same with radio.
Even so it has to beat putting both in same plane, and I'll use no more boost
than will reduce local RF noise acceptably. Actually I have a bunch of PV
panels and such out there on grounded mounts that any major reflections that
occur won't make it into the flat, it's more likely there would be a few
narrow opportunities for a direct signal to get past rather than many
reflections getting here.

Frank Turner-Smith G3VKI wrote in
:

For safety's sake, keep the TX and RX aerials as far apart as you
can.


I can quantify that relatively, which may help.. The distance between the
internal dipole to the radio will be about a tenth of the distance between
dipoles. Would that offer enough scope for avoiding feedback while boosting
20dB in the antenna amp? (If not I estimate I can get useful help with 10dB).