short antennae
 

On 31/10/14 19:05, Ian Jackson wrote:
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 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...

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.

Indeed. The figure applies under "free space" or "line of sight"
conditions. Reflections or "grazing" can reduce this amount.

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

short antennae
 

"Lostgallifreyan" wrote in message
. ..
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.



Theory say infinate, but in practice it won't hapen. Just too many
reflections, especially on the low bands where the signal reflects many
times over a long distance.

Here are some numbers if there are no reflections to upset theory.
deg differance in dB

20 54
30 1.25
45 3
60 6
70 9.32
80 15.2
90 infinity

This also holds up for right hand and left hand circular.

There is another modification if the antennas are seperated vertically or
horizontal. You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.




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short antennae
 

"Ralph Mowery" wrote in news:V5-
:

You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.


Nothing I can do to bring that about, nice idea though it is. The external
one is about 12 feet off the ground, and the inner one will be wherever it
gives best results, likely about 6 feet. Much less difference there than the
distance between them across ground.

This is a building surrounded by buildings, even the yard is partially
enclosed by walls two stories high. When I try it I won't be expecting any
ideal theoretical figures to match, not even remotely. I just hope that the
grounded solar panels out there will be more likely to deflect unwanted
signals between the dipoles than to aggravate things, because they form the
bulk of material in direct line netween dipoles, assuming the internal one
goes where I need it most. I'll set it up horizontally because it seems
likely that would be better than vertical like the outer one, no matter what
happens.

short antennae
 

On 31/10/14 21:22, Ralph Mowery wrote:
"Lostgallifreyan" wrote in message
. ..
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.



Theory say infinite, but in practice it won't happen. Just too many
reflections, especially on the low bands where the signal reflects many
times over a long distance.

Here are some numbers if there are no reflections to upset theory.
deg difference in dB

20 54
30 1.25
45 3
60 6
70 9.32
80 15.2
90 infinity

This also holds up for right hand and left hand circular.

There is another modification if the antennas are separated vertically or
horizontal. You usually get lots more isolation if the antennas are mounted
one above the other instead of horizontal.


Are you sure about the 20 degree difference? Should it have been 5.4dB?

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

short antennae
 

"Frank Turner-Smith G3VKI" wrote in message
...
On 31/10/14 21:22, Ralph Mowery wrote:
Here are some numbers if there are no reflections to upset theory.
deg difference in dB

20 54
30 1.25
45 3
60 6
70 9.32
80 15.2
90 infinity

This also holds up for right hand and left hand circular.

There is another modification if the antennas are separated vertically or
horizontal. You usually get lots more isolation if the antennas are
mounted
one above the other instead of horizontal.


Are you sure about the 20 degree difference? Should it have been 5.4dB?


Yes, fat fingered the thing and missed putting in the decimal point.

:-)



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Sussed it! (I think)
 

On 31/10/14 17:44, Ian Jackson wrote:
In message , Frank Turner-Smith G3VKI
writes
On 31/10/14 11:56, Ian Jackson wrote:
In message , Wayne
writes


"Ian Jackson" wrote in message
...

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

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

# -- # Ian

OK, but I'm losing touch with what the point is.
The figures referenced both times show voltage peaks of opposite phase
at the ends of half wave dipoles, thus voltage peaks of the same phase
at the end of full wave dipoles.

I'm assuming we have agreement on that.

You assume wrongly. A centre-fed fullwave is also fed 'push-pull', ie in
antiphase - except that it's a high voltage feed instead of high
current.

Just draw a diagram similar to the halfwave, - again showing both the
voltage amplitude and polarity. The lines you draw on each side for the
amplitudes are copies of each other - not mirror images.

However, it seems to a different matter if you feed the fullwave
off-centre, a quarterwave from one end. At least on my drawing, the
voltages at the ends ARE in phase - so I guess the radiation pattern
will be different from the centre-fed.

And with that, it currently is time here in CA for Lagavulin 16.

No such luck here. FWIW, it's nearly midday, and I'm having a (rather
late) strong 'coffee-bag' coffee, with two teaspoons of maple syrup and
one teaspoon (heaped) of dark drinking chocolate powder.

Ian, thanks for your help. There's just one bit of this I can't get my
head around.
I'm happy with the idea of 'push-pull' feeding the antenna. This, as
you say, will cause the amplitudes on each side to be copies and not
mirror images. This in turn causes the currents flowing in the legs to
be in phase, with about 1dB increase in the front lobe and a
subsequent decrease in the lobe's width.
This has to reciprocate, so what happens in RX mode to cause the same
phase inversion at the feed point?

Well..... Errrrrr....... It.... just sort-of does?

As you say, it has to reciprocate, therefore the incoming wavefront(s)
from where the 'radiation' lobes are pointing, induce volts and amps in
the antenna such that they end up being in anti-phase at the feeder
connection points. They then slide, in antiphase, all the way down the
feeder and into the receiver. Or something like that.

It DOES reciprocate! If you draw the dipole as two elements end-to-end
across the screen, the radiation lobes run up and down the screen, not
to the left or right. Incoming RF would induce from above or below the
array and would produce identical waveforms in each leg. However, the
feed point goes to the right hand end of one leg and the left hand end
of the other, so the inputs are in anti-phase!
Time for a drink, or several.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

short antennae
 

On Fri, 31 Oct 2014 00:44:08 +0000, Ian Jackson wrote:

Very few are now purely horizontal.

One of our local community FM stations has a folded dipole at 45 degrees!



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