Whoops! Forgot the pic

"Marky P" wrote in message
...
Anyway, to answer your question, the
DAT45 does out perform the LP in terms of gain.

I should bloody well hope so since it weighs three times as much and looks
ten times worse.

Bill

"Brian Wood" wrote in message
om...
Every time I see an aerial made by Televes there is a part missing,
usually one of the reflecters, this is no exception. Look closely and you
can see that the top boom has an element missing!

That's one reason I don't use them.

Bill

"Marky P" wrote in message
...
On Sun, 11 Feb 2007 00:05:25 -0000, "Bill Wright"
wrote:
Anyway, thanks for the pointers, Bill, always appreciated. This is an
easily accessible install, so improvements can be made if necessary.

Marky, you're obviously interested in aerials and stuff. Here's an extract
from a conversation in Another Place. It might give you food for thought.

Quotation starts

I have a problem coming up shortly in which Emley and Sutton C are

both received at roughly the same strength, and there is severe

co-channel interference (analogue on DTT is what's causing the

bother). So I took that as an example.



The relevant angle is 118deg. Assuming that the aerials are 1.5

wavelength...



Houston, you have a problem.



I don't think that the toed-in version is necessarily the best way to

solve this one: is 25dB reduction going to be enough for your

requirements, even assuming that you can get it that high from an

assembly of commercial components? But I have another suggestion to

make: one that will probably have you rolling around on the floor

screaming with laughter...



Basically it consists of nulling-out the unwanted signal using an

aerial dedicated to the purpose. It looks like this:



a) Install on Emley Moor a decent aerial having as good a rejection on

your 118deg angle as possible.



b) Install, behind it, a similar aerial lined up on Sutton. Ensure

that the line joining the so-called radiation centres of the aerials

is as far as possible on a line at right-angles to the Sutton

direction.



c) Using equal lengths of feeder, combine the two signals using an

attenuator in the SC feed.



d) Set up the rejection amplitude with the attenuator.



e) Set up the rejection phase by sliding the SC aerial closer to, or

further from, SC.



f) repeat d) and e) until optimum rejection is obtained, or until you

become dizzy and fall off the chimney.



The combining device that you use is going to be fiddly: any standard

splitter/combiner is going to introduce losses to the wanted signal

from Emley. What would be used at the relay-site end would be

something like a 10 to 20dB directional coupler, which would introduce

less than 1dB loss to the wanted signal. See BBC RD 1973/22 for a

design that could be implemented by a home constructor, though bear in

mind that those couplers (and indeed the toed-in aerial

system) were designed around 50ohm characteristic impedance, rather

than 75ohm.



Yes, I know... a lot of work, expensive, very fiddly and all that.

But it has been known to sort out an RBL on occasion, as a last resort

when all else has failed, although it's regarded a bit sniffily as

'rather unprofessional...'. Still, when the Devil rides,

or whatever.



Since this nulling-out is only going to be perfect at a single

frequency, setting up is going to be a bit of a compromise (centre of

band?). But then, it's unlikely that all four channels from SC will be

of equal amplitude, anyway: so optimise for the strongest

channels.

This is a variation on the method I described in 'Television' in 1978 (it's
on my website but it creaks!). The main difference is that the two aerials
are aligned on the wanted signal. First the two aerials are got into exact
phase regarding the wanted transmission by finding two nulls and moving one
aerial to the midpoint. Then one aerial is moved at rt angles to the wanted
signal. This retains the correct phase relationship re the wanted signal but
varies the phase relationship re the unwanted signal. It does work but it's
fiddly.



You may not have the time or the real-estate to employ this sort of

wheeze. I fully realise that working on a nice solid platform on a

mast several hundred feet up, with plenty of room to lay out your

lunch-box and thermos flask, is not quite the same as clinging to the

crumbling brickwork on a decaying chimney stack, thirty feet above a

nice concrete backyard with pointy steel railings all around it...

No, this one was money no object, and the working platform was a flat roof

surrounded by pitched roofs of varying heights. In the event I calculated

the theoretical spacing well in advance and fixed together two log periodics
accordingly. With my transmitter of dubious legality at one end of my field
firing into a log periodic (to minimise the Rx aerial picking up reflections
from the side when off beam) I rotated the test array and discovered that
the null was exactly right, at 118 deg. The problem was that it was only
exactly right on the calculated frequency, which I had chosen for two
reasons. One, it's in the middle of the desired group of channels, and 2,
it's the only clear channel in this area in the middle of Gp B. It was
channel 48. I soon discovered that perfect nulling out was terribly
critical, and that the benefits only one channel up or down were only in the
order of 5dB over a single aerial. Bugger. So then this idea of 'toeing in'

cropped up. I worked it out for a Blake log periodic and it's hopeless. The
two aerials would be miles off beam. But some makes of log are longer than
other for the same bandwidth, so I thought, I wonder if I could make a log
that's long for its bandwidth. Then the two aerials would not be so far off
beam. But is there a fundamental rule that dictates the physical length and
element spacing? I thought there was, but if so how come a Vision log is 50%
longer than a Blake one? -- and they have the same gain near enough.

I put that idea on the backburner. I just don't have time at the moment.

Then, today happened. We set sail for Nottingham with two logs, mounted on a
crossarm, with no toeing in. I was thinking, well, it can't be worse and it
might be better and at least I'll have done my best. When I got onto the
roof it was ****ing it down with rain, but after a bit it stopped, so I took
the mast down and swapped the TC18E (I keep a few of these obsolete aerials
for special occasions) for the two logs. I had noticed when I approached the
building that the roofline in the Emley direction was a couple of foot
higher than that in the SC direction, suggesting that a bit of screening
would be possible. To test this theory I pointed the thing at Emley and
lowered it. Three feet down the Emley signal reduced by 6dB, which would
have solved the problem with ease, except that when I tried SC at that
height the slight (2 or 3dB) reduction in signal level worsened the c/n
ratio to the point where the BER was worsening seriously. This effect would
obviously over ride any improvements I could get by killing the Emley
signal. I should have said that both Emley and Sutton are at levels just
about on threshold for DTT. So, back to a reliance on the directional
properties of the two logs. I thought I'd try Waltham again, just out of
madness, but it was just as crap as it had been a fortnight before,
unsurprisingly since there's a whacking great hill in the way. So, with the
two logs as high as I could get them I started making minute adjustments to
the direction whilst looking at the Sutton BER. The results were sort of
quite good really. I think the limiting factor was the signal level, so
maybe if this hadn't been good enough I would have had to stack two big high
gain aerials and put them on a guyed mast. But in the event it was OK. Not
brilliant, but a worthwhile improvement. I ended up with five muxes that I
had no worries about and one that I had slight worries about. In the
following I have only listed the BER powers, since the numbers were
fluctuating so much.

Oddly, the figures I measured on the 'old' aerial were much better than they
had been a fortnight before, and in fact if they'd been as good then as they
were today I probably would have risked leaving it alone. But today:

41: CSI went from 18% to 16%, level dropped 0.1dB, BER went from wobbling
about between n-5 and n-6 to a solid n-7.

44: CSI went from 32-36% to 32-34%, level didn't change, BER went from
wobbling about between n-5 and n-6 to an almost solid n-7 (which is odd
really, given the CSI)

47: CSI went from 29-31% to 27-29%, level improved 0.4dB, BER went from
wobbling about between n-5 and n-6 to a solid n-7.

51: CSI went from 22-30% to 18-22%, level improved 0.4dB, BER went from n-5
to a solid n-7.

52: CSI went from 26% to 21-22%, level worsened by 1.1dB, BER was a solid

n-7 before and after.

55: CSI went from 13% to 15% (worse!), level worsened by 0.6dB, BER was a
solid n-7 before and after.

In retrospect I think I was wrong to use two logs, because gain was actally
a bigger issue than I thought. I'm wondering now if two TC18s would have
worked better (4 or 5dB more gain versus less good directional properties).

But, to be practical, it's good enough now.



Quotation ends



Bill

"Marky P" wrote in message
...

Are you saying that this is an inadequate erection?

I couldn't possibly comment, as I have never had an inadequate erection here
!!

In message , Bill Wright
writes

"Marky P" wrote in message
.. .
On Sun, 11 Feb 2007 00:05:25 -0000, "Bill Wright"
wrote:
Anyway, thanks for the pointers, Bill, always appreciated. This is an
easily accessible install, so improvements can be made if necessary.

Marky, you're obviously interested in aerials and stuff. Here's an extract
from a conversation in Another Place. It might give you food for thought.

Quotation starts

I have a problem coming up shortly in which Emley and Sutton C are

both received at roughly the same strength, and there is severe

co-channel interference (analogue on DTT is what's causing the

bother). So I took that as an example.



The relevant angle is 118deg. Assuming that the aerials are 1.5

wavelength...



Houston, you have a problem.



I don't think that the toed-in version is necessarily the best way to

solve this one: is 25dB reduction going to be enough for your

requirements, even assuming that you can get it that high from an

assembly of commercial components? But I have another suggestion to

make: one that will probably have you rolling around on the floor

screaming with laughter...



Basically it consists of nulling-out the unwanted signal using an

aerial dedicated to the purpose. It looks like this:



a) Install on Emley Moor a decent aerial having as good a rejection on

your 118deg angle as possible.



b) Install, behind it, a similar aerial lined up on Sutton. Ensure

that the line joining the so-called radiation centres of the aerials

is as far as possible on a line at right-angles to the Sutton

direction.



c) Using equal lengths of feeder, combine the two signals using an

attenuator in the SC feed.



d) Set up the rejection amplitude with the attenuator.



e) Set up the rejection phase by sliding the SC aerial closer to, or

further from, SC.



f) repeat d) and e) until optimum rejection is obtained, or until you

become dizzy and fall off the chimney.



The combining device that you use is going to be fiddly: any standard

splitter/combiner is going to introduce losses to the wanted signal

from Emley. What would be used at the relay-site end would be

something like a 10 to 20dB directional coupler, which would introduce

less than 1dB loss to the wanted signal. See BBC RD 1973/22 for a

design that could be implemented by a home constructor, though bear in

mind that those couplers (and indeed the toed-in aerial

system) were designed around 50ohm characteristic impedance, rather

than 75ohm.



Yes, I know... a lot of work, expensive, very fiddly and all that.

But it has been known to sort out an RBL on occasion, as a last resort

when all else has failed, although it's regarded a bit sniffily as

'rather unprofessional...'. Still, when the Devil rides,

or whatever.



Since this nulling-out is only going to be perfect at a single

frequency, setting up is going to be a bit of a compromise (centre of

band?). But then, it's unlikely that all four channels from SC will be

of equal amplitude, anyway: so optimise for the strongest

channels.

This is a variation on the method I described in 'Television' in 1978 (it's
on my website but it creaks!). The main difference is that the two aerials
are aligned on the wanted signal. First the two aerials are got into exact
phase regarding the wanted transmission by finding two nulls and moving one
aerial to the midpoint. Then one aerial is moved at rt angles to the wanted
signal. This retains the correct phase relationship re the wanted signal but
varies the phase relationship re the unwanted signal. It does work but it's
fiddly.



You may not have the time or the real-estate to employ this sort of

wheeze. I fully realise that working on a nice solid platform on a

mast several hundred feet up, with plenty of room to lay out your

lunch-box and thermos flask, is not quite the same as clinging to the

crumbling brickwork on a decaying chimney stack, thirty feet above a

nice concrete backyard with pointy steel railings all around it...

No, this one was money no object, and the working platform was a flat roof

surrounded by pitched roofs of varying heights. In the event I calculated

the theoretical spacing well in advance and fixed together two log periodics
accordingly. With my transmitter of dubious legality at one end of my field
firing into a log periodic (to minimise the Rx aerial picking up reflections
from the side when off beam) I rotated the test array and discovered that
the null was exactly right, at 118 deg. The problem was that it was only
exactly right on the calculated frequency, which I had chosen for two
reasons. One, it's in the middle of the desired group of channels, and 2,
it's the only clear channel in this area in the middle of Gp B. It was
channel 48. I soon discovered that perfect nulling out was terribly
critical, and that the benefits only one channel up or down were only in the
order of 5dB over a single aerial. Bugger. So then this idea of 'toeing in'

cropped up. I worked it out for a Blake log periodic and it's hopeless. The
two aerials would be miles off beam. But some makes of log are longer than
other for the same bandwidth, so I thought, I wonder if I could make a log
that's long for its bandwidth. Then the two aerials would not be so far off
beam. But is there a fundamental rule that dictates the physical length and
element spacing? I thought there was, but if so how come a Vision log is 50%
longer than a Blake one? -- and they have the same gain near enough.

I put that idea on the backburner. I just don't have time at the moment.

Then, today happened. We set sail for Nottingham with two logs, mounted on a
crossarm, with no toeing in. I was thinking, well, it can't be worse and it
might be better and at least I'll have done my best. When I got onto the
roof it was ****ing it down with rain, but after a bit it stopped, so I took
the mast down and swapped the TC18E (I keep a few of these obsolete aerials
for special occasions) for the two logs. I had noticed when I approached the
building that the roofline in the Emley direction was a couple of foot
higher than that in the SC direction, suggesting that a bit of screening
would be possible. To test this theory I pointed the thing at Emley and
lowered it. Three feet down the Emley signal reduced by 6dB, which would
have solved the problem with ease, except that when I tried SC at that
height the slight (2 or 3dB) reduction in signal level worsened the c/n
ratio to the point where the BER was worsening seriously. This effect would
obviously over ride any improvements I could get by killing the Emley
signal. I should have said that both Emley and Sutton are at levels just
about on threshold for DTT. So, back to a reliance on the directional
properties of the two logs. I thought I'd try Waltham again, just out of
madness, but it was just as crap as it had been a fortnight before,
unsurprisingly since there's a whacking great hill in the way. So, with the
two logs as high as I could get them I started making minute adjustments to
the direction whilst looking at the Sutton BER. The results were sort of
quite good really. I think the limiting factor was the signal level, so
maybe if this hadn't been good enough I would have had to stack two big high
gain aerials and put them on a guyed mast. But in the event it was OK. Not
brilliant, but a worthwhile improvement. I ended up with five muxes that I
had no worries about and one that I had slight worries about. In the
following I have only listed the BER powers, since the numbers were
fluctuating so much.

Oddly, the figures I measured on the 'old' aerial were much better than they
had been a fortnight before, and in fact if they'd been as good then as they
were today I probably would have risked leaving it alone. But today:

41: CSI went from 18% to 16%, level dropped 0.1dB, BER went from wobbling
about between n-5 and n-6 to a solid n-7.

44: CSI went from 32-36% to 32-34%, level didn't change, BER went from
wobbling about between n-5 and n-6 to an almost solid n-7 (which is odd
really, given the CSI)

47: CSI went from 29-31% to 27-29%, level improved 0.4dB, BER went from
wobbling about between n-5 and n-6 to a solid n-7.

51: CSI went from 22-30% to 18-22%, level improved 0.4dB, BER went from n-5
to a solid n-7.

52: CSI went from 26% to 21-22%, level worsened by 1.1dB, BER was a solid

n-7 before and after.

55: CSI went from 13% to 15% (worse!), level worsened by 0.6dB, BER was a
solid n-7 before and after.

In retrospect I think I was wrong to use two logs, because gain was actally
a bigger issue than I thought. I'm wondering now if two TC18s would have
worked better (4 or 5dB more gain versus less good directional properties).

But, to be practical, it's good enough now.



Quotation ends



Bill



Sorry to leave everything in, but it's difficult to snip.
However, when you erect a second aerial (Aerial B) to pick up an
interfering signal, and then use that signal to null out the
interference on the wanted signal (from Aerial A), it isn't necessary to
have the two feeders exactly the same length. In fact, they can be quite
a bit different in length.

Obviously, when you add B to A, to cancel the interference on A, the
signal on B must be the same amplitude as the interference on A, and in
antiphase with it. You can change the B RF signal from antiphase to
in-phase and back to antiphase simply (well, not so simply) by varying
the feeder length by up to one wavelength, regardless of the relative
lengths of the two feeders. You don't have to move Aerial B.

The feeders don't have to be exactly the same length, but there is a
limitation to their difference. The difference has to be small compared
with the highest frequency of the MODULATION of the signals in question,
otherwise you won't be able to balance the amplitudes. For TV, we are
talking about a frequency of (say) 8MHz absolute maximum, so the
difference of feeder length must be small compared with a wavelength at
8MHz. I reckon that this will be around 60 in coax, so the problem is
academic.

The only problem is what to use as a piece of variable length feeder
(which only has to be one wavelength long). A crude way would be to 'cut
and try', or make up a set of jumpers of varying lengths to add to the
existing feeder. The best way would be to construct / buy / steal a
'sliding trombone' line so the length can be varied continuously. All
adjustments are, of course, carried out in the comfort of the room where
the TV set is.

Finally...
No - I haven't actually done this myself.
But the principles are sound.

Ian.


--

Brian Wood wrote:
"Jim" wrote in message
...
"kim" wrote in message
...
"Marky P" wrote in message
...
In my haste, I forgot to add the pic.

http://i22.photobucket.com/albums/b3...riodic0001.jpg
Was the log periodic erected before or after the recent gale force winds?

(kim)
Thats a obvious one! After the winds. It wouldn't look like this now if
it
had been erected before them.

Every time I see an aerial made by Televes there is a part missing, usually
one of the reflecters, this is no exception. Look closely and you can see
that the top boom has an element missing!

Brian




Really? Well prepare to be corrected. Here's a picture of my DAT75. It's
been up for 2 or 3 years now, survived the gales and has nothing missing.
http://farm1.static.flickr.com/146/3...6ac9be4d1b.jpg

On Sun, 11 Feb 2007 23:08:49 -0000, "Bill Wright"
wrote:


"Brian Wood" wrote in message
news:[email protected] com...
Every time I see an aerial made by Televes there is a part missing,
usually one of the reflecters, this is no exception. Look closely and you
can see that the top boom has an element missing!

That's one reason I don't use them.

Bill

That's only been up there for 3 years. When I bought it, it arrived
with 2 bent elements. One bent back ok, and the other one snapped
off. I fixed it back on by shoving a small wooden plug between the
boom & the element. The one that eventually fell off was the one I
bent back into place :-)

Marky P.

"Bill Wright" wrote in message
...

2. The tape holding the mast should be every 200mm max.

Have you got a thing about wasting tape then Bill?

F

"Ian Jackson" wrote in message
...
In message , Bill Wright
writes
Sorry to leave everything in, but it's difficult to snip.
However, when you erect a second aerial (Aerial B) to pick up an
interfering signal, and then use that signal to null out the interference
on the wanted signal (from Aerial A), it isn't necessary to have the two
feeders exactly the same length. In fact, they can be quite a bit
different in length.

Obviously, when you add B to A, to cancel the interference on A, the
signal on B must be the same amplitude as the interference on A, and in
antiphase with it. You can change the B RF signal from antiphase to
in-phase and back to antiphase simply (well, not so simply) by varying the
feeder length by up to one wavelength, regardless of the relative lengths
of the two feeders. You don't have to move Aerial B.
Yes, agreed. I don't know why he said they had to be the same. It could be
that he put the two aerials the same distance from the wanted Tx, for the
same of a simple explanation.

One method of rejecting from the rear has a 1/4wave difference in feeder
length.


The feeders don't have to be exactly the same length, but there is a
limitation to their difference. The difference has to be small compared
with the highest frequency of the MODULATION of the signals in question,
otherwise you won't be able to balance the amplitudes. For TV, we are
talking about a frequency of (say) 8MHz absolute maximum, so the
difference of feeder length must be small compared with a wavelength at
8MHz. I reckon that this will be around 60 in coax, so the problem is
academic.
That's an interesting thought though.


The only problem is what to use as a piece of variable length feeder
(which only has to be one wavelength long). A crude way would be to 'cut
and try', or make up a set of jumpers of varying lengths to add to the
existing feeder. The best way would be to construct / buy / steal a
'sliding trombone' line so the length can be varied continuously. All
adjustments are, of course, carried out in the comfort of the room where
the TV set is.
I think generally the idea is to avoid having to vary the feeder length.
It's easier to get the phase by moving one aerial.
Although the idea of fiddling about with the feeder lengths indoors is
attractive.

Bill

"Freddy" wrote in message
...

"Bill Wright" wrote in message
...

2. The tape holding the mast should be every 200mm max.

Have you got a thing about wasting tape then Bill?

Wot? Err, I said holding the mast when I meant holding the cable. Is that
it? Am I being thick?

Bill