LF roll-off problem

Before I start delving into this problem in earnest, I thought I'd see if
anybody here can give me any pointers...Here goes:

The signal and quality readings on the lowest channel 22 for DTV mux2 from
Crystal Palace are poor (the picture degrades from time to time), whereas
all other channels from 23 upwards are fine (signal and quality readings
perfect or near perfect, and no picture degradations seen). This problem
with those digital services (especially affecting the ITV programmes) is
common to all the Freeview receivers I have.

I'ver been living with this problem for years now. I really ought to try to
sort it out! The advent of a new digital TV set this year really prompted me
to do something: we're watching digital now rather than analogue!

I've a somewhat interesting setup here. Aerial (installed in 1994) is on the
only chimney that can 'see' Crystal Palace properly. Unfortunately that
chimney is at the other end of the house from the living room, so there are
some longish runs of coax to and from a distribution amplifier in the loft
feeding the living room, bedrooms and kitchen. Significant...?

My key question is, is there any common factor that could cause a drop-off
just at the band edge? Could the aerial be the main suspect? Could a poor
joint somehere result in such an effect?

Any helpful ideas would be most welcome! Thanks!
-Roy-

In article , Roy Hammond
rhammondATieeDOTorg wrote:
Before I start delving into this problem in earnest, I thought I'd see if
anybody here can give me any pointers...Here goes:

The signal and quality readings on the lowest channel 22 for DTV mux2
from Crystal Palace are poor (the picture degrades from time to time),
whereas all other channels from 23 upwards are fine (signal and quality
readings perfect or near perfect, and no picture degradations seen). This
problem with those digital services (especially affecting the ITV
programmes) is common to all the Freeview receivers I have.

I'ver been living with this problem for years now. I really ought to try
to sort it out! The advent of a new digital TV set this year really
prompted me to do something: we're watching digital now rather than
analogue!

I've a somewhat interesting setup here. Aerial (installed in 1994) is on
the only chimney that can 'see' Crystal Palace properly. Unfortunately
that chimney is at the other end of the house from the living room, so
there are some longish runs of coax to and from a distribution amplifier
in the loft feeding the living room, bedrooms and kitchen. Significant...?

My key question is, is there any common factor that could cause a
drop-off just at the band edge? Could the aerial be the main suspect?
Could a poor joint somehere result in such an effect?

Certainly the aerial is the most likely cause of the poor signal. The long
runs of co-ax will, generally, attenuate the lowest frequency the least.
However if there was somewhere on that long run with a severe kink in the
cable, that could cause probles with just the one channel - it would depend
on the distance between the kink and the source point (aerial or
amplifier). It could also be that due to lack of screening, one of the long
runs is picking up sufficient signal on one channel to partly cancel the
signal that is being picked up buy the aerial.

If you haven't any test equipment, I'd suggest trying to look with your
receiver at the signal before the aerial. That might mean taking the set
into the loft which might be difficult. If so, try joining the input from
the amplifier to any outlet - ideally the one with the shortest cable - to
see if the error still occurs on Mux2. At least, by trying the various
outlets you can eliminate the distribution amplifier and cables.

--
From KT24

Using a RISC OS computer running v5.11

In message , charles
writes
In article , Roy Hammond
rhammondATieeDOTorg wrote:
Before I start delving into this problem in earnest, I thought I'd see if
anybody here can give me any pointers...Here goes:

The signal and quality readings on the lowest channel 22 for DTV mux2
from Crystal Palace are poor (the picture degrades from time to time),
whereas all other channels from 23 upwards are fine (signal and quality
readings perfect or near perfect, and no picture degradations seen). This
problem with those digital services (especially affecting the ITV
programmes) is common to all the Freeview receivers I have.

I'ver been living with this problem for years now. I really ought to try
to sort it out! The advent of a new digital TV set this year really
prompted me to do something: we're watching digital now rather than
analogue!

I've a somewhat interesting setup here. Aerial (installed in 1994) is on
the only chimney that can 'see' Crystal Palace properly. Unfortunately
that chimney is at the other end of the house from the living room, so
there are some longish runs of coax to and from a distribution amplifier
in the loft feeding the living room, bedrooms and kitchen. Significant...?

My key question is, is there any common factor that could cause a
drop-off just at the band edge? Could the aerial be the main suspect?
Could a poor joint somehere result in such an effect?

Certainly the aerial is the most likely cause of the poor signal. The long
runs of co-ax will, generally, attenuate the lowest frequency the least.
However if there was somewhere on that long run with a severe kink in the
cable, that could cause probles with just the one channel - it would depend
on the distance between the kink and the source point (aerial or
amplifier). It could also be that due to lack of screening, one of the long
runs is picking up sufficient signal on one channel to partly cancel the
signal that is being picked up buy the aerial.

If you haven't any test equipment, I'd suggest trying to look with your
receiver at the signal before the aerial. That might mean taking the set
into the loft which might be difficult. If so, try joining the input from
the amplifier to any outlet - ideally the one with the shortest cable - to
see if the error still occurs on Mux2. At least, by trying the various
outlets you can eliminate the distribution amplifier and cables.

The gain of a UHF TV aerial is usually 'escarpment-shaped'. Starting at
the LF end of the frequency range, the gain rises progressively by a few
dB, reaching a peak at (say) a couple of channels below the HF end of
the frequency range. Above the peak, the gain drops very rapidly. It
would therefore be unusual for the aerial itself to cause a sudden
drop-off of signal level at the LF end, below Ch23.

However, what you might have is a 'null' in the RF field surrounding the
aerial. This happens where reflections partially null out the direct
signal. This effect can be very frequency-dependent, and moving the
aerial position by only a few inches may change things considerably. In
some circumstances, the null can be very sharp and deep. You can usually
see this effect on a spectrum analyser, if you observe simultaneously
the signal levels of all the channels 22 to 34 (or even 37). As you move
the aerial position and direction, the individual signal levels usually
go up and down like yo-yos, with some being high when others are low ,
then vice versa. If this is causing your problem, then the most obvious
cure would be to move the aerial a little (if you can).
--
Ian

I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken.

The latter were extremely prone to roll off in gain at the lf end,
presumably the designers decided that the loss of signal due to poorer
propagation at higher meant they needed to make their tuners rise in
sensitivity with frequency.
A bad thing, as out in the east of the UK and presumably other places,
people had problems with bottom of band signals of course. Enter mug here
who used to 'well re align was too good a word for it, twealk the innards to
get them more level is probably better... grin.

It makes me wonder exactly how level gains are on the current crop of tuners
in use? Obviously some aerials will be rolling off as well, but as has been
mentioned here before, its not the actual strength that should be looked at
but the quality, and it is conceivable that some reflection is causing this
not to be good at that particular channel.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Roy Hammond" rhammondATieeDOTorg wrote in message
o.uk...
Before I start delving into this problem in earnest, I thought I'd see if
anybody here can give me any pointers...Here goes:

The signal and quality readings on the lowest channel 22 for DTV mux2 from
Crystal Palace are poor (the picture degrades from time to time), whereas
all other channels from 23 upwards are fine (signal and quality readings
perfect or near perfect, and no picture degradations seen). This problem
with those digital services (especially affecting the ITV programmes) is
common to all the Freeview receivers I have.

I'ver been living with this problem for years now. I really ought to try
to
sort it out! The advent of a new digital TV set this year really prompted
me
to do something: we're watching digital now rather than analogue!

I've a somewhat interesting setup here. Aerial (installed in 1994) is on
the
only chimney that can 'see' Crystal Palace properly. Unfortunately that
chimney is at the other end of the house from the living room, so there
are
some longish runs of coax to and from a distribution amplifier in the loft
feeding the living room, bedrooms and kitchen. Significant...?

My key question is, is there any common factor that could cause a drop-off
just at the band edge? Could the aerial be the main suspect? Could a poor
joint somehere result in such an effect?

Any helpful ideas would be most welcome! Thanks!
-Roy-

I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken

The original ELC1043 did tune down to the 70cm ATV band without modification
Pity about its performance though.

Can I nominate the TAA550 as the world's most unreliable Zener?

(Zener with delusions about being an IC)


--
Graham.

%Profound_observation%

"Ian Jackson" wrote in message
...
In message , charles
writes
In article , Roy
Hammond
The gain of a UHF TV aerial is usually 'escarpment-shaped'. Starting at
the LF end of the frequency range, the gain rises progressively by a few
dB, reaching a peak at (say) a couple of channels below the HF end of the
frequency range. Above the peak, the gain drops very rapidly. It would
therefore be unusual for the aerial itself to cause a sudden drop-off of
signal level at the LF end, below Ch23.

But see a special case:

http://www.wrightsaerials.tv/article...r-syndrome.pdf

Bill


However, what you might have is a 'null' in the RF field surrounding the
aerial. This happens where reflections partially null out the direct
signal. This effect can be very frequency-dependent, and moving the aerial
position by only a few inches may change things considerably. In some
circumstances, the null can be very sharp and deep. You can usually see
this effect on a spectrum analyser, if you observe simultaneously the
signal levels of all the channels 22 to 34 (or even 37). As you move the
aerial position and direction, the individual signal levels usually go up
and down like yo-yos, with some being high when others are low , then vice
versa. If this is causing your problem, then the most obvious cure would
be to move the aerial a little (if you can).

Or move it a lot, away from the zone where there is a multiplicity of signal
paths. The other chimney?

Bill

"Brian Gaff" wrote in message
om...
I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken.

The latter were extremely prone to roll off in gain at the lf end,
presumably the designers decided that the loss of signal due to poorer
propagation at higher meant they needed to make their tuners rise in
sensitivity with frequency.
A bad thing, as out in the east of the UK and presumably other places,
people had problems with bottom of band signals of course. Enter mug here
who used to 'well re align was too good a word for it, twealk the innards
to get them more level is probably better... grin.

When ch21 started from Crosspool it emerged that some TV sets did not work
AT ALL on that channel. Tuning down to it, there would be a sudden change,
as if the power supply to the tuner had gone off. I know nothing of these
matters, but always wondered if the LO was failing to operate below a
certain frequency.

Bill

I'll nominate those bullet shaped Millard rectifiers as being the worlds
most unreliable rectifiers then. I had a phillips n1501 vcr which kep
stopping for no good reason until I found the rectifiers all had loose
wires.

Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Graham." wrote in message
...


I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken

The original ELC1043 did tune down to the 70cm ATV band without
modification
Pity about its performance though.

Can I nominate the TAA550 as the world's most unreliable Zener?

(Zener with delusions about being an IC)


--
Graham.

%Profound_observation%

In message , Bill Wright
writes

"Ian Jackson" wrote in message
...


It would
therefore be unusual for the aerial itself to cause a sudden drop-off of
signal level at the LF end, below Ch23.

But see a special case:

http://www.wrightsaerials.tv/article...r-syndrome.pdf

I think what you're saying is that, at the LF end, where the reflector
becomes less than a halfwave, the forward gain suddenly 'starts to go
into reverse' (if I may use such a highly technical description).
Interesting.
--
Ian

In message , Bill Wright
writes

"Brian Gaff" wrote in message
. com...
I'm going to go back to the 70s here, back to the infancy of varicap uhf
tuners. There were two makes we used, The mullard and the telefunken.

The latter were extremely prone to roll off in gain at the lf end,
presumably the designers decided that the loss of signal due to poorer
propagation at higher meant they needed to make their tuners rise in
sensitivity with frequency.
A bad thing, as out in the east of the UK and presumably other places,
people had problems with bottom of band signals of course. Enter mug here
who used to 'well re align was too good a word for it, twealk the innards
to get them more level is probably better... grin.

When ch21 started from Crosspool it emerged that some TV sets did not work
AT ALL on that channel. Tuning down to it, there would be a sudden change,
as if the power supply to the tuner had gone off. I know nothing of these
matters, but always wondered if the LO was failing to operate below a
certain frequency.

I've known a few valve radio sets which were affected by the local
oscillator suddenly cutting off on certain bands (or parts thereof). In
fact, I have a late 1950s Murphy in the loft which, as I remember it,
didn't work on the LF end of the VHF-FM radio band. One day, I'll get
around to restoring it!
--
Ian