Band pass filters -- what am I missing?

I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?


Cheers


Clem

Clem Dye wrote:
I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?

What make/type filters are they, and what are the two transmitters concerned ?

--
Mark
Please replace invalid and invalid with gmx and net to reply.

www.paras.org.uk

In article ,
Clem Dye wrote:
I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?


the filter will reduce the level of the unwanted, but probably, unless its
very expensive, not remove them altogether. You could try two filters in
series ...



Clem

--
From KT24

Using a RISC OS computer running v5.16

The box should automatically put the weakest signals in the 800 range, so it
sounds like your Group A signals are quite strong. Maybe you should be
pointing towards the Group A signals. Depending on which filter you've got,
the out of band rejection might be as little as 15dB so filtering is just
reducing the Group A signals to below the level of the Group B ones. Why not
just manually tune each mux, or unplug the aerial feed until it's passed the
Group A channels.



"Clem Dye" wrote in message ...

I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?


Cheers


Clem

charles wrote:


the filter will reduce the level of the unwanted, but probably, unless its
very expensive, not remove them altogether. You could try two filters in
series ...

Even very expensive filters won't do that. All real world engineering
devices have limitations.

On 27/08/2011 18:22, Clem Dye wrote:
I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?

A simple solution in some cases is to unplug the aerial when tuning for
the bit of the spectrum you are not interested in. Hence if wanting to
filter group A, start the scan with no aerial, wait until the progress
meter indicates you are up into the mid 30s (or guestimate it if its a
progress bar), and plug the aerial back in. Often simpler than trying to
sort out a mess of weak channels later.


--
Cheers,

John.

/================================================== ===============\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sat, 27 Aug 2011 19:22:01 +0100, John Rumm
wrote:

On 27/08/2011 18:22, Clem Dye wrote:
I'm in an area served by two transmitters, one group A, the other group
B. The weaker group A stuff is being picked-up from the back of the
aerial. As a result, whenever I re-tune my DTV boxes the weaker channels
get allocated the lower numbers, the correct channels the 800 range. If
I insert a group B band pass filter into the downlead before it gets to
the tuner I still get the group A channels, albeit that they now are in
the 800 range. I was under the impression that a bandpass filter was
supposed only to pass signals on the stated band. That's not the case as
I see it. what am I missing here please?

A simple solution in some cases is to unplug the aerial when tuning for
the bit of the spectrum you are not interested in. Hence if wanting to
filter group A, start the scan with no aerial, wait until the progress
meter indicates you are up into the mid 30s (or guestimate it if its a
progress bar), and plug the aerial back in. Often simpler than trying to
sort out a mess of weak channels later.

Surely even simpler is to find out the channel numbers for the
transmitter you want and do a manual scan for these specific channels?

http://www.ukfree.tv/transmittersmenu.php will provide the details.

On Sat, 27 Aug 2011 20:02:55 +0100, Bob Latham
wrote:

In article ,
Scott wrote:
On Sat, 27 Aug 2011 19:22:01 +0100, John Rumm
wrote:

Surely even simpler is to find out the channel numbers for the
transmitter you want and do a manual scan for these specific channels?

But what about devices that do not have a manual tune facility?

Bob.

That would be different. I thought they all had. Certainly 3/3 of
mine have manual scanning.

"Scott" wrote in message ...



That would be different. I thought they all had. Certainly 3/3 of
mine have manual scanning.

Ok so one does a manual set like I did but every day we turn on we are asked
to do a re-scan because of the over power full Belmont CH 22.
Regards
David

On Saturday, August 27th, 2011 at 20:02:55h +0100, Bob Latham asked:

But what about devices that do not have a manual tune facility?

Simple solution -- do not buy them.