Re-reading Svend's mail, he wants Channels 22 to 37, 40 to 44, and 49 to
64 (Antenna 1). He then needs to add Channel 21 and 38 + 39 (Antenna 2).
This should not be too difficult.
Obviously, the filters will not totally reject channels which are close
(for example, the 22 to 37 filter will not reject 21 very well). There
will be a certain amount of unwanted noise outside the passband of each
filter, and ghosting (due to pick-up on the 'wrong' antenna) may not be
totally eliminated. It may be necessary to fit one or two notch filters.
I would first fit a level-raising amplifier to the feed from each
antenna. There is nothing like having lots of signal level to play with!
At the output of Amplifier 1, use a 4*-way CATV splitter to obtain four
feeds. The loss will be about 8dB (for a splitter which uses ferrite
transformers). Each output will be isolated from the others (typically
by about 25dB).
[*Note: 2, 4 and 8-way splitters are 'standard building blocks' in the
world of cable TV. You can get 3-way and 6-way, but they are not easy to
find).
To three of the feeds, fit bandpass filter for Channels 22 to 37, 40 to
44 and 49 to 64. He can use the fourth output for monitoring.
At the output of Amplifier 2, use another 4*-way CATV splitter to obtain
4 feeds. To two of the feeds, fit bandpass filter for Channels 21 and
38+39.
He now has six separate TV feeds. If he wants, he can fit in-line
variable attenuators to each to equalise the signal levels.
The six TV feeds are then connected together using an 8*-way splitter
(as a combiner) or two 4-ways then a 2-way. There should be little
interaction between them. The loss will be about 11dB.
Assuming a loss of 2dB for each filter, the total loss of this system
will be 8 + 11 + 2 = 21dB.
All that remains it to design (or obtain). 'normal' filters for each
block of channels. There is a company in the UK which has a good range
of equipment for MATV and CATV:
http://www.taylorbros.co.uk
I don't see anything there which is exactly what Svend wants, but here
probably other companies in Europe which have can supply similar
equipment.
Ian.
In message , Crazy George
writes
Well, you know, it depends......
Optimally, one would like to have separate, independent antennas for each
channel, the way we constructed the first CATV head ends up in the hills of
Pennsylvania in the 1950s. But, then Scientific Atlanta and others
designed LPD antennas which covered multiple channels coming from one
general direction, and since tower space and loading was always a concern
(don't forget multiple runs of low loss coax enter that equation also),
multiple antennas gave way to fewer antennas and lots of signal processing
down at the base of the tower. Some sites had racks and racks of gear.
If the requirement is to provide signals to only one TV, a switch could be a
good choice. On the other hand, if the design is for a whole house, or
apartment complex or commercial building signal distribution system, then it
gets progressively more complicated. Since Svend didn't specify, one had to
conclude that he had already considered and rejected the easy solutions.
--
Crazy George
"Ian Jackson" wrote in message
...
With all those channels mixed together, it would be best to simply use
an A/B switch to switch between the two antennas.
Ian.
In message , SH writes
Thanks for both you answers its nice to now i stil remember some theory
eventhough its 10 years since i read some UHF theory (Ended up being a
programmer :-))))
I think ill go for the notch version. I have made a lot of them and they
are
easy to build but I've noticed that it seams that 3 notches (adjusted to
3
diff freq.) is some kind of limit. When i put in no. 4 it seams that the
overall loss increases.
Do I remember correctly when i state that 2 nothes on the same freq.
should
be a 1/4 wave apart???
Best regards
Svend Holby
"Crazy George" wrote in message
...
Ben Jackson has some good points. You need not worry about audio,
usually,
as the FM capture takes care of the weaker signal. So, filtering the
video
carrier and near sidebands is all that is necessary.
However, in answer to your question, the way you connect bandpass
filters
in
parallel is with power dividers, amplifiers, and power dividers again,
backwards as summers. Otherwise, controlling the out of band input and
output impedances of several in parallel is a monumental, but solvable
task.
Spice, anyone?
--
Crazy George
"SH" wrote in message
...
Hi
I have come up with one of those crazy DYI ideas which I would like
to
try
to implement but one thing still remains.
I have two UHF (TV) arial pointing in different directions and would
like
to
connect them together through BP filters to minimize noise.
BP = Bandpass
Antenna 1: UHF channel (Europe) BP 1 ch 22-37, BP 2 ch40 -44, BP 3 ch
49 -
68/ or high pass
Antenna 2: UHF channel (Europe) BP 1 ch 21 / or low pass, BP 2
ch38 -39,
BP
3 ch 47
I have found formulas to design each section of BF but how to you
connect
several BP filters in parallel??? I have investigated some old TV
filter
and
they have a coil (12 WDG) in and out of each BP section but is that
the
way
to do it and how does it influence the design of each BP.
The only approach that I can find is to split the signal into three (-
6dB )
then feed each into a BP filter and the Combine them again (-6dB).
This
will
result in each BP section seeing 75 Ohms as they shouland they would
not
interfere with eachother. Ofcouse i would need a 20 dB amplifier to
fix
the
loss.
But is there a better way.
Best regards
Svend Holby
--
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