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Help me with a long coax cable cabling

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seven_segment

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r f change over switch

Hi, hopefully this is the right section for this post! Anyway...

I have linked together all the VCR's in my house (along with the cable box) to allow them to be viewed on any TV in any room. However, the house is fairly big and the signal goes through at least 80m of coax, possibly more, before getting to the main distribution amplifier which splits to all the TV's. Currently the cabling is something like this:
Code:
Antenna > Booster > VCR 1 + Booster  > VCR 2 + Main distribution amp
                          ^                  ^
                         CATV              VCR 3
Note: CATV and VCR3 signals are injected through splitters
I do realise that having this many boosters in the system is asking for trouble on an analogue system, however I get a very poor signal without them as you would expect. Trouble is, although I get an acceptable picture with my current setup, it can be prone to inteference and the signal strength can change quite a lot throughout the week. I guess this might be due to variance in cable characteristics due to the changes in temperature and humidity.

Rather than wiring everything in a loop with the output of one VCR into the next, do you think the following setup would improve the picture quality?
Code:
    +-----Antenna----+
    |        |       |
VCR 1     VCR 2     VCR3     CATV
    |        |       |         |
    +---Main distribution amp--+
This setup would mean the signal has to go through a much shorter cable run, as it doesn't need to run through each VCR in turn. Instead, the signal from the antenna is split and fed to each VCR, and then all the signals are cobined again before the final distribution amp to the TV's.

Only possible problem is that the leads to/from each VCR will be different lengths, so the signal is going to take longer to propogate down one route than another. Is it possible that this would cause ghosting and interference, or would the delay be minimal and have little/no effect?

The final idea I've had is to simply recable the whole lot, using lower loss coax with better screening. This wouldn't be too hard, but could be costly in both time and money, so I don't want to do it unless it will offer a significant improvement. Currently I'm just using standard 75 Ohm coax downlead.

Any and all advice is much appreciated!
 

IanP

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Re: coax cable

In my oppinion your current cofiguration is better than the second proposal.
It looks like the level of signals, even with boosters, are quite low.
I suggest that you try this: disconnect devices on one-by-one basis and find out which one damps the signals' levels down. If you find one try to isolate/separate...
And the last suggestion is about co-ax cable: in extended networks such as yours you should use at least broadcast quality co-ax such as RG6 or similar..
 

seven_segment

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Re: coax cable

Thanks Ian, the cable I was looking at upgrading to states the attenuation as around 19db @ 890Mhz, which I think is marginally better than RG-6. Not sure on the specs of the cable I am currently using, but I guess it will be pretty mediocre compared to RG-6. I'll let you know how it went as soon as I get the cable and test things out.

Oh, and also, one downside with the current setup is that if one of the VCR's is unplugged then the entire system goes down. Would it be ok to permenantly wire a small length of coax between the input/output socket of each VCR? Since I'm working with RF signals, it shouldn't cause a short, right? Thanks again.
 

IanP

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Re: coax cable

Most of VCRs have 75Ω impedance at the RF in and a buffer to the RF out .. so you should not connect input with output. Maybe some kind of RF change-over switch will be solution to this..
 

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