Unterminated coax acting as an antenna?

[Terry]

Apologies if this isn't the right place for this but I couldn't think of anywhere else.

I need some help here!

I was reading through this thread over at uk.tech.digital-tv in which Dan is asking about the importance of fitting the terminator provided when he disconnects his VM Broadband internet feed.

Being usenet, the conversation wandered all over the place but no attempt to answer the original question! So Dan asked again, to which I replied:

"The reason it is required is simple: the band of signals coming into your house is split. The majority forms the downstream for all the TV muxes and the internet and it is easy to provide plenty of isolation to prevent interaction with other customers' feeds.

The remainder is for the upstream and is used by your modem to communicate with the router in the headend or hub. All page requests and emails are sent using it and anything else you upload, whether it be pictures, mp3 file, VOIP or video.

All of these upstream feeds are combined to feed the input of the router and any external interference entering an unterminated feed can severely degrade the service for everybody by making it difficult for them to communicate with the internet.

Effectively your unterminated feed will become a receiving aerial, injecting local interference, short wave radio, baby alarms, etc. into the network.

A number of unterminated feeds can severely escalate this problem very quickly so yes, that terminator is very important.

To which Tweed replied, not unreasonably: " How does an unterminated coax act as an antenna?"

May answer - and note particularly the first few words! - was "Probably needs a better brain than mine to give a detailed  explanation but I've noticed this: "a transmission line with a high SWR tends to act as an antenna, radiating electromagnetic energy away from the line, rather than channeling all of it to the load" in:

https://www.allaboutcircuits.com/textboo...resonance/

Except for the signal direction, there is no intrinsic difference between transmitting and receiving antennae.

Monitoring the return path feed in a CATV headend with a suitable receiver can be very enlightening to anybody who doubts this ...

There seems to have been a fair bit of opposition to my explanation, including my last comment regarding the differences between transmitting and receiving antennae (which totally ignores the fact that, in this case, the 'transmitter' has an output frequency range of 85 - 750MHz!) so, do we have "a better brain than mine" to bail me out here ...?

[ppppenguin]

Terry, you're quite right about antennas being reciprocal between TX and RX. The only practical difference is power handling.

http://www.antenna-theory.com/definition...rocity.php

[Terry]

Thank you, Jeffrey - just what I was looking for!

[reset]

There seems to have been a fair bit of opposition to my explanation, including my last comment regarding the differences between transmitting and receiving antennae (which totally ignores the fact that, in this case, the 'transmitter' has an output frequency range of 85 - 750MHz!) so, do we have "a better brain than mine" to bail me out here ...?

Warning I do wander off subject!
About 55 years ago when I was studying antennas I made a one tube 400 Mhz transmitter and a simple receiver which feed a vacuum tube voltmeter (VTVM). The course material included a graph paper which was a series of circles and radial lines representing the compass direction. I built different types antennas starting with a simple dipole. Placing the antenna on the graph paper and rotating it  in 5 degree step and recording the VTVM reading gave the data need to produce a radiation pattern for each antenna design. The course material said the radiation pattern and the receive pattern would be the same. Once the chart was plotted I would change the antenna on the transmitter and the receiver and retest to prove they were the same. I was using a yagi and a dipole and it made no difference if the yagi was transmitting or receiving. So yes you are correct in what you said.
(The transmission line were bare solid copper wire so I could measure the standing waves on it too. The good old days!)

BUT...

In the computer age we have more to look at than just transmission lines and sending and receiving over them. Even the simple fire alarm system will have diagnostics build into them to "prove" system integrity. The computer will look for the terminator and when it is not found sound an alarm declaring a system problem. The terminator is at the last alarm on the loop and the test proves no wires are broken between the computer and the end of the loop.

The industrial computer (Modicon) will check for the terminator on both ends of the cable. I had one up and running with no terminators. The engineer and designer came in and put a terminator one one end the Modicon still functioned. They put a terminator on the other end and it failed. I put a resistor acoss the first terminator and it came back to life. Even after explaining to them that the first terminator was open and proving it by putting a resistor across it they insisted on switching the terminators from one end to the other. Modicon wants one at each end or none at all.

I said all that to lead to this one possibility. Maybe the TV company charges but the connection and they pole the system to see how many sets are connected, how many to bill for.

follow the money.

When my brother-in-law passed I called and told them he now longer required their services and they wanted all the cards from the boxes. They said they would continue billing until they received them. I was tempted to let it go and see how they collected from a dead man but I complied with their request and returned their property to them.

I do ramble but it could be food for thought.

73

[Terry]

Maybe the TV company charges but the connection and they pole the system to see how many sets are connected, how many to bill for.

No, that isn't the reason at all. The signal from each amplifier is split, typically, 48 ways to feed 48 homes. As all the splitters are inductive and, therefore, directional, there is plenty of isolation to the forward path to prevent reflections causing interference to other users. In the return direction, however, those splitters are now functioning as combiners and all unwanted signals appearing on those 48 feeders are combined and fed back to the head-end.

As a typical fibre node feeds 2,400 homes, it doesn't take much unwanted interference to cause major problems!

If the service is terminated, it doesn't matter whether the subs equipment is still connected or not because, if it is, it will be deactivated remotely by the same billing system that activated it in the first place and is responsible for communicating all changes in service levels, such as adding sport or movie packages to the service and so on.

However, the customer's set top box is an expensive piece of kit, which is why Virgin want it back. At the same time, they don't want the feed left unterminated - thus the instructions to fit the supplied termination.

[reset]

OK. I guess I missed part of the original question. In my case the four boxes are still at BILs house. They just wanted the cards. Maybe he paid for the boxes when they were installed???
73

[Terry]

Obviously, cable networks in different countries - or even within the same country - operate in different ways.

In the UK the cable companies traditionally have always supplied the subscriber equipment.

There were very good reasons for this from the start of cable as our 625-line TVs have only ever been fitted with UHF tuners, VHF being reserved for the obsolete 405-line system. So, with early networks being limited to about 450MHz, no cable channels were directly receivable. Later advances to 500, 550 & 600MHz only allowed direct reception of a few channels.

Contrast this with Belgium where sets were fitted with both VHF and UHF tuners. As all the channels originally carried were simply the national broadcasts of Belgium itself - both Flanders and Wallonia - plus Britain, the Netherlands, France, Luxembourg and Germany - plus one satellite channel, RAI Uno, to cater for a large number of Italian immigrants - there was no need for any channels to be scrambled. There was one subscription film channel which required the use of a special decoder but this was the exception to the rule.

Prior to the introduction of cable, massive aerial installations were common in Belgium. On the coast, for example, aerials pointed across the North Sea to Dover, north to the Netherlands, south to France and inland to the local Flemish and French language transmissions. Most sets were multi-standard PAL/SECAM multi-system sets which were very expensive plus the cost of all that hardware on the roof (plus the insurance!). TV reception on that scale wasn't cheap!

When cable arrived, it was a no-brainer. Cheaper single standard sets could be used, all the hardware quickly came down from the rooftops and multi-channel TV became cheap!

Initially, converter boxes were supplied to allow reception of the out of band channels between Bands I and III plus the Hyperband which extended to 300MHz but so quick was the uptake that receiver manufacturers quickly developed multi-band tuners for their TVs and all the converters disappeared.

In the UK, the emphasis was on the new satellite channels from Sky and British Satellite Broadcasting, subscription film channels and so on, so the special set tops weren't simple converters but also complex descramblers with two-way communications back to the headend and all owned by the cable operator.