Long Freeview TV aerial extension cable - use F plugs & satellite cable?

[John Stumbles]

No. It's the wrong impedance.

[Ian Jackson]

Yes, the ethernet cable would be 50 ohms. I wouldn't use it out of choice, but there's a good chance it would still work. Try a run of it, and find out! Ian.

[The Natural Philosopher]

Not good. You will get standing waves - peaks and nulls

[Ian Jackson]

Please quantify. Ian.

[The Natural Philosopher]

Not possible without knowing cable length, specific losses and reception frequency.

[Ian Jackson]

OK. Let's say 10m, 20dB/100m @ 650MHz. F = 650MHz. Ian.

[The Natural Philosopher]

Bugger..its been 30 years since I had that sort of question in an exam..

Mm,. need to konow the relative propagation speed in the cable as well..assume its about .9c..so a quarter wave at 650MHz will be around

128cm..

So the 10m will be almost exactly 78 quarter waves..Hmm. You should get a modest peak around that length.

[Ian Jackson]

But how big will the peak-to-trough amplitude be?

What will be the worst-case slope across an 8MHz channel?

How much signal level will you lose compared with using 75 ohm cable with the same loss>

[To answer these question, you might choose to assign a notional worst case value to the STB input RLR .]

Will any of this have a noticeable impact on the quality of a digital signal?

Ian.

[Edster]

I swapped a 5m 75ohm TV cable for a 5m 50ohm ethernet cable with the ends cut off and twist on connectors on it, and didn't notice any difference.

[The Natural Philosopher]

The short answer is a 'dunno'. My background is from needing as flat a ferquency reponse as possible. We simply wouldn't introduce anything like that on principle. My gut feeling is around +-3dB over a wavelength..

[The Natural Philosopher]

In short cases you won't. In some cases you may. I prefer not to take the risk.

[Ian Jackson]

5m is still pretty long compared with the signal wavelength. At what point will length start to cause problems? Ian.

[The Natural Philosopher]

At any multiple of one eigth of a wavelength.

Think of a wrongly terminated cable as a classic example of multipath..one signal comes straight down the wire..one comes down the wire and bounces back to where it started and bounces back down the wire..and so on ad infinitum, with the strength of the 'ghosts' being a function of the cable loss over the length and the mismatch of the impedance at each end which generates some fraction of the power as a reflected wave. I can't remember the formula.

[Ian Jackson]

It really depends on how good the input RLR of the STB is. I don't know what is typical. For analogue CATV boxes, 6dB worst case (3:1 SWR) was typical, so on certain frequencies. It could actually be a better match for 50 ohms than for 75. I wonder what DTT boxes are like?

So, with the correct 75 ohm cable, there could be quite a lot of the signal which gets reflected back from the STB towards the aerial. You are relying on the aerial being a good 75 ohm match to prevent the reflection from being re-reflected back again from the aerial. If you use 50 ohm coax, a good 75 ohm aerial looks like an SWR of 1.5 (14dB RLR) wrt 50 ohms, so there will obviously be a reflection 14dB down. The result is that the reflection rattles back and forth along the cable, being progressively attenuated by the return losses of both the STB and the aerial, and the cable loss.

As I said, I wouldn't use 50 ohm cable out of choice. But, if you do feel the need to use it, things might not be as bad as you think they would be.

Ian.

[kenknott]

cables

see where RG6

Excellent link on cables. Thanks.