Is TV coax impedance important?

[Forrest]

I've noticed that TV coax is 75 ohms. I have seen other spools of coax at
Fry's that have a different impedance. Can it be used for TV antennas? The
reason I ask is that I have a spool of it laying around and a TV or two that
could use wiring to the antenna.

 

[Jerry G.]

If the impedance is not matched, the length is short, and the signals are
strong, you will not see any losses. But, these errors will add up. If the
length is going to be more than about 50 feet, I would certainly want to use
the right type of wire.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


I've noticed that TV coax is 75 ohms. I have seen other spools of coax at
Fry's that have a different impedance. Can it be used for TV antennas? The
reason I ask is that I have a spool of it laying around and a TV or two that
could use wiring to the antenna.

 

[Gary Woods]

If the impedance is not matched, the length is short, and the signals are
strong, you will not see any losses. But, these errors will add up.

There's a worse problem: If the line is not 75 ohm and the length is
non-trivial, you'll get reflected energy that will show up as ghosts.

Ungood.

 

[Andrew Rossmann]

[This followup was posted to sci.electronics.repair and a copy was sent to
the cited author.]

I've noticed that TV coax is 75 ohms. I have seen other spools of coax at
Fry's that have a different impedance. Can it be used for TV antennas? The
reason I ask is that I have a spool of it laying around and a TV or two that
could use wiring to the antenna.

One problem with the wrong cable is that it may not be the right size,
and you won't be able to install connectors. Or if you force it, you may
get a physically poor/loose connection.

For a short run of a few feet, you can get away with alot. Even a plain
piece of wire could work. Once you start getting about 10-20ft or so, you
may start to notice problems, especially at higher frequencies.

 

[Jerry G.]

I myself have used 50 ohm coax of the same type of diameter and gauge as 75
ohms for a short run, mostly temporary. Therefore I had no connector
problems. I was never able to see any reflection problems on even 100 ft
runs. I only had losses, especially in the high band channels. I use a
spectrum analyser to check my work when doing CATV jobs. Infact, even the
connectors should be rated at 75 ohms for TV work.

The 50 ohm system is generally used for radio communications. The 75 ohm
impedance type is mainly used for CATV work.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================

If the impedance is not matched, the length is short, and the signals are
strong, you will not see any losses. But, these errors will add up.

There's a worse problem: If the line is not 75 ohm and the length is
non-trivial, you'll get reflected energy that will show up as ghosts.

Ungood.

 

[Asimov]

"Gary Woods" bravely wrote to "All" (21 Nov 03 16:36:18)
--- on the heady topic of "Re: Is TV coax impedance important?"

GW> From: Gary Woods <[email protected]>
GW> Subject: Re: Is TV coax impedance important?
GW> Xref: aeinews sci.electronics.repair:12107

If the impedance is not matched, the length is short, and the signals are
strong, you will not see any losses. But, these errors will add up.

GW> There's a worse problem: If the line is not 75 ohm and the length is
GW> non-trivial, you'll get reflected energy that will show up as ghosts.

GW> Ungood.

One can match a load to a different impedance using a 1/4 wave length of
a different cable impedance, the value of which is the square root of
the former products, but the 1/4 wave only works at one frequency.
Another related method is the "series-section transformer" (the
equations are too big to include here). But for broadband one needs a
transformer with the right ratio.

Asimov
******

.... Reactance: your imaginary friend.

 

[Bob]

For a short run of a few feet, you can get away with alot. Even a plain
piece of wire could work. Once you start getting about 10-20ft or so, you
may start to notice problems, especially at higher frequencies.

My cable company thinks it is important enough to use good cable
and connectors, that they wil give me the cable and connectors I
need, even putting the connectors on for me if I can tell them
what length I need. I've had several cable employees tell me not
to use radio shack stuff.

Bob

 

[Daniel L. Belton]

Even a plain


or so, you



My cable company thinks it is important enough to use good cable
and connectors, that they wil give me the cable and connectors I
need, even putting the connectors on for me if I can tell them
what length I need. I've had several cable employees tell me not
to use radio shack stuff.

Bob

Cable companies has found out that it is cheaper for them to provide the
material and time to install the cable rather than having someone put in
the wrong stuff and start sending signals back over their entire grid,
causing all kinds of interference problems...