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900MHz Cable Feeds

J

Joe G \(Home\)

Jan 1, 1970
0
Hi All,

I have a 900MHz radio system and I want extend 50ohm coax cable from 3m to
20metres.
(well.... I mean - replace the 3m with a complete 20metre section with
proper termination coax connectors etc)

What losses and reflections should I watch out for?

Eg if the cable is a multiple of the 900Mhz wave length or 1/4 wave length
etc... is there refection effects.

Obviously there will be losses rated in db per metre (foot) etc....


What else should I pay attention to?


** PS the antenna is std 50ohm termination and the transmitter is 50ohm
termination and the cable is 50ohm coax style - already.

** Transmission is spreadspecturm **


???? Is this the right NG? If not - please advise which NGs might be more
appropriate.

Thanks in advance.

JG
 
Joe said:
Hi All,

I have a 900MHz radio system and I want extend 50ohm coax cable from 3m to
20metres.
(well.... I mean - replace the 3m with a complete 20metre section with
proper termination coax connectors etc)

What losses and reflections should I watch out for?

The obvious question is: how much loss can you *stand* in this
application?
(X-posted to more appropriate groups)
 
G

gravity

Jan 1, 1970
0
The obvious question is: how much loss can you *stand* in this
application?
(X-posted to more appropriate groups)

hardline might work. quite expensive.

i'm not sure what the losses from the connectors would be in a hardline
system.

Gravity
 
J

Joerg

Jan 1, 1970
0
Hello Joe,

I have a 900MHz radio system and I want extend 50ohm coax cable from 3m to
20metres.
(well.... I mean - replace the 3m with a complete 20metre section with
proper termination coax connectors etc)

What losses and reflections should I watch out for?

If both ends (or at least one) are terminated with line impedance there
should be no reflections. Losses will be listed in the cables specs.
There should be a graph, losses per meter versus frequency, or at least
a table that contains losses at 1GHz. If frequencies that high aren't
listed you need a better cable. If there are no loss specs at all don't
even consider buying the cable.

Let's say you arrive at 3dB which would be kind of high for 20 meters.
That would tell you that half the transmitted energy doesn't arrive at
the other end and that your SNR would be 3dB worse than if the receiver
had been directly connected to the antenna.
 
J

Joerg

Jan 1, 1970
0
Hello Joe,
Why is the SNR 3dB worse?

When the cable loss is 3dB then the signal received at the antenna will
drop by 3dB before it reaches the receiver. The receiver's noise
performance, however, will remain the same unless it's mismatched.

If the other station had a similar cable added you'd lose a total of 6dB
because its transmit energy would have dropped by 3dB before reaching
its antenna.
 
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