Transmission line stuff
 

HI all,

A few questions have arisen in my mind since some guy came around to
install a wi-fi system at my girlfriend's apartment. This system uses
a dish and the signal frequency is 5Ghz. I asked the installer what
type of fancy, low-loss coax he was going to route the down feed with
and he told me "CAT 5" which I've never heard of. Anyway, it turns out
this stuff is essentially just telephone wire!
There must be some serious gaps in my understanding about transmission
lines, because on further enquriy it transpires the loss with this
wire is pretty low and the installer told me he can have a run of 100
meters before he has to worry about signal loss.
What's the deal here? How come I have to have some fancy, military
grad coax that costs a fortune and must be as short as possible just
for the 23cm band when this guy can use what is basically just phone
wire at 5Ghz and slop the stuff over half the building without a worry?

Transmission line stuff
 

phaedrus wrote:
HI all,

A few questions have arisen in my mind since some guy came around to
install a wi-fi system at my girlfriend's apartment. This system uses
a dish and the signal frequency is 5Ghz. I asked the installer what
type of fancy, low-loss coax he was going to route the down feed with
and he told me "CAT 5" which I've never heard of. Anyway, it turns out
this stuff is essentially just telephone wire!
There must be some serious gaps in my understanding about transmission
lines, because on further enquriy it transpires the loss with this
wire is pretty low and the installer told me he can have a run of 100
meters before he has to worry about signal loss.
What's the deal here? How come I have to have some fancy, military
grad coax that costs a fortune and must be as short as possible just
for the 23cm band when this guy can use what is basically just phone
wire at 5Ghz and slop the stuff over half the building without a worry?

I believe there's a converter/demodulator at the feed point, so all
that's being sent along the cat 5 cable is digital data at 100 MB/s or
less. There's no reason you couldn't use cat 5 cable as transmission
line up to 100 MHz or so, if you're willing to put up with the loss.
It's specified as 24 dB/100 meters at 100 MHz, which is about twice (in
dB) of the loss of RG-58. Loss of cat 5 cable would be extremely high,
and it would likely radiate pretty badly, at 5 GHz.

Roy Lewallen, W7EL

Transmission line stuff
 

Roy Lewallen wrote:
phaedrus wrote:
HI all,

A few questions have arisen in my mind since some guy came around to
install a wi-fi system at my girlfriend's apartment. This system uses
a dish and the signal frequency is 5Ghz. I asked the installer what
type of fancy, low-loss coax he was going to route the down feed with
and he told me "CAT 5" which I've never heard of. Anyway, it turns out
this stuff is essentially just telephone wire!
There must be some serious gaps in my understanding about transmission
lines, because on further enquriy it transpires the loss with this
wire is pretty low and the installer told me he can have a run of 100
meters before he has to worry about signal loss.
What's the deal here? How come I have to have some fancy, military
grad coax that costs a fortune and must be as short as possible just
for the 23cm band when this guy can use what is basically just phone
wire at 5Ghz and slop the stuff over half the building without a worry?

I believe there's a converter/demodulator at the feed point, so all
that's being sent along the cat 5 cable is digital data at 100 MB/s or
less. There's no reason you couldn't use cat 5 cable as transmission
line up to 100 MHz or so, if you're willing to put up with the loss.
It's specified as 24 dB/100 meters at 100 MHz, which is about twice (in
dB) of the loss of RG-58. Loss of cat 5 cable would be extremely high,
and it would likely radiate pretty badly, at 5 GHz.

Roy Lewallen, W7EL

I believe that standard fast ethernet (100Mbps) uses full duplex at
31.25 MHz with 4B5B encoding on top of MLT-3.

MLT-3 gives an effective rate of 4 bits per cycle, and 4B5B means you
need an extra 25% cycles. 100/4 - 25. 25 * 1.25 for the 4B5B - 31.25
MHz base frequency.

tom
K0TAR