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cable attenuation and signal frequency

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frankie
Ex Member


Dec 20th, 2013 at 7:51pm  
Please could someone explain a little about the cable attenuation measurement, and in particular how to check whether a cable is performing as it should be?

I'm trying to find some appropriate numbers in a cable attenuation chart but am not sure which frequency ranges to look at.

I understand (sort of) that the KA-SAT signal is 20 or 30 GHz, but I don't understand whether this means that the signal in the coaxial cable between the TRIA and the modem is also at those frequencies or whether it is converted to another frequency.

Anyway, I haven't been able to find an attenuation chart that goes up to 20 GHz so perhaps I'm looking for the wrong thing!

~Frank
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Reply #1 - Dec 21st, 2013 at 2:14pm  
TX freq 29.5 - 30 GHz
RX freq 19.7 - 20.2 GHz

Coax cable frequencies:
RX 300-800 MHz.  model TR4000N-10
RX 1000 - 1500 MHz.  model RM4200N-10
TX 1800 - 2300 MHz. 

Regarding the cable:
DC resistance maximum value = 2.27 Ohms
Attenuation maximum values 9.17 dB at 1000 MHz and 14.3 dB at 2200 MHz

Using good copper core cables, properly terminated, maximum lengths.
30 m using RG59 Cable
50 m using RG 6 Cable
100 m using RG 11 Cable.

If you want to view the spectrum in the cable with a high loss coupler make sure not to short circuit the cable or allow 30V DC to the spectrum analyser.

Any measured spectrums please tell me your location and sent to eric@satsig.net

There are four beam colours and I think these refer to four different frequency usages, to minimise interference between beams:
For example:
19.7-19.95  RHCP   29.5-29.75  LHCP
19.95-20.2  RHCP   29.75-30    LHCP
19.7-19.95  LHCP   29.5-29.75  RHCP
19.95-20.2  LHCP   29.75-30    RHCP 

This arrangement provides 250 MHz bandwidth per beam.
Considering the receive spectrum you will probably see several steady carriers, each 27 MHz wide with bit rate 67 Mbit/s in clear sky.   Towards beam edge locations, you should see lower level carriers in the adjacent beams.
On transmit, you may see fast transient, narrower bandwidth, bursts, whenever your site transmits.

Your receive C/N depends on the signal in the air as it arrives at your dish plus the G/T of your terminal.  The G/T depends on dish gain and receive noise temperature of the LNB. 75cm dish has G/T=+17 dB/K.   The C/N is hardly affected by cable length, until you go beyond the extreme maximum lengths.  If you have too high DC resistance, ant thus low supply volts/amps at the dish, the transmit and receive qualities will both deteriorate.
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