Sunday, 9 March 2014

Coax Cable Loss

Antennas receive the signal, receiver makes it understandable. Between these two coax cable carries said signal, which gets more and more tired the further it travels (coax length) and the bumpier the road (coax quality). 
Being tired means you lose energy, so poor signal cannot tell you all the information.
Less info is called signal loss, how much depends on coax cable quality and length. 

To improve a signal you can:

1) Decrease the length of coax cable - less travel, more signal,
2) Improve coax cable quality,
3) Add a preamplifier - this is like an energy drink to signal.



Get rid of coax cables altogether...

No coax cable mean no costly connectors, expensive low-loss coax and the hassle of connecting everything together. 
Simply use a Raspberry Pi 2 - if you're not familiar with microcomputers, my upcoming RTL-SDR book contains a separate chapter dedicated to installing and setting up a RasPi-based server.

How to calculate signal loss


Coax cable loss increases with frequency. Online resources give this loss in decibels, which is logarithmic, so 3 dB is half the power  - transmitting or receiving makes no difference. 
Decibels and logarithmic stuff is complicated, so measure the

Coax cable loss, attenuation, RTL-SDR, RTL, SDR, AIS, Marine, sdrformariners, SDRSharp, improve reception, VHF, marine, yacht, sailing
length of coax cable, note the writing on it and google this plus "attenuation". Attenuation means weakening of signal in radio speak. So if writing says RG 6, google "RG 6 attenuation". 
Get a decibel figure with this online calculator. Enter values on top - results in feet will be on the left, meters on the right, you get a figure in decibels.
To understand decibels in a "this is how much percent of info reaches my receiver ", use this online calculator. It simply converts decibel to understandable percentages, with 1 being 100 % of signal received at the antenna.
Coax cable loss, attenuation, RTL-SDR, RTL, SDR, AIS, Marine, sdrformariners, SDRSharp, improve reception, VHF, marine, yacht, sailing
Enter number to the left of dB with a minus sign, select energy size on the right, then press calculate. 
For example, common RG 58 in 20 metres 4.4 dB is lost: only 36 percent of signal reaches the receiver.

Decrease the length of coax cable


More length = more loss.
Coax cable loss, attenuation, RTL-SDR, RTL, SDR, AIS, Marine, sdrformariners, SDRSharp, improve reception, VHF, marine, yacht, sailing

Use the minimum length of coax required between antenna and receiver; excess lengths will not only lose more signal but will also pick up electrical noise.
Finding an antenna location is a compromise, in an urban setting getting away from household electricity, gadget interference and TV radiation can be challenging. 
In the illustration the difference between signal strengths might be small on screen, but that -2 dB means 37 percent of the signal is lost.

2) Improve coax cable quality
Coax cable loss, attenuation, RTL-SDR, RTL, SDR, AIS, Marine, sdrformariners, SDRSharp, improve reception, VHF, marine, yacht, sailing


Better coax will lose less signal. Four times less loss is four times the cost, so selecting a cable is entirely up to your budget.
LMR-400 is commonly used, which is the upper limit in terms of cost and flexibility for mortal souls. 
Performance improvents will be only realized with long cable runs, and / or high frequency applications such as ADSB.
Check the table to see the LMR-400 has much lower dB loss figures - but performance comes at a price. 

Use a preamp
Coax cable loss, attenuation, RTL-SDR, RTL, SDR, AIS, Marine, sdrformariners, SDRSharp, improve reception, VHF, marine, yacht, sailing


For receive - only applications mount a preamp at the antenna to overcome long lengths of coax cable signal loss. 
Punters seeking signals above 800 MHZ - such as monitoring trunked emergency comms in the States - or ADSB enthusiasts hunting 1090 MHz will need a preamp with cable runs over 30 feet / 10 metres. Most of these require line of sight / unobstructed view of surrounding terrain hence necessitate long cables to mount antenna on top of the house.

If you enjoyed this article, please pre-order my RTL-SDR
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