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Microwaves and Decibels

This article demonstrates the need for decibels in the microwave world.

Written by: G4RFR ( FRARS ) Wednesday, 3 April 2002

Microwaves and dB's

Recent articles on Microwave systems and equipment by Paul Marsh and John Fell concerned themselves with generating a microwave signal and getting it from point A to point B. This got me to thinking: How much power is needed? What about cable losses? What happens to the signal when it leaves the transmitting antenna? What sort of calculations are needed to predict whether or not a planned microwave Microwave system will work?

Take for example the transmitter to antenna coaxial cable losses at 2GHz. RG-58 has an attenuation of 0.35dB per foot. If the cable run is 30 feet the attenuation would be 30 x 0.35 = 10.5dB. This of course would be intolerable for a microwave system at 2GHz. But the point I am making here is that by using dB for the calculations, the calculations are fairly easy. Doing it the other way where 0.35dB represents a ratio of about 0.92:1. This means that 92% of the power at 2GHz gets through 1 foot of RG-58. Now to work out 2 feet of cable 92% of 92% of the power gets through. For 4 feet the power throughput would be 92% of 92% of 92% of 92% of the input power. How about carrying on out to 30 feet of cable. No way! Using the old tried and true maths here is not such a good idea. I personally prefer the dB method.

How about microwave antennas? Their gains typically run from about 3 to 36dB. (I have seen them as high as 50dB). Here again, these are ratios running from 2:1 to 4000:1 which translates to gains of from 2 to 4000. Besides being a very wide range of numbers, the gains would have to be multiplied by the power from the coaxial cable to get EFR (effective radiated power). Using dB, all I have to do is add cable losses and antenna gain to the transmitted power to get the EFR stated in dBW. Did you ever wonder why they started specifying maximum allowable power in dBW rather than Watts? Well-----

Lets have a look at what the signal is doing traveling through space. The signal is not actually attenuated in free space (vacuum) In air it is negligible. Actually it is its spreading which results in signal intensity decreasing. The rate of spreading happens to be the inverse square of the distance and is determined by the antenna gain. It's easy to see that working this out with the old "tried and true) maths would be horrendous to say the least. Again, the dB to the rescue.

At the receiver end we are concerned with what the signal level is. Being use to micro volts, calculations
using them would still be a problem. Since most modern receivers have a common input impedance of 50 ohms, there is no reason why we can't specify input signal levels in terms of power rather than volts. The dBW, or better yet dBm (dB with respect to one milliwatt) seems to fit the bill rather nicely.

So there we are, all of the relevant microwave parameters specified in dB. No more multiplying awkward numbers, just add the dB's to predict a microwave system performance. An example of such a set of calculations can be found on the link below called "MICROWAVE CALCULATIONS" . .

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