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TERMS: FOB Cambridge, Mass. Send check or money order, included postage, average wt. per package Vi lb. Allow for COD. Minimum order $2,00,

P.O. BOX 74, SOMERVILLE, MASS. 02143

Ray Thrower WA6PZR 964 Montgomery St. San Carlos, Calif.

So, you want to go on amateur microwave? Fine! And, you have your equipment built? Excellent! And it tests out fine on t ie bench? Wonderful! But you can't communicate over a full sized path? Too bad! But don't feel lonesome. Many hams share the same Fresnel zone, to coin a phrase.

Almost without exception, articles dealing with amateur microwave communication have dealt solely with construction of the equipment. Relatively little, if anything, has been said about the path the beam must take in getting from transmitter to receiver. Just because you have a line of sight path does not mean you will be able to communicate via microwave.

Unlike on the amateur high frequency bands, (160-10 meters) where little consideration must be given to the path the signal takes, microwave communication takes something a little more than just putting together a pair of transmitters and receivers. Consideration must be given to such things as

1, Path Loss

2, Transmission Lines

3, Fade Margin

4, FM Improvement Threshold

5, Reflections, Refractions, Diffractions and Fresnel Zones

6i Antenna Gain

7: Antenna Orientation and Polarization

8, Passive Repeaters

9; Path Profiles

Singly, none of the items above are difficult to understand. And collectively they can save a lot of headaches trying to figure why a microwave system won't work.

Ray is a transmission engineer. He has thirteen years experience in commercial and military communications:

Path loss

Let s start at the head of the list with Patli Loss. This is the calculated free space path loss that will occur to the beam going from transmitter to receiver. Path 'ajss is not attenuation (reduction) of signal strength due to the beam encountering any obstacle such as air, dust particles, clouds and the like (although these will cause attenuation I. Path Loss is the result of the normal spreading ot the beam that occurs to all radiations, including light.

In microwave work we must try to approach or achieve free space conditions, since trees, shrubs, buildings and the like can absorb from 12 to 46 dB per mile, or more, of the microwave signal. In most cases this would render microwave communication out of the question.

Free space path loss is easy to compute if you have a basic knowledge oi logarithms. If you don't, a high school course in trigonometry with it s associated Study of "logs" is recommended, Anyway, to compute path loss, use the following formula:

where D = distance between antennas in miles i fMIJÄ = the frequency in megahertz and

Path Loss is expressed as a logarithmic ratio, in dB,

As an example, assume a pair of transmitter receiver terminals (let's call them a system), are operating on 1213 MHz and the distance between the terminals is 30.0 miles. Then we have a Path Loss equal to

36.6 + 20 X L477 + 20 X 3.085 = 36.6 + 29.5 + 61.7 - 127.8 or about 128 dB

So, we have a Path Loss of 128 dB. Of course, if using the formula is too hard, you can al ways refer to the nomograph in Fig, 1 and come just about as close.

Transmission lines

Well, 128 dB is a lot of loss and there is some additional "system loss" in the coaxial cable from transmitter to antenna and from antenna to receiver. For instance, RG-8/U, as used in our hypothetical system, has 3 dB loss for about 30 feet at 1215 MHz. This is the same as cutting your transmitter power in half. So, if you are going to the trouble of getting on microwave, its recommended you use something better than BG-S/U, such as RG-17/U. It has only 1.3 dB loss at 1215 MHz for 30 feet. Remember, i£R costs only a little more to go First Class/*

Fade margin

In the design of your system, you will need to consider the degree of reliability you want. This will be a function of your Fade Margin Fade Margin is the arbitrary signal margin established by the designer and is a function of Reliability. The fade margin establishes the amount of "cushion" you will have to fall back on, in case of a deep fade, before your receiver starts to controlled by noise rather than signal. To determine your Fade Margin, first decide the amount of reliability you want by referring to Fig, 3,

If you have built your equipment, you should know your receiver FM Improvement Threshold (FMIT), If you don't know your FMIT it is easy to calculate,

NF = receiver noise figure in dB and

Ln our example, assuming a noise figure of 13 dBm and a bandwidth o1 3 MHz, we have an FMIT = -104 + 13+10 log 3 =

Rounding this off we have an FMIT of —86 dBm. This is the knee of the inherent thermal noise characteristic of your receiver, the "crossover" point where internal thermal noise of R1 siena! strength becomes the controlling factor of receiver usefulness.

Assuming wre would like a 20 dB Fade Margin for 99*£ reliability w ith our FMIT at —86 dBm. our median received signal must be — 66 dBm or more (by more, we mean going in a less negative direction; — 65s —64, etc.). To get a signal of —66 dBm we have to

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