We explain, as simply as possible (remember it was written for us), some of the mysteries of the wireless world and the terminology used as it relates to the various ISM & UNII (i.e. 'licence-free') bands. If you are not comfortable with the terms Spread Spectrum, Frequency Hopping or Direct Sequence we have an outrageously biased overview.
In the wireless world you need to know some stuff (actually quite a lot of stuff) before you can be certain of a successful link. We try to explain the stuff you need to know as simply as possible and to give you some rules of thumb to work with. Finally, if you just want to see if your proposed link might work (down hill with a following wind) or not, try our wireless calculators and if you don't understand a *!?! thing come back here.
This explanation and our wireless calculators use approximations and generalisations that purists may find objectionable. To mitigate this objection we offer two explanations. First, we have included, in the side-bar links section, references to the real stuff (serious equations, explanations, tutorials and other brain-hurting material from people who really know their stuff). Second, at the gross level we are working most of the detail amounts to some decimal points in various calculations. If the decimal points matter you are already in big trouble and the decimal points, one way or another, are not going to save the day. The rule of thumb for all wireless systems is have at least 30% more POWER than you need with a minimum of 5 - 10 dB. We call this the SAD (Seriously Approximate Deviation) Factor - mostly this is called the System Operating Margin (SOM).
A MARGINAL RADIO LINK IS A VERY BAD IDEA™.
This section covers Power Budgets, Free Space Propagation, EIRP, the deciBel or dB), antennas, cables and connectors, Fresnel Zones or Effects and finally some more outrageous 'rules-of-thumb'.
HEALTH WARNING: This information is relevant ONLY to low-powered wireless systems running in the ISM or UNII bands.
The term 'Power Budget' (a.k.a. EIRP) is widely used in the wireless world. Simply put the power budget is the sum of all the gains and losses in the pieces that make up your radio system and which confusingly may be expressed in various terms.
We convert all these pieces to a common base (the deciBel or dB) and then perform some simple arithmetic.
You create a power budget to make sure that you have enough legal power to achieve the desired result. Note: Legal in this context simply means within the limits imposed by the various regulatory bodies.
Let's first look at all the 'pieces' in our wireless system and their typical 'power' values:
In the above diagram everything Red'ish is bad (it loses power) and everything Green'ish is good (it gains power).
Radio power (more properly transmit - TX - power) (A and G above). Usually expressed in milliWatts (mW) which adds power (use this table or this calculator to convert mW to dB). Radio Power always has a maximum value set by the regulatory bodies.
When a radio is receiving (A and G above) the important parameter is its sensitivity which is usually expressed in negative deciBels (dB). Typical values will be -70 to -110dB. The bigger the negative number the more sensitive the receiver.
The antenna cable which loses power (B and F above). Uusually expressed in dB. The outrageous rule-of-thumb is antenna cable will cost you between 0.25 and 1 dB'ish per 3 meters (~9 feet) at 2.4 GHz so runs should always be kept as short as possible. Cable info.
The various connectors (B and F above), which lose power, from the radio source to the antenna. Normally expressed in dB.
The antenna gain (C and E above) usually expressed in deciBels (dB sometimes in dBi) which adds to the power output of the radio.
Free Space (jargon for air) Propagation (D above) over which line-of-sight distance you want to send your wireless signals.
LoS or Line-of-Sight - jargon for being able to see the receiver from the transmitter. Unfortunately LoS is not enough - radio waves have a bad habit of bumping into things and scattering all over the place. You also need to make sure that you have a minimum amount of space around the LoS. This is called the Fresnel effect after the French mathematician.
Note: You will see the word 'Attenuation' all over wireless specs and documents, for the purposes of this explanation - it's bad and means 'losses'.
To calculate the EIRP (Effective Isotropically Radiated Power), more commonly called the Power Budget, of our wireless system we are going to get the power ratings of all the 'pieces' of our system and convert them to a common value (the dB) so we can perform some simple arithmetic. Use the links for each radio system 'piece' above to find the various values for your system (or our Power Budget calculator) and then perform the following calculation:
Radio power(dBm) + antenna gain (db) - cable loss (dB) - connector loss (dB).
Assume a 30mW radio (= 15 dBm from here), with a 12 dB Yaggi antenna connected with a 30 foot cable run of RG58 cable (= 10 dB from here) and with an SMA connector on one end of the connection and an 'N' type on the other (= 0.6 dB from here). Our Power Budget is:
15 dbm (radio) + 12 (antenna) - 10 (cable) - 0.6 (connectors) = 16.4 dB.
Radio Transmission is a bit like conception, if you explain the process to someone and all the things that can go wrong, you end up considering it nothing short of a miracle that anything happens at all!
This is the story about what happens to a young and innocent radio wave when it ventures out for the first time.
Radio waves (or signals if you prefer) start from a certain point (typically the transmit antenna, dummy) and with a certain level of power (or energy) defined by the 'Power Budget' and expressed in milliWatts (or Watts if you are a big macho operator). Assuming your antenna is omni-directional it radiates power equally in all directions, called in the jargon isotropic and assuming perfect conditions (oh yeah!!) then your friendly radio waves continue in an ever expanding circle until finally overcome by exhaustion - actually they contine forever, well past the point that they can be received by anything sensible. Because the energy is finite, at any distance from the transmitter the same (original) energy is now spread over a MUCH larger space. The signal is said to have attenuated or gotten much weaker (or smaller). The analogy frequently used here is the stone dropped in the middle of a pond - the waves at the edge of the pond are much smaller than those close to the dropping point.
Somewhere on the edge of this circle we must have a radio, with its receiving antenna (oh, that's the problem!) which is capable of receiving and making sense of this much smaller signal. The level at which the receiver is still capable of interpreting the incoming radio signal is known as the Receiver Sensitivity and is typically expressed in negative deciBels (-dB) .
Now the world is not perfect and most of these friendly little radio waves, who never did anyone any harm, do not have a pleasant life. They tend to bump into things (buildings, leaves, people) and either get absorped or bounce off again in some random direction (just like a pool ball). They get pushed off their path by forces greater than themselves (gravity, Fresnel Effects) and succumb to mysterious aliments (Rayleigh fading etc.).
So our tidy picture of regular circles of friendly radio waves starts to look pretty chaotic with crazed radio waves flying all over the place (this is no exaggeration) and with apparently random 'kamikaze' radio waves arriving at our Receiving Antenna from all directions and at different times (or with different phases) 'cos they traveled different paths to get there. This phenomena is called MultiPath and is the single biggest problem for radio wave reception.
This is the end of our sad tale about the short lives of the friendly little radio waves and what happens to them when they venture out into the big, bad world. Bit like life really...
Free Space Propagation losses can be calculated here and generally the higher the bandwidth the more the loss over any given distance.
EIRP is the Effective Isotropic Radiated Power (Isotropic means 'in all directions') and essentially means the total power of your system. EIRP always has a maximum value set by the regulatory bodies.
The more interesting (but much tougher) questions are: how far will it go? or how much power do I need to get this distance? Our System Performance calculator may help you here.
There are - broadly - three factors that determine distance (or at least factors over which you have some level of control):
The EIRP of your system, that is, its total power output expressed in dB.
The sensitivity of the receiver expressed in dB (this time it's a negative dB)
Line of Sight (LoS). Are our friendly little radio waves going to get battered to death or have an easy run at the receiving antenna. If you have a clear arc of vision of about 3 to 5° round your target antenna you are unlikely to have problems (this clears the Fresnel Zones). Less than that and you are starting to take a risk. Don't forget:
Winter and summer. Trees tend to do strange things in the summer like grow and put out leaves.
Ice build up on antennas - especially Yaggis can knock 50% off your power. A radome (a plastic cover) is typical with Yaggis to minimize this problem.
We are told by our friends in seaside locations, that guano (polite word for bird *!*?) build-up is a very serious source of radio power loss in non-radome Yaggis. We have no experience ourselves of this phenomena!!
As you get past 3 -5 Km our friendly planet may get in the way (explanation of Fresnel Effects and Zones) or use our Fresnel Zone calculator which also tells you the earth's height/bulge at the link mid-point.
When you have calculated your 'Power Budget' add the SAD factor (Seriously Approximate Deviation) = 30% more. HAVE AS MUCH POWER AS YOU CAN AFFORD IN RESERVE.
Keep your antenna cables to the shortest possible runs - cable losses are serious - outrageous 'rule-of-thumb' is 0.25 to 1dB per 3 metres (~9 feet).
If you must have long cable runs use the lowest loss (= most expensive) cable you can find.
Have clear, unobstructed line of sight all year round including the 60% Fresnel Zones (and watch those low flying sea-gulls).
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