Determining distance of a transmitter
by trixter on Aug.20, 2009, under Radio, Zombies
I have been asked by a few people to explain how you can tell the distance of a transmitter with only a scanner. I have explained this a few times, and feel that it would be better to write it once more, and then just refer people to this page and be lazy about it 
The only thing that you need to determine the distance is a scanner or other radio receiver with a signal strength indicator. The finer the resolution of the signal strength indicator (or RSSI meter) the more accurate you can be. While this method is not 100% it will get you fairly close. If you take samples in a few different locations you can even estimate where the transmitter is to know better where to look.
This is by far not the most efficient or quickest way to locate a transmitter. It is effective if this is the only method available to you because you do not have a directional antenna, are not close enough to look for the ‘null’, or do not have something like a doppler system. It is something you can do if you have minimal equipment and are willing to try some trial and error efforts.
First in order to understand why this is possible you have to understand how radio waves dissipate over distance. There are two main types of fields given off by a radio transmitter, the near field and the far field. Most communications occur in the far field. The near field dissipates using the inverse cube rule, simply put this means that P=1/X3. The far field dissipates using the inverse square rule, simply put this is P=1/X2. P is the relative field strength, or power, and X is the distance.
This assumes an ideal world. Buildings, mountains, even the atmosphere itself can affect readings. This means that it is not highly precise, but can be generally relied upon to get you close enough that you may be able to use other methods to locate the transmitter.
If you have a scanner and the RSSI meter reads “10″ (for this example it does not matter what that means so long as the scale is linear), and you double the distance from the transmitter, then P=1/22 or P=1/4 or P=0.25*10 (our original reading). The RSSI meter should read 2.5 at double the distance. Conversely, if you are half the distance closer to the transmitter it would be P=1/0.52 or P=1/0.25 or P=4*10. If you notice that the power will quarter or quadruple when you double or halve the distance.
You do not have to double/halve the distance, you can use any sufficiently large enough distance to ensure a reasonably accurate reading. Just replace X with whatever the distance modifier is and you should be able to solve this easily. Most S-meters make doubling/halving the distance much easier to deal with, which is going to be explained in the next section.
But S-meters are not linear
In order to keep things simple the above example assumed that the S-meter reading was linear. This makes the math easier for explaining the algorithm. The reality is that S-meters are not linear, they are logarithmic.
Without getting into too much detail on what S-meter specs are since we do not care about absolute values but rather relative values very simply every S unit is 6dB. So the difference between S1 and S2 is 6dB. This goes up to S9. Above S9 there is usually a 10dB over S9 and a 20dB over S9 indicator and little else. Some low end radios do not even have this.
You must know what your radio actually does though, some radios may not use this scale, although if it is a ham radio odds are it does. For the enthusiast, the IARU started agreeing on standards as far back as 1981. Also know that the values displayed may not be accurate, especially for absolute values, but are accurate enough for relative values.
What is a decibel?
A decibel or dB is a logarithmic scale. It is always in relation to something else. Because the S-meter is providing us with all the readings we are using, the readings will always be relative to the same thing, so we can basically ignore what it is relative to.
A dB is a system based on 10 (deci). 10dB is 10, 20dB is 100, 30dB is 1000 and so on. Many people will say 3dB is double, however this is only an approximation and not accurate (its really really close though). This also does not help if you need to find out what 5dB is. The algorithm is not that complex, X=10(Y/10) where Y is the dB you want. Using this we can have the following:
| dB | Formula | Change |
|---|---|---|
| -6 | 10(-6/10) | 0.2511 times |
| -3 | 10(-3/10) | 0.5011 times |
| 3 | 10(3/10) | 1.995 times |
| 6 | 10(6/10) | 3.981 times |
| 10 | 10(10/10) | 10 times |
| 12 | 10(12/10) | 15.848 times |
As you can see it is extremely easy to solve this. You should be able to do this with a pen and paper although some people will need to use a calculator.
Since we know that the difference from S6 to S7 is usually 6dB, if we are receiving S6 and we drive half the distance to the transmitter the meter will tick up to S7. Likewise if we are driving away from it, it will drop by one when we have doubled the distance.
If we have a known point on a map with a known signal strength, we can drive around and collect other points and signal strengths. If we mark points that are North, South, East and West of our original location you can figure out the direction the transmitter must be, as well as the distance it likely is.
This is where trial and error comes in, and this can be time consuming. Odds are you will be able to get a general idea of where the transmitter is without going to all 4 compass points from your original location. This can save some time.
Now what?
You have gotten as close as you can with this method, but you want to pinpoint that transmitter even more. Well the only solution that you can try, and this may not work with all radios, is to drive around until you get the strongest signal possible. Generally if you are 20dB over S9 you have to use alternate methods to home in on the transmitter. If your scanner has a built in attenuator (generally 20dB) you can enable this, which will make the received signal strength weaker as the radio sees it. This lets you wander around a bit more trying to get even closer to the transmitter. If the signal strength goes down, you are going away, if it goes up you are getting closer. By mapping those points, you can home in even closer.
If you do not have an attenuator, or if you have enabled it and still are getting really strong signals, you can try to look for the ‘null’. One common method is to hold the radio to your chest, so your body will block the transmission in one direction. Then slowly rotate monitoring the S-meter. Look for the direction that it is the lowest. This is 180 degrees from the transmitter. Looking for the peak is difficult, so looking for the null is recommended. If you cannot locate the null, remove the antenna on your radio, if it is telescoping and non-removable collapse it as much as possible. See if you can get even the slightest weak signal. If so try looking for the null.
By doing this you can locate some transmitters in your area. It is not the best way, but it is better than nothing when you just have to know where someone is transmitting from. If you get bored, locate a local radio station or TV station. Find where their transmitter actually is (the studio where they produce the content may not be the same location as the transmitter). Try to hunt for the transmitter using this method. If you know where the transmitter is, it can be helpful initially so you know if you are getting totally wrong readings.
Once you have located known transmitters, try to locate some unknown transmitters. Remember though, you can only hunt for transmitters while they are transmitting. Occasional transmissions require a lot more effort, and patience and are not recommended for the first few hunts that you do.
A directional antenna is a really good idea to help you locating the general direction, and can save you a lot of time. In a future article I will discuss a method of making one out of scraps that may be laying around your house. It is not an ideal antenna, but it is quick, cheap, and easy to do without permanantly modifying your radio or antenna.
