The principles of Scanning - Part 1

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Originally compiled by Milairman on the website.

THE PRINCIPLES OF SCANNING VHF and UHF (Part 1) milairman

There you are, you have just obtained your first Air band Scanner, plugged it in, pushed a few well known frequencies into a bank, started the scan and heard exactly nothing ! It has happened to us all so you are not alone. So let us go back to square one and try to fathom out why we don't always hear what we hope to.
There is no need to delve too deeply into the way radio waves travel through the atmosphere. The easiest principle to remember is the 'line-of-sight' rule. In general terms signals may be lost to your receiver if there is a major 'obstacle' between it and the transmitter. Such obstacles may be 'high ground, or even high buildings. Rather like selling a house its a case of location, location, location. For example when following Air to Air Refuelling it makes a huge difference to the quality of the received signal depending on the height that AAR takes place. The main area I follow is some 100 miles from my location. AAR at say 21000 ft results in good communications where both sides [chicks and Tanker] are heard easily. Drop the AAR height to say 17000ft and results can be, at best, mixed. Below that threshold no signal may be received here at all. I have a range of hills to the East and these effectively block incoming signals once the aircraft get too low.
So let us {censored} that distance and height are working in our favour, there are 2 further factors to consider, both of which are absolutely critical to obtaining good reception. These are the sensitivity of your scanner and the quality of your antenna. I personally rate a good antenna as being far more important than a good scanner. The rubber-duck type antennas that come as part of the standard kit with most scanners are by their very nature, a compromise. Nothing that small can hope to achieve consistent gain across the frequency range we are interested in....say 117MHz > 400MHz. If you are sat beside an airbase and only interested in transmissions in the immediate area then such antennas are fine. If on the other hand you are sat at home say 50 miles away from the base you are interested in then it is almost inevitable that you'll hear very little of any ground station transmissions. So whilst inbound aircraft may be heard as they start their descent, facilities such as Ground, Tower, Approach, Director, PAR etc originating on the base may simply not be audible. This results in a 'one-sided' conversation, a phenomenon that we are all familiar with.
Conventional wisdom suggests that you should opt for the best available antenna you can finance. It should be fitted externally, and mounted as high as you can safely get away with. There are as many opinions as to the best outside antenna as there are models available. I don't propose to get into heated arguments as to which antenna does the best job. It is a case of horses for courses. In general terms either a specially designed vertical antenna or a broadband discone are equally likely to produce good results. I actually use the latter in the form of a Skymaster Broadband discone and most days it does the job.

Next, although it is stating the obvious, buy the best [and in most cases that means the most expensive] scanner you can afford. The price range available is enormous, varying from say £80 right through to £1500. Like everything in life we finish up with what we pay for.
It is no function of this article to engage in a lengthy review of all the scanners available. Plenty of sources exists on the World Wide Web for folks to do their own research.

Now to more practical techniques that will improve your enjoyment of the hobby.

It is a somewhat perverse truism that the more frequencies you scan, the less you hear. Take for example the accepted range of UHF frequencies. To round the figures up we'll use 230MHz> 400MHz. On the usual 25KHz channel spacing there are 40 channels per MHz so 40 x 170 =6800 individual channels. [Some of these are now dedicated to Police TETRA transmissions] but in general terms let us take a ball park figure of 7000 channels. If your scanner is capable of searching say 40 channels per second then its going to take about 3 minutes to scan the entire range {censored} that no local sources of interference cause the scan to stop. So this is the order of magnitude you encounter if you are SEARCHING for a frequency that you know has changed but don't know which new one has been allocated. Remember also that the 'new' frequency has to be 'active' for it to be heard. The needle in the haystack theory applies here. It is true to say that a 'new' frequency may be more likely in some areas of the range than others, but over the years such allocations have become more random so even this is no longer a useful guide.

In the case of the usual VHF range say 117>136MHz the situation looks simpler. Only a 19MHz spread, so 19 x 40 = 760 channels. However the recent introduction of 8.33 KHz channel spacing means that the 40 channel per MHz rule no longer applies. Also in this range there are many [certainly the vast majority] Civil allocations so much time can be wasted listening to frequencies that are not 'military'.
Take the case of an 'enemy' wanting to listen in to say Tower frequencies. If they were all allocated in the range 335.0000 to 337.5000 then new allocations wouldn't take a lot of discovering. 100 channels...say 2.5 seconds to scan. So we can't reasonably expect the powers that be to make our job any easier and quite rightly so.
There again the number of monitors who actually run a SEARCH rather than a scan is miniscule. Most rely on the folks that do SEARCH to spoon feed them with their newly discovered frequencies without ever doing an iota of work themselves. This has always been the case and seemingly always will be.
Most modern scanners allow the programming of individual banks which may be linked. Take the case of UK 100 series TADS. Again in round figures there are about 155 of these. {censored} a scanner bank will hold 100 frequencies then 155 into 100 won't go. So at least 2 linked banks must be utilised to cover them. Oddly enough this is the one case where banks are usually NOT programmed in ascending frequency order. It makes sense to use banks 0 and 1. Why ? well TAD 01 goes in bank 0 channel 1 through to Bank 0 channel 99. Note Bank 0 channel 00 is left unused. This covers the TAD range 01 >99. Then starting at Bank 1 channel 0 > Bank 1 channel 55 we only need to read off the Bank number and add the channel number to know which TAD the scan has stopped at. Simple really once you know the technique. It doesn't take a mathematician to realise that if your scan stops at Bank 0, channel 88 then that is TAD 088 etc.
The same logic applies to those who follow STUDS. Program a bank of your local base frequencies on the same basis. Since STUD 01 is almost invariable Ground, 2 = Tower etc then you know the studs from the displayed channel number if you adopt a logical approach to frequency entry. Here again it implies that frequencies are not in ascending order.
The exact opposite applies for those who follow say Mil Radars. If you decide to program a bank of say LONDON MILITARY EAST radar frequencies then the scan will be more efficient if you take the time to add these in order of ascending frequency. Manual programming is a painstaking and digit aching process so it pays to program alternate channels in a bank so that additional frequencies can be entered without having to reprogram the entire bank. It always pays to plan ahead and not open a frequency guide and immediately plunge in to shoving frequencies into your scanner. Some scanners allow the use of 'text entries' for station names. Here it pays to use a dedicated bank for your local airbase. That way you don't have to enter the name of the base multiple times, just the normal abbreviations GND, TWR, APP, DEP, DIR, PAR1, PAR2 etc. That way it makes life and programming a lot easier and tied in to STUD by channel it is a lot more informative and makes any change immediately noticeable.