Dummy Guide to Scanning Antennas
Its a minefield out there... and as a newbie to the hobby it can be very confusing and overwhelming. I've put together this simple (ish) guide to help you understand some of the basics and to help you choose the correct antenna for your setup.
You've probably heard phrases like "What's the best antenna to buy", "What antenna do I need" etc etc... There's not a single answer to these questions as it all depends on many factors.
You may also hear people say "There's not a single antenna that can cover the whole frequency range of your scanner" This is somewhat true but I'll explain below...
Different services and radio users transmit on different frequencies and bands. When you hear a term such as 'Air Band' or 'Broadcast Band' this refers to a group of frequencies within that band. A good example is the FM broadcast band, it starts at 88MHz and finishes at 108MHz and it contains lots of musical radio stations. Whilst driving you might scan for radio stations using your car stereo. The car stereo will scan all the frequencies between 88Mhz and 108MHz and stop on a frequency if it detects a signal such as BBC Radio4. A radio scanner or SDR receiver works in the same way but on other bands and frequencies. Here are a few common Bands and their frequencies:
Civil Aviation Air Band 118MHz - 137 MHz
PMR Walkie Talkies 446MHz - 447MHz
Pagers & Telemetry 150MHz - 165MHz
Marine AIS Band 158 - 163 MHz
Military Band 220 - 400 MHz
To receive any of the bands and frequencies above you require an antenna. Technically speaking anything conductive such as a metal coat hanger will act as an antenna and will receive signals from the radio spectrum, but how efficient it is at 'receiving' is the question?. You want to be capable of receiving the most faintest & distant radio signals as possible, not just the really strong ones local to you. This is where the correct antenna comes into play!
If you take a coat hanger and connect it to your antenna socket you will receive signals from the whole radio spectrum, after all the metal hanger is conductive! For example you would easily receive your local BBC FM radio station on 98.8 MHz or a nearby Walkie Talkie on 446MHz, BUT if you tried to receive the same frequencies from several miles away where the signal strength is much weaker it would perform poorly and you probably wouldn't receive anything at all (even though the signal is present in the air)
This is because the coat hanger is not Resonant at the frequency you wish to receive. When you hear the term 'Resonant' or 'tuned' this simply means that the antenna is manufactured for optimal reception at a specific frequency.
Unfortunately some manufacturers may label this coat hanger as a 'Wide band scanning antenna' because technically it does receive all the frequencies stated however it receives them poorly. DONT be fooled by cleverly worded or false advertising and marketing. All of Ceecom antennas have technical datasheets and S11 readings to illustrate how well each antenna performs at specific frequencies.
How do you know what antenna you'll need? This all depends on what you want to listen to?
Lets imagine you're only interest is listening to Civil Aviation within the Air Band (You must be authorised to do this by the way). A single band antenna such as an 'Ceecom Air-Stick' that is specifically tuned to the Airband frequencies (118-130MHz) will perform great and and really kick ass!!!. It will receive weak and distant signals outperforming an antenna designed for a wider spectrum of frequencies such a Wideband Ceecom Discone Antenna (25-1300MHz) or an antenna design for a different band such as a PMR 446MHz radio.
A wideband antenna is always a compromise over a single band antenna. An easy way to understand this is to remember that the wider the frequency range the less efficient the antenna becomes at receiving a single particular frequency. Only buy a wideband antenna if you wish to scan a wider band of frequencies such as Airband, PMR and business radio all at once, it will still perform very well. If you only want to scan a single band such the Airband then buy an antenna designed for this band only !
An example of this is as follows. If we try and use a Ceecom Airband Air-Stick Antenna to receive a different band such as 'Business Radio Band' (around 460MHz). The Air-Stick antenna would receive frequencies from the Business Radio Band however not very efficiently, similar to the coat hanger scenario explained above. You may not hear the weaker transmission from the business band, where as a dedicated Business band antenna would.
So what the heck now? its impractical to have a separate antenna for each band so what should I do?
Lets imagine you want to listen to the 'Air Band' and the 'Business Radio Band' ? A serious scanning enthusiast may have two separate antennas installed, one for 'Airband' and one for the 'Business Radio Band', each connected to a switch box to toggle between antennas. As a beginner this becomes costly and impractical, you'll probably want an antenna that can cover both bands in one at a slight compromise. A perfect antenna for this scenario would be a Wideband Ceecom Discone 1300 Antenna. It will receive at a slight compromise as the antenna is no longer a single band antenna and therefore performance of each band (Airband & Business Band) will diminish slightly, however you'll be able to scan from about 50mhz right upto 1300mhz. Being wideband does not mean you cannot receive the signals, they are still received and perfectly audible on your scanner but just a little weaker than a single band tuned antenna.
To measure how well an antenna performs at a specific frequency we use a measurement called s11 Return Loss. The better the s11 decibel return loss reading the better the tuning of the Antenna. All of Ceecoms antennas have Return Loss readings so you can clearly see what bands and frequencies the antenna will operate at and how well. This way you can see if you're buying a coat hanger or a quality tuned Antenna !
To help you decide I've made an Antenna S11 Return Loss Comparison page, simply click on the antenna of interest and it will show you how efficient it is at particular frequencies.
Now hopefully you're not too confused because it's about to get a little more confusing. We are now going to talk about Gain...
Its great to have some gain on an antenna as it can effectively amplify the incoming signal, however gain is only a good thing if the gain of the antenna is at the angle we want to receive at.. Angle you say, what do you mean? Lets explain
So a basic dipole antenna has 2.15dBi gain or 0dBd gain. These are both the same figure.
The dBi figure means gain in comparison to an isotropic radiator (A dummy antenna in space, used for modelling purposes) and the dBd figure is in comparison to a Dipole. Because a Dipole is a Dipole it cannot be compared it to itself and therefore has 0dBd gain (remember dBd is compared to a dipole).
A dipole in free space has a doughnut shape radiation/reception patter with about 70-degree beamwidth. To achieve gain we have to take power from somewhere in this doughnut shape and add it to another part of the doughnut to achieve the gain. If you take a doughnut and squash it between your hands it will flatten out and the doughnut will become wider and flatter, the area where the doughnut becomes wider is where we achieve the gain. The Doughnut is now flatter and therefore we might only have a 40 degrees beamwidth radiation pattern (it was 70 degrees before we squashed it). Depending on how we squash the doughnut the wider parts might be at different angles, and this is what we call the radiation angle. If you live on a very high hill an antenna with low angle of radiation and high gain probably wont benefit you as it will only have a very narrow window of reception and might not be able to hear signals below it very well, unless of course the next town along is on a Hill then we would receive this fine.
Below is a very basic diagram explaining this (image based on freespace radiation patterns to keep things easy to understand, it shows a slice through the doughnut looking in from the side). This is why one antenna might perform excellent for one person yet perform extremely poorly for somebody else, you often see this when people ask on facebook groups what the best antenna is to buy, there is not a correct answer as it depends on location and what you want to listen to.
Are you confused yet? Theres one more thing to consider. Dual Band Antennas and angle of radiation.
We often see antenna advertised as covering different bands, let's take our 2m Band Dipole. This is tuned for 145 MHz however it also tunes very nicely on 70cm Band at 435 MHz. This is because 435MHz is a harmonic of 145 MHz (its 145 x 3= 435).
Now although this tunes up very nicely on 435MHz and it can be used to transmitting and receiving it has a very different radiation pattern on the higher frequencies. If you're using a 2m Dipole for 70cms it'll work but the power is not focussed where you want it. Same goes for an Airband Antennas tuned to 125MHz, some sellers would tell you this also works on military bands and its does, but its angle of radiation on the military bands is higher and lower rather than focused directly outwards into the horizon.
Below is an example of a 145MHz dipole being used up on 435 MHz, you can clearly see the different radiation patterns, this also applies for reception when scanning. The image shows a slice right through the middle of the dougnut looking in from the side with the antenna in the centre.
Are you confused yet... :-) I did say its a minefield at the start. If you need any help please don't hesitate to contact me and I'll be happy to help