Correct two-way radio system
Choosing the correct two-way radio system
The difference between radio frequency bands (frequencies behavior)
|LF Low band = 4~50Mhz||High||Furthest + Skip||Rural|
|VHF Midband = 66~88 MHz||Medium||Furthest||Rural/ semi urban|
|VHF High Band = 136~174 MHZ||Fair||Far||Rural/ Urban|
|UHF Ultra High = 400~480MHz||Low||short||Urban|
From above table the lower the frequency the further the communication and the more frequency noise. Therefore, you can image the following, try and walk with a 10-meter pole inside a house, it will be difficult than to walk with a ½ a meter pole. This rule applies basically the same to the wave lengths of the frequencies. A 30 MHz frequency have a wave length of 10 meters and a 480 MHz frequency have a wave length of 630mm. Therefore, a shorter pole (higher frequency) will perform much better in a built up area as lower frequency.
From above simulation graphs. The difference between midband 70.0000 MHz, high band 142.0000MHz and UHF 442.0000MHz on above calculations = Same sites; same output power; same antenna gain. This is a system design with frequencies versus each other side by side.
|100.55 dB||106.69 dB||116.56 dB|
|Blue line basically covers mountains||One mountain peak above blue line||More mountain peaks above blue line|
|There will be good communication||Very scratchy communication||No communication at all|
Above will apply on base station and mobile radio using unity gain antennae as per ICASA licenses.
When choosing a portable radio, the scenario changes completely, let I explain: -
Radio propagation work as follow, ground plane plus antenna. These two are very critical in an antenna design. You can purchase the best two-way radio portable on the market, but it boils down to the helical rubber duck antenna on top of your portable and the ground plane. (portable physical size, die-cast chassis)
See table below.
|Unit gain antenna length||+- 1070mm||530mm||170mm|
|Normal length of rubber duck antenna on portable radio||+- 200mm||+- 200mm||+- 200mm|
From above we can learn that the UHF 442.000 MHz portable radio is the closest to reality. Therefore, it will outperform the other frequency band radios, as simple as that.
This will also work in practice. Radios below with the same output power.
|Distance between to two portables in urban area||+- 300 meters||2 km||3 km|
|Distance between to two portables in rural area||3 km||4 km||6 km|
Distances above can vary due do obstructions and area.
Please remember these tests are based on simplex radio systems and a repeater network is completely another story and above will not apply. On a repeater network the system is design to the client’s requirements
Analogue versus Digital
Analogue or commercial two way radios as we know them. The comparison is only made between a general analogue radio and a general digital radio. There are three main digital systems CDMA (Code division multiple access), TDMA (Time division multiple access) and FDMA (Frequency division multiple access). I do not plan to go in detail on the various systems, we will leave this for another day.
There is one common rule that we must not forget and this applies to both analogue and digital. The older people will still remember that our radios use to be on 25Khz channel spacing and the South African Authorities change or channel spacing to 12,5 KHz in the year 1991. The radios on 25Khz use to speak further than the new modified 12.5KHz. All that has happened is that we send less information over the air using a smaller bandwidth. This also applied to data transmissions, slower data speed.
Therefore, the larger the bandwidth the further the communication and the better the data throughputs.
The analogue radio are straight forward and communications are instantly, no delays. With other words when you press the press to talk button (PTT) the communication is at once.
Whereas on digital communication when you press the press to talk button the voice gets compressed and made up in packets (protocols) to be send over the air, on the other side the radio that is receiving needs to unpack the packets and convert is back to voice. This normally takes a break of a second and we call this latency. The busier the system the longer the time and then we start the queuing process.
On a community repeater system, if one group communicate, using analogue will show busy and you need to wait until that group end their conversation, whereas the digital radio will communicate at once, no waiting. Only the third group onwards will experience latency and queuing.
|Can you receive scratchy signals||Yes||No|
|Can two groups speak at once||No||Yes|
|More spectrum efficient||No||Yes|
|Lighter on battery power||No||Yes|
|Messaging||No||Available on certain models|
|Status Calls – predetermine messages||Yes||Yes|
|Channel busy||No additional communication||Communication or queuing|