802.11a Band C – 5.8GHz Wireless Links

The 802.11a band C was released in the early part of 2004 and offers the possibility of greater ranges and bandwidth than 802.11b/g systems. The techniques used are largely the same as working at 2.4GHz, however extra care has to be taken with connectors and cable runs.

Improvements in 802.11a

Whilst the range per mW of output power is reduced with 5.8GHz systems, the power levels are much higher and will give a greater achievable range. The maximum EIRP is 100mW for 802.11b/g systems and 2W (2000mW) for 802.11a band C.

One of the other improvements offered by 802.11a is its improved tolerance to reflections and multipath interference (like ghosting on the TV). This has been achieved by improved electronics an modulation techniques in the equipment. This will mean that in cluttered areas where previously a circularly polarised signal would need to be used, the normal horizontally or vertically polarised aerials can be used.

The equipment is often described as 'non line of sight' – this leads people to think that the signal will pass round obstacles, unfortunately this is not true and just means the entire Fresnel zone does not need to be clear and it will tolerate multiple reflections. Some manufacturers claim non line of sight and in fact are refering to the ability to use an adjacent building as a reflector.

Another improvement is that the signals will not be subject to as much interference. The 802.11b/g band suffers from interference from Bluetooth equipment, portable phones, video senders and microwave ovens.

As the Fresnel zone at 5.8GHz is smaller compared to that at 2.4GHz, it also means less of the link needs to be clear and the aerial height above ground can also be reduced.

Equipment Requirements for 802.11a

Whilst there is a reasonable amount of equipment for 802.11a band C available in the United States (who for once will have a lower power limit than the UK). There is very little equipment available that will meet the UK specifications. The UK specifications have three requirements which are different from the USA.

TPC Transmission power control. This is where the output power of the link can be adjusted to the minimum level required to provide a working link. This is useful as it allows lower transmit output power from the bridge and therefore a higher gain aerial can be used which will allow greater distances to be achieved (because with the higher gain aerials, the received signal is effectively amplified by the antenna)

DFS Dynamic Frequency Selection. This is a mechanism where if the bridge detects an interfering signal such as RADAR on the same channel, it will change the frequency of the link to an interference free channel.

Frequency Range The frequency range 5795 to 5815 MHz can not be used in the UK. Equipment should have the capability to notch out this frequency range in order to protect RTTT devices. This will leave four channels available in the UK.

Low gain Omni directional antennas are unlikely to be legal in this country at the higher power levels as there is a requirement that the signal must stay largely horizontal to prevent interference with satellite systems, however I suspect this is unlikely to be enforced.

At the moment, we have found four suppliers of equipment which can be made to meet the proposed requirements. The price varies considerably, as does the performance.

Types Of Equipment

The currently available equipment falls into three categories.

The first is the lowest cost and consists of an access point/bridge in an indoor housing with a seperate aerial connector. This equipment is the lowest cost and can be coupled to any aerial. These systems often appear to give the poorest performance as there is a tendency to mount the unit away from the aerial and the corresponding cable loss makes the links unexpectadly poor. The transmitter units tend to be based on standard chipsets.

The second is the integrated aerial/access point systerms. These again have standard chipsets and their performance tends to be slightly better than the first type – mainly due to the lack of cable losses. These units are also available with external aerial connectors.

The third is the integrated aerial/access point system with custom RF front ends. These units tend to be five times the price of the other types, however the improvements in RF sensitivity mean that these units are capable to very long ranges and working in difficult conditions.

Cost of Equipment

The equipment cost is likely to be greater for this equipment for the foreseeable future. For instance:

Equipment 802.11g cost in UKP802.11a band C cost in UKPNotes
Main Transmitter Unit85800 
Slave Unit85450 
Cable and Connectors502205.8GHz requires LMR900 cable
Adapters240no adapters required for 802.11a
Aerials90600both 2' dish
Equipment Total3342070** converted from US price excl. shipping
Range at 5Mbs real throughput4-8Km*10-15Km*Depends on manufacturer
Range at 10Mbs real throughput2-4Km*5-10Km*Depends on manufacturer

As can be seen, the achievable range is improved for an 802.11a system.

However in real use, the 802.11a outperforms the range even further due to its improved interference rejection and the fact that the band is much clearer. The practically achievable Range at 5Mbs real throughput for 802.11b is typically 5-6Km in open countryside and very much less (2-3Km) in towns. The smaller Fresnel zone at 5.8GHz will also mean that aerial heights can be lower.

Aerials and Cables

At the moment there is not such a wide range of manufacturers and suppliers available for 5.8GHz aerials as there is for 2.4GHz. Accordingly the prices are very much higher when comparing like for like. All the common aerial types – dish, yagi, patch sector and omni are available but at higher prices.

One point to be careful of is that the quoted gains for some aerials are not at all accurate – often the only way to get a true idea of what an aerial is capable of is to try it yourself. We have seen as much as 8dB difference between different aerials that claim the same gain.

It would normally be assumed that because of the higher frequency, the aerials will be smaller. This is true in that a 1' dish aerial at 5.8GHz gives a gain of 22dBi and a 2 foot dish would be required to get the same gain at 2.4GHz. However, the extra gain is required to make the distance, so for a given link the aerial size is likely to be the same in both cases.

Because of the greater cable loss at higher frequencies, to match the loss of a run at 2.4GHz, the cable size needs to be increased. For example a 10m run at 2.4GHz of LMR400 (10mm diameter) has a loss of 2.2dB. At 5.8GHZ LMR600 cable would be required, this is much more expensive and requires more expensive connectors.

As time goes by, the cost of all the equipment will fall considerably.


In the UK, whilst this is considered a license free band, there is a licensing cost current of £1 per year per transmitter with a minimum charge of £50. There are parts of the country where 'exclusion zones' are in place which are slowly being removed to avoid interference with outside broadcast equipment.

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