DME – Welcome to the future
By Pyers Easton September 2019
It’s no secret that PMSE is losing the band between 700 and 800MHz next year. To their credit Ofcom have come up with a solution to ease crowding in the remaining spectrum; the DME band.
Developed in Australia in the early 1950s DME stands for ‘Distance Measuring Equipment’ and operates in the band 960 to 1215 MHz. Aircraft ‘interrogators’ send a signal to a ground based ‘transponder’ which replies to the aircraft. The aircraft measures the time it takes between the interrogation and reply and calculates its distance from the ground transponder, which is displayed to the pilot. An aircraft can use multiple DMEs to determine its position.
After extensive testing and working with the CAA (Civil Aviation Authority) to ensure that PMSE can share in the band without interference to aeronautical services, Ofcom made the bands 961 to 1015, 1045 to 1075 and 1105 to 1154 MHz available for use by wireless mics under a coordinated licence.
This frequency spectrum is not a million miles away from the currently used UHF spectrum, so propagation (basically how far the signal goes and how much of the signal is attenuated by obstructions such as bodies, walls etc) is similar to what we are used to. Attenuation is slightly higher at these frequencies, but this is more than mitigated by the reduced noise floor up there.
The UHF spectrum is beset with interference from many sources; obviously there are all the digital terrestrial TV transmitters (DTV) which use up 8MHz blocks everywhere, these transmitters are often up to 200kW in power so can severely reduce the operating range of radiomics in the UHF band. On top of this there is the scourge of SDI (Serial Digital Interface) video. This happens to produce a lot of RF in the UHF band, peaking around 600MHz, which is sub-optimal when poorly screened cables or cheap SDI to HDMI converters are used, this can lead to loads of noise being sprayed over your signal. Add to that noise from large video walls and you have a fine mess.
I have had recent experience this kind of noise in a very large TV studio full of video walls and SDI amongst other things near London in an area with lots of DTV. It was very encouraging to see the average noise floor of DME was around 20dB lower than in UHF. This translates to massively improved range. Attenuation in the DME band is roughly 4 to 6dB worse than UHF, but the reduced noise floor gives a net gain of 14 to 16dB. When you consider that 6dB equates to a doubling of range then you can see that you could expect more than quadruple the range using the DME band compared to UHF in a noisy environment. This is pure gold for PMSE users.
So, what about interference from DME transponders in aircraft and on the ground to PMSE users? Well, because DME is such an old standard developed decades ago when receiver performance was not what it is now the DME channels are spaced at 1MHz intervals and only use a relatively small bandwidth. This means that channels on 300, 500 and 700kHz used for PMSE are basically occupying ‘virgin’ spectrum.
To enable the use of these frequencies, as they are 200kHz apart, the transmitters and receivers need to use “narrow band” technology using only 100kHz of bandwidth, instead of the usual 200kHz for a wireless channel. The benefits of using narrow band are twofold, firstly, you get double the channels in a given slice of spectrum, secondly the reduced bandwidth in the receiver provides an improvement to sensitivity; halving the bandwidth translates to a 3dB improvement in carrier to noise (i.e. sensitivity). This is equivalent to DOUBLING the transmitter power.
This is all very good news for PMSE operators. The DME band is offering at least 400 high performance, brand new channels to choose from.
At the moment the only manufacturer who is able to supply equipment with narrow band and DME is Wisycom from Italy who deserve to be commended for embracing this new technology whilst there is currently only a market in the UK. They have a range of gear available for use in this new spectrum including single and dual battery transmitters along with portable and rack mounted receivers.
Contact Raycom on 01789 777040 or firstname.lastname@example.org for details and a quote for this.
What about using the DME band abroad? As mentioned previously DME is currently only licensable in the UK, however DME is by its very nature is a global standard. Also, as DME gear is deployed in almost every plane from small private aircraft to the largest of jetliners it is not going to go away anytime soon. This is something that the UK have been pioneers in and it can surely only be a matter of time before other countries follow suit. Germany is actively preparing for tests right now, and more will no doubt follow.
How do I license it? Licensing can be arranged by contacting the PMSE licensing team on 020 7981 3803 or by email at email@example.com.
The only conclusion I can come to here is that DME is a fantastic resource which mitigates most of the issues thrown up by the reduction of usable UHF spectrum. It offers a huge number of channels in very clean spectrum which will offer the longest range and freedom from interference. I think that it is well worth considering for future needs, especially if you are deciding where to spend funding gained from the return of old obsoleted UHF gear in the Ofcom buyback scheme.
Finally, you can see a video of me talking about DME at IBC by following this link: