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29/06/2012

Antenna Setup Tips For Wireless Receivers

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Pyers Easton, Managing Director of Raycom Ltd shares his expert knowledge
I am frequently asked about the best antenna setups for wireless receivers to maximise range, whilst remaining as practical as possible to implement, so here are some tips:
If you are using a receiver (RX) with whip antennas on top then they are on an imperfect ground-plane (the RX body), and with other wiring around it will cause a net loss when compared to a dipole reference antenna in free space, let's assume maybe 3dB loss, maybe considerably more if RX is buried in a bag.
Now let's take a 'sharkfin' antenna, say the Lectrosonics ALP500, this has a stated gain of 4dBd (4dB relative to a dipole), but you need to factor in other losses, cable loss, assuming RG58 (typical cable for this application) is about 0.5dB/m at 700MHz, also, if you are feeding two RX from one antenna (one half of the diversity pair) then you will need a splitter, a two way splitter has a theoretical loss of 3dB (0.5 x power), but in the real world has a loss of 3.5 to 4dB. So see below for gain calculation:
• Antenna gain: 4dB
• Feeder loss assuming 3.5m cable (including 0.5m jumper from splitter to RX): -2dB
• Splitter loss: -3.5dB
• Net loss: 1.5dB
You can see from the above that you are losing 1.5dB relative to a dipole assuming no other losses, but you are 1.5dB better than the first case of 3dB loss, PLUS you would have more gain from having the antennas mounted higher up, so you will see a good benefit.
Active (amplified) systems can be very useful, but only if used properly, there are many disastrous pitfalls that can occur if not. Assuming you have an amplified antenna such as the Lectro ALP650 which you can jumper for 5dB or 8 dB gain, you could offset the losses in both the feeder and the splitter, getting you back to the nominal 4dB gain of the antenna itself. This is a net 6dB gain relative to the first case of whip antennas on RX, providing a theoretical doubling of range.
A couple of important points relating to active systems:
1: NEVER use more gain than you need to make up for the feeder and other losses between the antenna output and RX input terminals. Using more gain will not improve range, it will actually work against the RX which will be designed to have a specific internal gain structure to provide the best possible sensitivity and intermodulation performance, overloading the RX front end will compromise the intermodulation performance of the RX, reduce the amount of usable channels, and make it more susceptible to overload (blocking) by unwanted signals.
2: The weakest link effect - Always use a good quality amplifier with a high dynamic range in active systems, or the performance of the whole system will suffer. As an RF designer one of my pet hates is designing a receiver that is as good as you can only to have someone stick a cheap amplifier in front of it, thus rendering a £2000 RX no better than a £10 FM radio! The bottom line is that the RF performance of the whole system is only as good as the weakest link in the chain.
Also remember that active sharkfin antennas are wideband, so, unlike a filtered input of the RX are more susceptible to out of band signals, for example a mobile phone going off in your pocket next to the antenna whilst receiving a distant wireless mic. signal is likely to cause problems.
Personally I would always use a passive system where I can unless I really need maximum range, as the design performance of the RX is preserved and uncertainties are minimised, though once you go above 2 way splitters the splitter losses start racking up rendering active systems more attractive.
One last point about choice of antenna with gain: there are two main choices, 'Yagi' or 'Log Periodic' (sharkfin), Yagis tend to have more gain, but are more directional and have lots of 'ripple' (gain nulls, sometimes very deep) in their gain pattern so are best suited to fixed point to point links, log periodics have a good front to back ratio and a very smooth pattern, so if talent is moving around in front of the antenna they are much less likely to enter a deep null and drop out.
• Pyers Easton is Managing Director of Raycom Ltd and has been involved in RF and audio design for over 30 years.
VMI.TV Ltd

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