In this guide: When the whip runs out · Antenna types · What gain really means · Placement · Splitting and distribution · Antenna cable · IEM combiners · Coordination basics · Fault-finding · Common mistakes · FAQ

When the rear-panel whip stops being enough

One channel, receiver on an open shelf, performer twenty feet away: the two whips on the back panel are the whole RF system you need. Almost nothing about a working show looks like that.

• More receivers. Two receivers means four antennas fighting for one rack panel. Eight means sixteen.
• A metal rack. A whip inside a closed 19 inch rack is inside a metal box. Shure’s guidance is that receivers with fixed antennas “should be placed on an open surface or shelf, in line-of-sight to the transmitter”. Front-mount kits and antenna panels exist because whips and racks do not mix.
• Distance and bodies. UHF wants line of sight, and a crowd absorbs rather than reflects. Shure recommends receive antennas sit at least 2 metres up and above the audience.
• Where the rack lives. Front of house, side stage, a cupboard: chosen for cable runs and sight lines, rarely the best place in the building for an antenna.

Two numbers to start with. Shure recommends at least about 5 metres between transmitter and receive antenna, because a transmitter parked on a receiver generates intermodulation in its front end. And given a choice, “it is better to have the antenna/receiver combination closer to the transmitter (and run a long audio cable) than to run a long antenna cable”.

Antenna types and what each one is for

Quarter-wave whip, and the ground plane

Wavelength in metres is roughly 300 divided by frequency in MHz, so around 610 MHz a quarter wave is about 12 cm: the whip in the box. The catch is that a quarter-wave antenna is only half an antenna. Its efficiency depends on “a ground plane, that is, a metal surface at least 1/4 wave long” connected to the receiver ground, in practice the receiver chassis. Screwed to a receiver, a distro or an antenna panel, a whip has what it needs. Hung on the end of a coax in mid air it does not.

Half-wave dipole

A properly designed half-wave antenna needs no ground plane, which makes it the default for getting antennas out of a rack and onto a stand or wall. Shure puts half-wave types at up to 3 dB above a quarter wave. Sennheiser’s Half Wave Dipole rods publish typ. +2 dBi, 50 ohm, omnidirectional, BNC male with no DC path, in band-specific versions including 606 to 694 MHz for Channel 38.

Directional paddles (log-periodic)

The flat triangular paddle is a log-periodic dipole array: a boom of dipole elements whose size and spacing vary in a logarithmic progression, so at any frequency some are active while the rest act as reflectors and directors. That is how one paddle covers most of the UHF band rather than one TV channel. Shure gives typical log-periodics 6 to 8 dB of forward gain over roughly 120 degrees, rejecting off-axis signals by as much as 30 dB. Sennheiser’s passive ADP UHF covers 470 to 1075 MHz at typ. 5 dBi, apex angle approximately 100 degrees. Shure’s active UA874WB covers 470 to 900 MHz at 7.5 dBi on axis, 70 degree beam width, with a switchable +12, +6, 0 or -6 dB amplifier.

One detail catches anyone who has fitted a TV aerial: mount log-periodic and Yagi antennas with their transverse elements vertical, because the transmitters they listen to are vertical.

Helical and circularly polarised

Helicals are the specialist option: broadband, up to 12 dB forward gain over only about 60 degrees, and with no preferred polarisation, so a helical is “equally sensitive to incoming signals polarized at any angle”. Sennheiser’s A 5000-CP publishes circular polarised helical, 450 to 960 MHz, minimum 8 dBi at 650 MHz, 50 W power handling. That combination is why they usually end up on the transmit side of in-ear rigs.

What “gain” really means on a receive antenna

A passive receive antenna contains no amplifier and adds no energy. Its dBi figure describes shape: how much sensitivity has been concentrated in one direction at the cost of every other. A 5 dBi paddle is not a louder antenna, it is a fussier one, which is why one pointed the wrong way underperforms a plain dipole. Amplification is separate, and more is not better. Shure’s UA874 guide: “Too much gain actually reduces reception range and the number of available channels”, and receivers perform best “when the sum of signal gain and cable loss equals 0 dB”.

Placement: free, and it fixes most problems

• Real line of sight. “A clear line of sight does not just mean the antennas are visible”: glass, Perspex, water, chain link fencing and cages all count as obstacles.
• Height. At least 2 metres up, above audience head height.
• Out of the metalwork. In open air, or perpendicular to racks and grids, and at least a quarter wavelength from any parallel metal structure. At UHF that is only 10 to 12 cm, but a whip lying against a rack rail fails it.
• Away from reflectors. Receive antennas “should not be positioned closer than one wavelength to any reflecting surface”.
• Away from noise. Clear of dimmers, digital processors, computers, network switches and network cables.

LED walls earn their own line, because Shure has measured them. With an image displayed “the average power of the noise floor is raised by approximately 6 dB, with many strong peaks of EMI”, and the peak frequencies “fluctuate randomly” with the content. Their warning about the obvious mounting spot: it is tempting to put the antenna on top of the wall for line of sight, “but in reality, your antenna now has to fight through a whole load of extra noise”. Use a directional antenna with a null facing the wall. An omni cannot ignore anything.

Diversity spacing, and the V

Diversity works because two antennas in genuinely different places are unlikely to be in a dropout at once. Shure’s figures: a quarter wavelength is the minimum for significant benefit, “about 10 cm for UHF systems”, improving up to about one wavelength and no further. Sennheiser gives working dimensions of roughly 10 to 20 inches for omnis and 30 to 60 inches for directional antennas.

Crossing two paddles into a V is not decoration. A handheld ends up at whatever angle the performer holds it, so two vertical antennas can both be badly polarisation-mismatched at once. Shure recommends “adopting a V shape between AB receive antennas”, at 315 and 45 degrees for a 90 degree included angle, capping worst-case polarisation loss at around 3 dB. On whips, angling the pair up to 45 degrees apart does the same job.

Why you cannot just T two receivers together

An antenna system is an impedance-matched chain: “all components should have the same impedance: that is the antennas, cables, connectors and the inputs of the receivers”, and in wireless audio that is 50 ohm. A bare BNC T-piece is not a splitter. It parallels two receiver front ends across one cable, presents the wrong impedance to the coax, and lets each receiver see the other’s oscillator leakage.

A real passive splitter provides “RF impedance matching for minimum loss”, but halving the power is what splitting does: “a single passive split results in about a 3 dB loss”, and “multiple passive splits are impractical due to excessive signal loss”. Two receivers, a passive splitter is fine. Three or more, “active splitters are strongly recommended”.

What a distribution amplifier does

Three jobs at once, using the Shure UA844+SWB’s published specs as the example. It splits and makes up the loss, amplifying “to compensate for insertion loss that results from splitting signal power to multiple outputs”, at -1 to +1 dB from input to any output. It isolates the receivers, publishing 30 dB typical output connector isolation. And it sends DC up the coax: 15 V at up to 150 mA per RF input, so a remote active antenna needs no separate power run. Four 14 to 18 V outputs also power the receivers.

Distros cascade, feeding the primary unit’s cascade outputs into the secondary units’ A and B inputs. Shure’s limits: a single UA844+ “can support up to five wireless receivers”, and “a maximum of five UA844+ systems may be used in a two-tiered configuration”. More generally, “two or possibly three levels of active splitters is the maximum recommended”. Receiver cascade ports chain “up to as many as ten units”, with two catches: every receiver must be in the same band, and one unit losing power kills everything downstream.

Antenna cable: loss, length and the 75 ohm trap

Coax surprises people because audio cable does not behave this way. Shure: “significant losses can occur in relatively short lengths of cable. The loss is a function of the cable type and the frequency of the signal.” The mechanism is skin effect, whose severity “increases with frequency”, and loss scales with length too. For scale, Shure cites Belden’s 9913 RG-8/U at 11.8 dB loss per 100 m at 700 MHz, and that is a chunky low-loss cable. The UA844+SWB manual says “use 50 ohm low-loss coaxial cable, such as RG-8U”.

The budget is small: “cable loss of less than 3 dB is usually acceptable”, and beyond that an active antenna or inline amplifier can compensate. With the gain rule above, the whole question reduces to one sentence: work out your loss, and if you must amplify, amplify by about what you lost.

Connectors, and the 75 ohm mistake

BNC at 50 ohm is the standard across receivers, distros, splitters, boosters and antennas, which is why our RF (50 ohm) cables and BNC connectors sit apart from anything video: “50-ohm cables are typically used for audio RF systems, and 75-ohm cables for video distribution”.

Worth being precise about why the mistake happens. A 75 ohm BNC plug physically mates with a 50 ohm socket, so nothing stops someone grabbing a video patch lead. Shure notes mismatch losses “are negligible compared to the losses due to antenna cable length”, and that quality 75 ohm RG-6 can stand in where loss per unit length is comparable. So the mismatch is rarely the villain. The villain is what people reach for: thin, floppy video patch cable with high UHF loss and unknown terminations.

• Shortest run that does the job. Every extra metre is loss you pay back with an amplifier, and the amplifier lifts noise with it.
• Match the A and B leg lengths. Loss scales with length, so 20 m on A and 2 m on B hands the receiver two different signal levels.
• Right construction. Solid centre conductor for permanent installation, stranded and flexible for portable rigs. Our cable care guide covers storage.
• Fewest connections. “The number of connections in the antenna signal path should be kept to a minimum.”
• Watch for adapters. The A 5000-CP terminates in type N with a BNC adapter supplied. Other connectors are in our pro audio connectors guide.

The transmit side: combiners for in-ear monitors

In-ear systems run the other way: the rack transmits and the performer wears the receiver, so you want many transmitters feeding one antenna. That device is an antenna combiner. Sennheiser’s AC 41 “operates up to four wireless stereo transmitters from the ew series using one shared antenna” at 50 ohm, powering the SR IEM G4 transmitters over the BNC cables. Shure’s PA411 takes up to four PSM 300 transmitters over 470 to 865 MHz with 50 dB typical input port isolation, but powers them through a separate 1-to-4 cable, not over the coax.

That isolation figure is the real reason combiners exist. Shure explains the mechanism: “the signal from each transmitter generates IM products in the output stage of the other”. Four transmitters with their own whips in one rack are close proximity by any definition. A combiner stops them seeing each other while putting all four onto one antenna: fewer antennas and cleaner spectrum.

Keep transmit and receive apart. For a 10 mW transmitter into a passive omni receive antenna, Shure says “maintain at least 3 m of separation between antennas”, more for higher power, and where monitor transmitters are in use “it is strongly recommended that they be mounted in a different rack”. One hard rule: receive-only active antennas must never transmit. The UA874 guide says “do not use this antenna for transmitting”; use something rated for it, such as the A 5000-CP at 50 W. Our in-ear monitor systems guide covers the rest of the chain.

RF coordination: enough to stay out of trouble

Coordination is a job for software and the manufacturer’s preset banks, but the mechanism explains the rules. Start with separation: Shure gives the recommended minimum difference between two operating frequencies as “300 kHz to 1.5 MHz, depending on receiver selectivity”. Then intermodulation. Non-linear stages generate new frequencies from the ones present, and the strongest are the third-order products of two carriers f1 and f2, at (2 x f1) minus f2 and (2 x f2) minus f1. Shure’s example: 590 and 595 MHz may generate products at 585 and 600 MHz, and anything tuned there “would be at risk of experiencing IMD”. If the interval is F, products land one interval below the lower carrier and one above the upper, and most manufacturers want at least 250 kHz between any product and any frequency in use.

That is why evenly spaced frequencies need thought rather than faith. On a system that generates products, three carriers at equal intervals put the third straight onto a product of the first two. Digital systems designed not to generate significant products invert this: Sennheiser builds equidistant grids into Evolution Wireless Digital deliberately, publishing 600 kHz spacing for EW-D and EW-DX standard mode and 300 kHz in Link Density mode, and Shure’s ULX-D High Density mode cuts spacing from 350 kHz to 125 kHz. Which world you are in depends on your kit, which is why frequency banks exist.

Then scan properly, because most people get the on/off part backwards. Shure’s receiver manuals say to switch off all transmitters so they are not logged as occupied, and to “turn on any devices that might produce interference during the show”, listing other wireless systems, computers, large LED panels and effects processors. Scan from more than one position: a scan at the rack is not a scan at the far wall, and a quiet Wednesday will not predict Saturday night.

Capacity is the last constraint, and no antenna fixes it. Ofcom publishes a ten-channel frequency plan for Channel 38, so around ten systems is the realistic ceiling in the UK shared band at one site. Beyond that means coordinated assignments in other bands: see the Ch38 and Ofcom licensing guide, and the wireless microphone buying guide for band choice.

Fault-finding a distributed RF system

One channel or all of them? If every channel misbehaves at once, suspect something shared: the antennas, the coax, the distro, or its power supply. Check the distro is powered and bias is reaching an active antenna, then substitute one antenna leg at a time. If only one channel misbehaves, the suspects are its frequency, its patch lead, or its coordination. Retune it to a known clear frequency: if the fault follows the channel it is hardware, if it stays behind it is RF.

Read the receiver. Watch the RF level meter and the A and B diversity indicators together. An antenna whose indicator is consistently lower or never selected is telling you about that leg: a damaged cable, a poor connector, a passive antenna on a bias port, or far more cable on one side. Shure’s troubleshooting table maps intermittent audio with low RF at long range to insufficient antenna gain, excessive cable loss, multipath or obstructions. Overload is the same symptom from the opposite cause: if dropouts appear when a performer walks towards the rack, take gain away.

Walk the room before doors. Shure is unambiguous: “always perform a walk around test to verify coverage” before the show. Walk everywhere a transmitter will go, including behind the drum riser, in the wings, out into the crowd and behind the LED wall, watching the RF meters rather than listening for a dropout. A small repeatable dead spot is almost certainly multipath, and the fix is moving an antenna, not turning something up.

Common mistakes

• Whips left inside a closed metal rack. The commonest fault in the trade, and the cheapest to fix.
• Antennas behind the stage or a speaker stack. A wall of metal, magnets and bodies in the path.
• Two receivers T-ed off one antenna. Matched passive splitter for two, active distro for three or more.
• Mismatched A and B cable lengths. Diversity comparing two antennas that never had an equal chance.
• 75 ohm video patch coax in RF service. The connectors mate, so nothing warns you; the UHF loss does the damage.
• Paddle pointed at the audience. Throws away the gain and the rejection at the same time.
• Amplifier gain as a cure-all. It lifts noise and interference by exactly as much, and costs you channels.
• Passive antenna on a bias port, active antenna without one. Check the datasheet for a DC path.
• Mic receive and IEM transmit antennas on the same bar. Separate them, transmitters in a different rack.
• No scan before doors. Transmitters off, LED wall and lighting on. Any other way measures the wrong room.

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Frequently asked questions

Can I connect two receivers to one antenna?

Not with a bare BNC T-piece. Use a matched passive splitter, which costs about 3 dB per split, or an active distribution system, which splits at roughly unity gain and isolates the receivers. Shure recommends passive for two receivers, active for three or more.

Do I need paddle antennas, or will half-wave dipoles do?

Dipoles suit most small and medium rigs: omnidirectional, no ground plane, easy to get out of a rack. Move to paddles for long throws, to reject a noise source such as an LED wall, or where coverage is clearly in one direction.

How far apart should diversity antennas be?

At least a quarter wavelength, around 10 cm at UHF, improving up to about one full wavelength and levelling off after that. Sennheiser’s working figures are 10 to 20 inches for omnis and 30 to 60 inches for directional antennas.

Why are paddle antennas crossed in a V?

To cover both polarisations, because a handheld ends up at whatever angle the performer holds it. Shure recommends 315 and 45 degrees, a 90 degree included angle, capping worst-case polarisation loss at around 3 dB.

Can I use 75 ohm video coax for antenna cable?

You should not, even though 75 ohm BNC plugs mate with 50 ohm sockets. Shure notes the mismatch loss is small next to length-related loss, but what people grab is thin video patch lead with high UHF loss and unknown connector impedance.

Can one antenna serve both my radio mics and my in-ears?

No. Shure suggests at least 3 metres between a 10 mW transmit antenna and a passive omni receive antenna, and housing monitor transmitters in a different rack. Combine the IEM transmitters onto one transmit antenna and split the mic antennas to the receivers, as two separate systems.

Does an antenna booster increase my range?

Only by cancelling losses you already have, because it amplifies noise and interference along with the signal. Shure states that too much gain “actually reduces reception range and the number of available channels”. Real range comes from placement, antenna type and cable discipline.