Taming the Antenna Farm

In Uncategorizedby tfwm

Most Houses of Worship start with a simple need – a wireless for the worship leader. But then we begin to experience wireless creep. First the assistant worship leader wants a another wireless, then another. Youth asks for a few and before you know it, your sound booth or altar area is bristling with antennas like the bridge of a battleship or Mt. Wilson outside LA. Called the “antenna farms” by the broadcast industry, this imposing array of antenna whips can be visually disruptive and a contributor to RF problems in your wireless microphones – especially if, buried in the mess, is a broadcast antenna for wireless auditory assistance systems or in ear monitors.

How do we tame this mess? In this article we’ll address antenna types, signal splitting, combining, cabling and the various traps you can encounter.

Antennas can be made in a variety of manners. In the ideal and perfect theoretical world, the isotropic antenna is the perfect component. Entirely independent of wavelength, the isotropic antenna is a infinitesimally tiny point in space which perfectly radiates in all directions and is perfectly receptive in all directions. Of course, we cannot build such an antenna and so that leads us to the first basic antenna, the whip. All antennas in the real world require an active element and a ground plane. Typically made in 1/2 or 1/4 wavelengths, the whip is attached directly to either a transmitter or receiver. The whip is the active element and is dependent on the body or ground of the receiver (or transmitter) to act as the ground plane. One of the most common errors in attempts to improve wireless performance is to extend the whip on a cable. This in fact separates the active element from the ground plane, potentially placing them in divergent phases of the signal and actually reducing antenna performance. The loss of signal can be from 15 to 30dB – a fatal loss if you have low powered transmitters. The whip should always remain attached to the receiver. If a remote antenna is needed, you will need to add one of the following antennas.

Dipole – The dipole antenna is basically two whips, one active and the other attached to the ground plane, both via the coaxial cable. Both components are in the same phase or wavelength of the RF signal and thus you get effective pickup, the RF equivalent of a balanced signal. The dipole has just the two elements and while basic, is a very effective receptor. The length of each element will be the same and will limit the receiving bandwidth of the antenna – an attribute which can be very useful. The pickup pattern of the dipole is a toroid – a doughnut shape aligned perpendicular to the antenna elements (which are aligned opposite each other. The lowest sensitivity is aligned along the vertical axis. Because of this, this type antenna should be placed with elements vertical.

Similar to the dipole, the ground plane has an active vertical element but uses multiple legs for the ground plane. These multiple legs, by their angle, determine the antenna impedance.

Log Periodic Dipole Array (LPDA) – The LPDA is simply a series of dipoles of various lengths that are capable of receiving a wider range of frequencies and also possess a directional pickup pattern not too different than that of a cardioid microphone. Such antennas have gain – they boost the RF signal and work well to improve pickup – especially at a distance. These are excellent for churches where the sound booth may be situated at the rear of the sanctuary and the pickup pattern is suited to the altar/antenna positions.

The dipole is commonly found in two styles, one is the “paddle” or “shark fin” style which is formed from a printed circuit board. These are rugged, transportable and can be painted to match the interior scheme (avoid paints with metal flake). The other style is the rod style antenna, made of metal rods. Equally effective, the rod style is good in permanent installations.

Keep in mind with a LPDA, you can have up to 6dB increase in signal – the equivalent of boosting your transmitter output 4X. A 50mW transmitter will be effectively the same as a 200mW transmitter – but remember that interfering signals will also be boosted the same amount. The LPDA does mean an increase in range if all other factors are good.

All of the above described antennas are passive – ie they require no outside power. Active antennas have an on-board RF amplifier, require external power (more on that later) and boost the signal immediately after reception. Active antennas can help assist when long cable runs are required from the antenna to the receivers. The boost of the signal will help make up for losses in the cable. The same result can also be achieved with an in-line RF amplifier, a device which can be combined with a passive antenna. A good in-line RF amplifier will be filtered so you are not boosting out-of -band signals which can increase your interference problems.

OK, you have selected a remote antenna. Now, how do you connect it? Cable selection can be critical. Wireless microphone systems are based on 50 OHM impedance components so in the ideal world 50 Ohm cable is highly recommended. But the impedance is not the only factor you must consider. Review the loss rating for the cable for the frequencies of your wireless microphones. Example – RG58 in the UHF band loses nearly 17dB per 100 ft of cable. The loss over a 2400 foot run of cable would require nearly the power output of the sun to get 100mW out of the receiving end. For UHF frequencies, RG8 is recommended. It is, however, very bulky, rather difficult to terminate and more expensive than RG6, the 75 Ohm equivalent. Can you use RG6? Technically speaking, you will get better performance from the RG8 but good RG6 will serve well in all but the most difficult circumstances. The difference in gain over a 100 foot run is only about 1.5dB and you can save considerably with RG6 cable. So, select a good quality RG6 or equivalent RF cable with low loss numbers per 100 ft and you will be in good shape.

Be careful about cable length. It is not uncommon to place the antennas so far from the receivers that you lose more signal via the cable than you do over the air.

Now you have selected cable and antenna. Next you need to somehow serve multiple receivers from single pair of antennas.

There are two approaches that can be used at this point. First, you can construct a passive network using RF splitters. Passive splitters have the advantage of not adding any RF signals to the signal the antenna picks up. They have the disadvantage of signal loss. A splitter that splits the signal 8 ways (one in, 8 out) will lose over 9dB, a considerable reduction of level – (50mW at the antenna will be about 6mW after splitting).

If you have four receivers, the antennas are fairly close to the receivers with short low loss cables, and you are fairly close to the altar, then the antenna splitters can be an effective solution, giving you a clean installation.

In the example of passive 4 channel system, the 4 way splitters will lose about 6db. Add up the loss in the cables based on the cable specification, add that to the 4 way loss and you have your total system loss. Depending on the range and the distance of the antenna to the receiver and the antenna from the transmitter, this could be a perfectly acceptable situation.

But what if greater range is involved? Then an active antenna distribution system becomes important. In-line RF amplifiers can boost the signal before the cables run and increase signal strength – thus making up for cable and splitter losses. In line RF amps should always be placed immediately after the antenna – adding it to the receiver end in the rack only boosts the noise picked up along the way and will be completely ineffective.

In this illustration, the antennas (passive) first feed in-line RF amplifiers, then go to two way splitters which feed downstream 4 way splitters. No net loss as the 10dB loss of the passive splitter networks has been compensated by the in line RF amps.

Active multicouplers (splitters with on board RF amps), when properly designed, can be effective methods for supplying RF to large wireless microphone arrays. Well designed units have limited passbands with strong filters to weed out intermodulation products. Look for a good third order specification (6Bm is good, 27dBm is great). Poorly designed (read low cost here) units can actually generate intermodulation problems and create more problems than they solve. Multicouplers will give you clean installations without cabling loss. They will be more expensive than a passive network but in extreme installations (lots of channels in large churches), they may be the only solution.

As you can see, through careful design, a clean antenna system can improve your RF reception. Eliminating multiple antennas will clean up the sight line, eliminate altar or sound booth clutter and increase your system reliability. Clean up that antenna farm!