Answers to Frequently Asked Questions about ultralight/microlight aircraft communications

As a manufacturer and supplier of intercoms and radio communications interfaces to the ultralight and microlight industry, we get occasional calls from folks with questions about intercom systems (both ours and other manufacturers) and radio communications. Here are the most common questions and our replies. Please feel free to contact us on +61-(0)3-95536445 if you have comments or questions!

Index:

• Faint transmissions
• Best antenna position
• Antenna lengths
• Squealing in headsets
• The Icom A22 (or A3) VHF doesn't work with my intercom
• My new Icom A6 or A24 radio squeals when I transmit
  

I can't hear my passenger very well on the intercom. There's a lot of noise in the background with the engine running.

The first question we always ask for any problem is: has it always been this way?
If it was working fine before, then obviously something has changed for the worse. You should think about any changes to aircraft systems you've made about the time the problem started. Chances are, the change and the problem are related.

If turning the intercom off doesn't make any difference to the, background noise, then you probably need either a better set of headsets with better sound insulation, or a better muffler.

If placing your hand over the microphones eliminates the problem, then either you have the volume of the intercom set too high, your lips are too far from the mikes, or both. Or it may be that the microphones are inferior - they should be quality noise cancelling units. On the volume front, the intercom volume should be set to the lowest level appropriate for clear communication, and when a radio is used, the squelch control should be appropriately set to trigger on reception only.

Often, folks get tempted to put huge foam wind-shields on their mikes. However, noise cancelling mikes work best if the mike element is almost touching the lips, so a huge muff can make them ineffective simply by separating the mike and lips. In windy cockpits, you can use a big mike shield, but grind a flat on it on the side that faces the lips.

The culprit could be a dirty DC supply line. This can be caused by bad contacts in plugs and switches, or it may be that your regulator is on the way to failing. If the problem has always been there, it may be you have a poor regulator. The standard Rotax regulator does not have a good reputation for clean DC output, hence the keen market for other regulator manufacturers.  A good alternative to the standard Rotax regulator is the PowerMate made by Claus Grimm in Australia.. You can click here to find out more.

The first test is to try running the intercom from a separate battery from the rest of the electrical system, assuring that all radio leads are also disconnected. If the problem goes away, you need to look into the DC power. A simple filter choke and capacitor in-line with your DC to intercom and radio may solve the problem (the parts for this cost about $8)

If the problem is still there, it's likely that the noise is being picked up from the ignition system either by the headset leads or the intercom itself. You should try routing all leads to the intercom well away from ignition system components. The intercom itself should also be clear of things like kill-switch or tacho leads. This should be an uncommon problem.

Other people tell me that I'm transmitting just fine on the radio, but I can hardly understand other stations. They come through all distorted and noisy or faint.

This could be for several reasons. You could have a faulty intercom. This is easy to test. Tune into a local strong VHF signal (eg: ATIS) on the ground with the engine off and with the "rubber duck" antenna on the radio. If it comes through with reasonable volume and clarity, the intercom amplifiers are fine. If you are hearing your passenger OK as well with the engine running, you can probably discount the intercom itself as the cause.
 

Next, do the same test (still connected to a separate battery) with the radio hooked up to your external antenna. If the signal is poorer, particularly if there's more "hiss" in the background, which indicates that the radio is working harder to amplify the signal, you should start looking for corrosion, shorts or other problems with the external antenna system. Sometimes it's difficult to tell that the signal is degraded if the source is strong, however, so this is not a conclusive test.

This particular problem can be difficult to track down. the most likely culprits are a bad antenna system or a poorly shielded or routed ignition system. Try routing all cables and leads well away from ignition leads. Assure the antenna itself is as far from the engine as possible. See the answer to the next question for advice on where the antenna is best placed.

Where is the best place for my VHF antenna?  

Most antennas used on ultralight are basically a simple 1/4 wave whip antenna or base loaded whip with or without an effective ground plane. The diagram shows the sensitivity of these kind of antennas to transmissions coming from different directions. (the greater the arrow length, the higher the sensitivity on receive or strength of the transmitted signal.

As you can see, both antennas transmit and receive signals poorly when the other station is in line with the antenna. For most purposes, if only a very small groundplane is available, as is the case in most ultralights or microlights, the patterns are very similar.

On trikes, the best positions are either on top of the kingpost or, if there is room, below the engine, pointing straight up and down. These positions place the engine in the area of least sensitivity and provide good coverage for distance communications.

Some people place the antenna pointing upward on a gear strut or on the pod. The former position is convenient, but is a recipe for reception of excess ignition noise. And both placements virtually guarantee a "blind spot" so that reception and transmission in the direction of the engine will be substantially weaker.

Remember also that the strength of radio signals, including inerference reduces as the square of distance from the transmitter, so making an effort to get the antenna as far away from the ignition system as possible is well worth it!

On 3-axis aircraft, there are usually more positions to choose from as well as opportunities to include a substantial ground plane. Common successful installations include the cabin roof or pointing downward under the fuselage.

How long should my antenna be?

The diagram at the right shows the practical length for quarter-wave simple antennas for the air band. As you can see, you have to make a compromise.
 
Most folks tune the antenna for the common CTAF frequency 126.7 Mhz, which leads to an antenna length of about 57cm for the average whip. This should be measured from the point at which the whip exits the co-axial cable, so it includes part of the base in most cases. For the typical CB base, this means cutting the whip antenna to about 55 to 56 centimetres in length. For most purposes, this will work quite acceptably.

I get a squeal in my ears when I try to transmit

The squeal is usually feedback from RF signals getting back into the intercom system, being demodulated, then amplified, then put back into the radio - creating a feedback loop. Look for cabling problems. You may have put extra-long headset leads on the system, or run the radio interface lead close to the antenna. You may even have put the radio itself right next to the intercom. The problem can also occur on some systems if the radio and the intercom are hooked up to the same battery and the battery is low or there is a problem with poor power contacts.
 

Other people hear a squeal when I transmit. But it sounds fine to me through the headset

This is probably feedback from the radio/antenna getting into either the intercom's radio interface lead or interface circuit. It can also be a symptom of a badly matched or badly connected antenna. Check all cables, routing interface leads well away from antenna leads. Check all antenna connections and mounts for corrosion. Make sure you are using an antenna of the correct length and type, and that you are using a good quality 50ohm co-axial cable. (Don't use 75 ohm TV cable!)

I just bought a new Icom A22 radio, and it doesn't seem to work with my old intercom. What's going on?

In mid '95, Icom introduced the A22 VHF transceiver, a marvellous piece of miniaturisation that works really well. But the "external Mike" connection is very different to the older A20 model, even though it looks> the same. The A20 is designed for a simple electret mike. The A22 expects a GA-style headset to be connected to it. Hence not only the impedance (we're talking dirty in electronics-speak) of the microphone is different, but the actual plug that goes into the radio is wired up differently and works in a different way.
 

If you have a UltraCom intercom, you will need a new interface lead (available for between $99-140 from their distributors or us. We can upgrade our own older Aerial Pursuits systems also at a nominal fee (parts + labour).

My A6 or A24 radio squeals when I transmit!

Miniaturisation of handheld radio transceivers reached a new level with the intriduction of the A6 and A24 radios.  Unfortunately, to miniaturise the radio Icom also had to miniaturise the batteries, and to get enough battery life, they had to go to a 7.2 volt battery pack.  The problem is, most GA headsets are designed to expect about 9 volts available from the radio, and without this voltage, many do not work well, and a side result is usually that RF energy from the radio antenna gets into the mike circuit, causing a squeal or hum that may or may not go out on the transmitted siganl making it unreadable.  Icom do know about the problem, but there is no fix.  Some people find that fitting a remote antenna and getting it as far away from the headset and leads as possible reduces or eliminates the problem.  We do NOT recommend this transceiver for use with a headset or intercom system.