This article was written by Keith Pickersgill, aimed at helping pilots to effectively use radios while flying.
Background
There are a limited number of frequencies available in South Africa and the demand far outstrips the availability of frequencies. (Note, Hz pronounced Hertz, simply means “per second”, so MHz or Mega-Hertz, means a frequency of x millions of cycles per second, while kHz means x thousands of cycles per second)
SAHPA cannot get exclusive use on any frequency because there are no unused frequencies in the entire VHF Commercial Band (136 to 174 MHz).
However, SAHPA has obtained permission for pilots to use two roving frequencies, which gives pilots the use of the same frequencies throughout the country. This is an extremely rare arrangement and hails back to the days before cellphones, when ICASA was convinced of the need to have country-wide comms due to the high risk nature of our sport and the remote areas we often fly.
In summary, we therefore have two VHF Commercial band frequencies available to us:
- 141.600MHz
- 141.625MHz
Achieving a small degree of privacy
There are several different methods of achieving some degree of privacy on any frequency. These methods do not prevent other groups on the same frequency from hearing you if they are not using any form of privacy, but it does silence (Squelch) other users so that you do not hear them.
We elected to use the Continuous Tone Controlled Squelch System (CTCSS), simply because it is the most widely available system that most modern radios can do. This is also known as Tone Squelch, or Tone-control or just QT.
How Tone Squelch works
When you transmit, the radio first generates the Radio Frequency (e.g. 141.625 MHz). It encodes a sub-audible frequency tone for the group. Human ears cannot hear this very low frequency. It then also encodes the actual voice data.
When enabled on both transmitter and a receiver, the receiving radio does not open the speaker if it receives a carrier wave without the correct sub-audible tone, i.e. it squelches the signal.
If the speaker emits any audio at all, you then know its someone in your group talking, unless another group in the area happens to use the same sub-audible tone, in which case you tell your entire group to change to a different sub-audible tone.
Implementing Tone Squelch
Program all radios in your group as follows:
- Channel 1 on 141.600MHZ Open, i.e. you can hear EVERYONE on that frequency.
- Channel 2 on 141.625MHz Open, i.e. you can hear EVERYONE on that frequency.
- Channel 3 & 4 are on the same radio frequencies, but using a sub-audible tone of 97.4kHz
- Channel 5 & 6 are on the same radio frequencies, but using a sub-audible tone of 179.9kHz
- Channel 7 & 8 are on the same radio frequencies, but using a sub-audible tone of 241.8kHz
If you are using Channel 1 and there are farmers or delivery drivers chatting, ask your entire group to change to Channel 3 which is the same frequency, but filtering out anyone who is not using the sub-audible tone of 97.4kHz
Better yet, before launching, just set a channel for the day for everyone in your group.
Channel 3 and 4
I’d suggest that everyone just use Channel 3 as default for general flying, perhaps reserving Channel 4 for when you are on the ground talking to your recovery vehicle (if no phone signal), or for instructors training students.
NB: If the group is using Channel 3, filtering out everyone who is not using the correct QT, and a pilot has a radio that has not been programmed as above, he will be using the frequency in open mode, basically the same as Channel 1. He will hear everyone on that frequency, inlcuding the pilots on Channels 1, 3, 5 and 7, as well as other groups, regardless of whether they use any form of channel privacy or not. However, he will not be able to talk to anyone on any channel other than Channel 1. Take note of this. Read it again if it did not make sense the first time.
If you find an areas where Channel 3 is actually used by another group, then simply get everyone to use channel 5.
If a radio can do 16 channels, then we repeat the first 8, with some other settings changed, such as Busy Channel Lockout (which prevents you from transmitting while someone else is already transmitting), or shifting the intermediate frequency (used when other electronics you have on you is causing radio interference – older Garmin’s were major culprits here.), or changing the power level between High and Low.
When using sub-audible filters like this, it is essential to also enable BCL on each of those channels. This is the Busy Channel Lockout feature. Basically if you try to transmit while someone else is already transmitting, the radio will refuse to transmit and give you a loud audio alert (e.g. Beep-Beep or more commonly a Dit-Dit sound).
Using the monitor button
While someone else is transmitting on your frequency, with or without the sub-audible filter enabled, you should either have a green light showing on the radio, or RX visible on the screen. If you do not hear them, but you want o listen in to see who it is, or get an idea of how long they will be, there is usually a Monitor button, which you hold in to temporarily disable all filters to hear anyone else on that frequency, regardless of what privacy system they use.
You can also use the Monitor button to listen to a very weak signal, as it also opens up the background-noise squelch (that irritating stating hiss when a frequency is not used). So if your buddy has landed far away and his signal is choppy, or you landed and cannot hear your recovery vehicle clearly, hold in the Monitor button while they are speaking.
Range
Many people complain to me their radio does not reach very far. The cheapest radio will reach as far as the most expensive radio. For example, I have one radio with an 8 Watt transmitter, and another smaller radio with a transmitter of only 0.23 Watts.
That tiny radio can transmit just as far as the one with 35 times more power! And the same with receiving, which does NOT depend on the transmitter power. Factors which have the greatest impact on communications range are:
- The antenna in use
- the orientation of the antenna
- the position of the antenna.
I wish to address each of these three issues.
The ideal antenna
The ideal antenna is one quarter wavelength long. To get very rough idea of the wavelength of any frequency, divide 300 by the frequency. So 300 / 141.625 has a wavelength of around 2.12 meters, of which a quarter-wave antenna would be somewhere around 53cm long (not exactly, just roughly). It is not practical for a handheld radio to lug around a long whip of 53cm length, so they cheat a bit and coil the antenna inside a rubber sleeve. This reduced the transmitting signal by around 80% or more, and receiving even worse than that, but it is a compromise to keep the radio portable and handheld.
Orientation of your antenna
For clear and long-range communications between two radios, both antenna need to be parallel to each other. The ideal would be to hold these antennae horizontally, but that is impractical as you would need to know where the receiver is and “aim” your antenna perpendicular to the other radio, while the other pilot does the same, aiming at you.
Instead, the best option is vertical antennae. No matter what your orientation is to other pilots in the group, all your antennae would be parallel to each other.
Common mistakes in orientation
Mistake 1: If you hold your radio to your head like a telephone, the antenna is likely at 45 degrees. If the other pilot is trying to hear you doing the same, but facing a different direction, with the two antenna then perpendicular to each other, they will be completely out of phase with each other. You would have to be quite close to communicate effectively and it would be more effective to use a megaphone.
Mistake 2: Pilots often mount their radios lying flat on top of their lap-cockpit (with a remote speaker/mic or headset perhaps) which is same problem.
No radio will work well if you make these two mistakes. Find a way to have your antenna vertical whilst in use.
Interference
- The position or location of the antenna can seriously affect your range, especially during transmitting. It cannot transmit through your head or through your body. You have a radio-shadow on the opposite side of your body to where the antenna is.
- If the antenna is within about 25mm of wet flesh (your body, head, etc), the signal will be “soaked up” by that wet flesh and not much gets out.
- If the antenna is very close to metal components or hardware, the signal can be distorted and “shaped”, essentially reducing the amount being radiated in certain directions.
If you take care of these issues, you should have unlimited line of sight communications.
Understanding Line of Sight
On the ground, you will find that trees, rocks, buildings and other obstructions will block the signal in that direction. Even in flight, if you are below ridge-top, your signal might not reach pilots on the other side of the ridge, unless they are high enough to see you.
Radio signals are no different to light “rays”, in fact, they are EXACTLY the same thing, just at a different frequency. You eyes can detect only a very very small slice of the Electro-Magnetic (EM) Spectrum, what we call Violet to Red and all the colours inbetween, each just a different frequency.
I’ve seen a number of recovery vehicles with very powerful mobile radios pushing out 50Watts or more, with an enormous external antenna fitted to the vehicle. In many cases, my little quarter-Watt radio with its tiny rubber duck antenna will reach further. Why? Because I practice the above three aspects.
Long Range Antenna for vehicles
Many pilots have been suckered into buying a “long-range” antenna for their car.
In our application, ground-to-air (car to pilot), that antenna delivers dismal performance.
Why? Because these “long-range” antennae sacrifice vertical performance to optimise horizontal performance. These are recognised by being 5/8 of a wavelength, so for SAHPA frequencies, will be about 1.3 meters in length. Instead of propogating an isotropic signal (a sphere expanding outward in every direction), they produce a turoidal signal, i.e. a donut-shaped signal which radiates outward horizontally, with little to no vertical signal. Sometimes the pilot is can actually see the recovery car from the air, but cannot hear them trying to talk to them, because he is too high above their radio signal.
The solution to your range-anxiety, is to get rid of the rubber-duck antenna while flying, use it only while on the ground for short-range chatting with pilots around the launch or landing area.
Instead, fit a decent quarter-wave antenna to your wing.
Xplorer Wonder Whip
Twenty five years ago, I designed a long-range antennae for pilots, which many thousands of PG/PPG pilots around the world have used. I still get emails and WhatsApp messages from new pilots thanking me after using one. I no longer manufacture them, but have published the design online for anyone to make, or manufacture for commercial purposes.
The design is an end-fed, quarter-wave dipole. A dipole usually means to separate rods or whips, one reaching upward and one downward from the connector (i.e. center-fed). This is the most practical omni-directional antenna for VHF use. In order to avoid the center-fed issue which makes it awkward to mount on a paraglider, I went for an end-fed dipole. There is no real benefit to making it longer, or a half-wave, because you will be airborne when using it. A half-wave length is often used on the ground simply to get a portion of the antenna high enough above the trees and rooftops.
This antenna is a very thin wire (RG74 Co-axial), that runs up one of your inner B-Lines.
It is attached to the B-line by some very thin and flexible spiral-binding.
The antenna in no way affects your ground-handling and will still allow you to do B-line stalls if needed.
Now consider the three range issues previously mentioned:
- It is a much better antenna than the rubber-duck that the radio manufacturer fitted – in fact, several orders of magnitude better!
- it is held almost vertically in flight on an inner B-Line
- it is raised above your body and head, so the signal is not blocked by you, and it is clear of any metal parts (except the maillon at the top of the riser, which is incorporated into the antenna design itself and aids in its performance)
You may find the design and details to build this here. Measurements are given for both SAHPA frequencies and for airband radio.
If you fly both PG & PPG, and use the same wing, and want to use the same antenna for both radio types like I do, simply average out the two sets of dimensions. You sacrifice a bit of performance, but its will still be heaps better on both bands than your rubber-duck antenna.
Troubleshooting interference from a paramotor
If you are flying a paramotor and find that you are experiencing interference on your radio, you can try the following troubleshooting tips
- Check that your spark-plug has an R in its model number, or that the cap has a 5 kilo-Ohm resister inside (measure with a multimeter).
- Ensure that the ANR is enabled on the radio (Active Noise Reduction).
- Try a different model headset, as the headset cable acts like an antenna if the headset circuitry does not have sufficient RF filters.
- Add an RF choke on your headset cable if nothing else helps. You can find a free one on some AV cables. Look behind your TV, Hi-Fi, etc or on all your spare AV cables lying in a drawer of box somewhere.
- Your kill-switch wire from the coil to the hand-control might be radiating RF energy from the ignition coil. To diagnose and potentially eliminate this possibility, disconnect both kill-switch wires at the engine end and test-fly. Have another plan to kill the engine after landing (Choke, fuel-cock, HT plug, etc).
- If none of that works, then temporarily shield the entire HT cable and spark-plug cap by wrapping it in kitchen aluminium foil, which you must earth onto the engine to dump the RF energy.
- If you find its your kill-switch wire that is radiating RF, the solution is replace it with coaxial cable, with the centre wire going to the coil, and the braid going to the Earth connection.
- If you find its the HT cable that is radiating too much, then remove the aluminium foil which was just temporary to test that possibility, and replace it with the braided wire sleeve that you can cut from RG58 cable, or TV antenna cable or any similar diameter coaxial cable. Remember to earth the braid onto the motor.