Radio Technique

Radio Technique

George M. Myers 70 Froehlich Farm Rd., Hicksville, NY 11801

ABSTRACT: Monitoring and battery cycling.

Monitors and monitoring

Mr. Floyd Reed, of SPARKS (South Pinellas Aeronautical Radio Kontrol Society), St. Petersburg, FL, writes that the club president borrowed a scanner and, while tinkering at the field, discovered a local radio station broadcasting on RC48 and “a lot of chatter” on RC22. These are channels that have been troublesome in the area for months. What gives?

When performing tests, always ask, “How can this instrument mislead me?” Practically any low-priced scanner sold in the U.S. would have to be modified to scan the 72 MHz RC channels, and modifications often introduce problems. In this case I suspect the scanner has a rather wide front end. Even a dual-conversion receiver with a precise frequency display can be fooled by image frequencies if its front end or local oscillator placement is inadequate. You should confirm whether the scanner has been modified.

Because the club president could understand the voices he heard, it is likely the scanner was receiving an image of a local FM broadcast station rather than true on-channel RC interference. A simplified example of how an image can occur:

• RC48 = 72.75 MHz (the RC control transmitter frequency)
• 10.7 MHz = first IF
• 83.45 MHz = local oscillator frequency (high-side)
• 10.7 MHz = first IF (offset to the image frequency)
• 94.15 MHz = local FM radio station (the image the scanner is actually receiving)

Commercial FM stations are spaced 0.1 MHz apart and each occupies about 0.08 MHz, so a scanner with poor front-end filtering can easily look right into the middle of an FM broadcast channel. Many scanners under $1,000 do not scan the 72 MHz band reliably because of these image and wideband problems. RC22 would behave similarly.

Signal strength is another important clue. A good scanner has a signal-strength meter. A reading of S1 is barely detectable and represents no threat; S9 is quite strong. Note, however, that an RC transmitter at normal model distances may well show S9 (± about 30 dB). That +30 dB margin is generally enough to protect safe flight.

Another possible cause is someone illegally using an old wideband AM RC transmitter (for example on Red/White, 72.240 MHz). A wideband transmitter can hit wideband, single-conversion receivers on RC22 and RC48 simultaneously through image or harmonic problems. Narrow-band, dual-conversion RC receivers that I’ve tested generally have not shown 21M/image problems, but strong nearby transmitters or an inadequate receiver at the field can still cause symptoms.

Advice and precautions

• Don’t bother the FCC or a radio station until you verify with a proper test instrument. The problem may be the scanner, not an on-channel interferer.
• If possible, confirm the scanner’s frequency behavior by checking the range of frequencies the unit can scan without changing configuration, and by noting whether the scanner has been modified.
• Your best protection is to buy the highest-quality receiver you can afford and keep it well maintained.
• Club project idea: convert obsolete receivers into personal interference monitors so pilots can monitor their own channels at the flying field.

Batteries and cyclers

All of you got a set of batteries and a charger with your RC system. Is anything wrong? Do you need a cycler?

The short answer: nothing is wrong, and you don’t strictly need a cycler. However, someday you may want to know whether there is enough charge left in your batteries to finish another flight. The only reliable way to measure the remaining charge in a nickel-cadmium (Ni-Cd) pack is by discharging it under a known load and measuring the total charge removed.

Why voltage readings can mislead

• Measuring pack voltage, even with an expanded-scale voltmeter (ESV), tells you almost nothing about remaining charge. An ESV can only show that the pack is producing voltage and that the cells are not completely dead.
• Example: remove one cell from a fully charged four-cell pack and an ESV may show the pack as discharged, yet a three-cell pack may still run your servos long enough for normal flying. Conversely, a five-cell pack may read “enough” voltage on an ESV even when it’s actually exhausted under load.

How cyclers work

• Typical cyclers discharge a four-cell pack at a roughly constant 300 mA current down to an end voltage of about 4.4 volts, timing how long it takes to reach that point. Manufacturers select cells that will last two to three hours in average applications, which corresponds to roughly 200–250 mA continuous draw, so a 300 mA discharge test is conservative.
• Cyclers can be reasonably accurate when used according to the manufacturer’s instructions, but they will give misleading results if you use the wrong number-of-cell setting.

Practical use: establish capacity and estimate remaining time

1. Cycle a pack at home to establish its full capacity (e.g., 500 mAh).
2. Recharge and fly as usual; note the flight time used.
3. After flying, go home and discharge the pack completely with the cycler. The remaining measured charge is what was left when you stopped flying.

Example calculation

• Pack capacity from cycling: 500 mAh.
• Flight time before testing: 2 hours.
• Remaining charge after returning: 50 mAh.
• Discharge rate while flying = (500 − 50) ÷ 2 = 225 mAh/hr.
• Remaining time at that rate = 50 ÷ 225 = 0.222 hr = 13.3 minutes.

This method lets you evaluate several packs in an afternoon without ever running out of power.

Practical notes, pitfalls, and safety

• Use the correct cycler unit for the number of cells. Some manufacturers (e.g., RF-Tronics) make dischargers labeled and color-coded for specific cell counts; using the wrong unit “tells lies.”
• Many dischargers have special plugs keyed to the number of cells to prevent mistakes. If you follow the recommended wiring procedure, you avoid connecting the wrong pack.
• You don’t need two cyclers in many cases. You can test the transmitter battery simply by extending the transmitter antenna, turning it on, and checking until the transmitter-meter shows a drop. If you test at night when nobody else is flying, one cycler can suffice for the flight packs.
• If a cell shorts during discharge, the discharger should shut off, the beeper will sound, and the reduced runtime will warn you to locate the bad cell.
• Don’t forget to record start and finish times of the discharge; otherwise the test is wasted (but the battery won’t be damaged).
• Don’t leave a discharger running unattended. Many units beep when they reach the end-of-discharge condition; if you can’t hear the beeper, you might leave a pack on the discharger for a week. Some dischargers will draw a small current (e.g., 1 mA) while beeping; after prolonged connection this can flatten Ni-Cd cells to zero volts, which is harmful.
• Because the back of a discharger can get hot, place it on a ceramic saucer during use.

Recommended practice

• Cycle batteries at home to know their true capacity.
• After flying, record flight time, then discharge the pack to determine how much remained when you left the field.
• Keep a cycle log for each pack so you can detect gradual capacity loss and replace weak packs before they fail in flight.

Final notes

• Cyclers are useful diagnostic tools and for capacity estimation, but they must be used correctly.
• For monitoring interference at the field, use proper test equipment and avoid jumping to conclusions based on inexpensive or modified scanners. Converting obsolete receivers into monitors can be a productive club project.

Transcribed from original scans by AI. Minor OCR errors may remain.