RF interference at GCRC’s Field.

As some of you are aware, we have recently been experiencing some apparent interference problems at our field and requested a frequency analyzer from District 8, which by happenstance arrived Thursday. Friday morning I lost the third plane in the last two months to apparent interference. Later Friday John Kling and I took the analyzer to the field to try and see if we could identify anything. After surveying the field and not finding any significant (over 1-2 dB) on channel 24 (the channel I was operating on) at ground level, we went on the adjacent property which used to have a city police firing range and has a berm about 15 feet high. There we found that we were picking up an intermittent signal on channel 24 that ranged from 2 -5 dB. The signal would be present for about 2-3 seconds and then disappear for as much as 30 seconds. At that point, since we had measured the transmitter output at 65 dB, this did not seem to be a signal sufficiently strong enough to cause any disabling interference.. However this was at close range and the spot which I experienced the hit was about two hundred to two hundred fifty yards from the pilot station. Also the radio I was using was an Airtronics PCM with fail-safe on. Since these readings didn’t really show any significant interference I decided to take the analyzer home and run some tests.

First, I ran a range test with the antenna fully extended. The analyzer has a ¼ wave collapsible antenna the same as the transmitters. The transmitter was turned on and positioned with the antenna vertical. I walked away from the transmitter holding the analyzer antenna vertical. At 100 yds., the signal strength had dropped from 65 dB to 45 dB. At 200 yds. the signal strength was at about 30 dB and fluctuating. At 250 yards, the signal strength was at about 20 dB and fluctuating. In addition, the position of the antenna relative to the transmitter could cause a variation in signal strength of plus or minus 15 dB. This test was repeated later to better define the relative changes with distance and position

Next, I positioned the analyzer next to the receiver with their antennas in the same plane. I placed a second transmitter with the antenna collapsed so that the analyzer was reading a signal of 10 to 12 dB and then with the primary transmitter walked away, working the controls to observe servo movement, until they quit working reliably. I then turned off the second transmitter and read the signal strength on the analyzer at the distance where the servos quit. Reading was 20 dB. I repeated this several times with the second transmitter at a signal strength of 2, 5, 10, 15 and 30 dB.

Consistently the fail-safe would activate and servos would lock any time a secondary signal would exceed 30% to 40% of the primary transmitter signal. In addition, the servos would not operate for about 3 seconds after the second signal was turned off.

I repeated the same series of tests with three other transmitters and receivers on FM-PPM. The results were not too dissimilar in the relative signal strength necessary to begin to see their effect. However, with ppm, until the secondary signal exceeded about 60% of the primary, the system would still operate, but there was some chattering of the servos. At signal strengths of over 60% relative, the servos would not respond to the primary transmitter inputs.

Saturday morning I took some additional field measurements with the analyzer antenna taped to a 20’ pole and connected to the antenna with a coaxial cable extension. Measurements taken from the top of the berm on the adjacent property. The pole this gave an altitude of about 35’ and was above the surrounding tree level. The signal on Ch 24 was still intermittent as before, but the strength varied from 5 dB to a maximum of about 18 dB. Turning my transmitter on and placing it at the pilot station where I had been flying and taking measurements where I had first observed that I had no control, signal strengths of 20 to 35 dB were measured, depending upon the orientation of the analyzer antenna relative to the plane of the transmitter antenna. The distance from the pilot station to where the problem began was approximately 225 yds.

This leads me to the conclusion that the interfering signal on 24 was great enough, relative to my transmitters signal strength at the location of the plane, to cause the fail-safe mode to activate. Since this was the first flight and the plane was not completely trimmed, the 3 second time for the system to reset was not long enough at 100’ of altitude to regain control. Probably if I had been flying FM-PPM I could have flown through the interference without complete loss of control since the field measurements indicated that the interference was only present for about 2 seconds.

In addition to monitoring Ch 24, I checked all of the available channels to determine which had any RF present and which did not. On Saturday morning the results were as follows:

Readings Saturday 09/23

Where two channels are listed, the RF was observed between the two and not directly on the frequency of either.

Channel Signal strength Comments

11 5 - 8 dB Present constantly, variable

19-20 0 - 40 dB Intermittent, variable

24 0 - 18 dB Intermittent, variable

39-40 0 - 45 dB Intermittent, variable

53 0 - 8 dB Intermittent, variable

54 0 - 5 dB Intermittent, variable

No RF was observed on any other channels.

Readings Monday 09/25

Channel Signal strength Comments

11 15 - 20 Present constantly, variable

12-13 0 - 8 Intermittent, variable

19-20 10 - 35 Intermittent, variable

22-23 0 - 3 Intermittent, variable

24 0 - 20 Intermittent, variable

37-38 0 - 8 Intermittent, variable

39-40 0 - 20 Intermittent, variable

53 0 - 8 Intermittent, variable

Readings on Monday were taken during the period 9:00am through 10:30 am. In addition, the following channels were observed to have no RF, either on or between channel frequencies, present at any time during the period which the reading were taken:

13 through 18

25 through 36

38

41 through 52

54 through 60

The only channels on which RF was present exactly on the channel frequency are 11, 24 and 53.

While I was at it, I ran three other tests. First, reduction of signal strength due to lowering transmitter battery voltage and a re-check of signal strength based on relative position of transmitter and receiver antenna to simulate the changing relative position of a plane in flight. Third, with a fully charged receiver battery, the lowest signal strength that the receiver system would operate without any problems.

For the battery voltage test, I used an Airtronics(old), HiTech (new prism) and a Futaba (6AX6 - new). The Airtronics had the old style output meter and both the HiTech and Futaba indicated battery voltage. All three transmitters were fully charged and started out with the following signal strengths. The analyzer was placed 100’ away for the test.

Airtronics Ch. 24 – 70 dB

HiTech Ch. 41 - 75 dB

Futaba Ch. 56 - 70 dB

None of the transmitters varied in strength until the battery voltage dropped to 9.6 volts and the Airtronics meter was at the upper edge of the yellow range. At 9.4 volts the HiTech and Futaba had dropped to 60 dB and 55 dB respectively. The Airtronics signal dropped to 55 dB when the meter was at the lower edge of the yellow range. This would seem to indicate that signal strength is acceptable down to battery voltages of about 9.5-9.6 volts. I would like to have conducted a similar test with receiver battery voltage, but as of yet have not figured out to do it.

Two versions of the antenna positioning test were run. First, the transmitter antenna was placed vertical. Measured signal strength at 50’ was 74 dB. At 200 yards (about the distance from the pilot stations to the west fence), the position of the receiver antenna was varied from vertical to horizontal, perpendicular to the transmitter then parallel to the transmitter antenna. The average signal was 37 dB with a -3, +4 variation.

The next version was with the antenna placed horizontal, and repeated to receiver positions of the first test. At a distance of 200 yards, the average signal strength was 23 dB with a -7,+7 variation. The practical aspects of this information is that if the plane is flying along the west fence and you point the transmitter toward the plane you are going to get a 40% reduction of signal strength. One additional test was run with the transmitter horizontal but parallel instead of pointing toward the receiver. The results of that were essentially identical to the first test.

The operation strength test was performed by setting a transmitter with the antenna collapsed and walking away with the receiver and analyzer. I had a neighbor move the sticks on the transmitter as I moved away. With no interfering signal, the Airtronics, Futaba and HiTech systems continued to perform without any glitches or chattering until the signal strength dropped below 8 - 10 dB.

Thinking about this information, I have reached several conclusions.

1. I will no longer operate on channel 24.

2. Operation on PCM fail-safe should be only on channel which have been shown to be completely clean since PCM fail-safe will initiate at a lower interfering signal to primary signal ratio.

3. Be aware of how far the plane is from you. The greater the separation the lower the signal strength of your transmitter. Also, at greater separation it takes less interfering signal strength to cause problems.

4. I would recommend that anyone operating on Channel 11, 24, and 53 consider changing to a frequency which is clear. Be aware that where there is RF on the even frequency between RC channels a cell phone transmitter/repeater is the likely source. If they have bandwidth problems it would be possible for interference. A search of frequency assignment database for 72-73 mhz in Harris county indicated that there are at least 8 cell towers within 5 miles of the field.

5. Be aware of the relative position of the transmitter antenna and the planes antenna. Separation greater than two hundred yards increases the potential for a problem. This is particularly so for those who fly with their transmitter in a tray, since much of the time the antenna is pointed toward the plane.

6. Monitor your transmitter voltage and don’t fly if the voltage is below 9.5 volts.