1 of 4 

Radio Interference Test Procedures

FS/OAS A-30

Revision: Original

September 10, 2018

Standard testing verifies that radios meet specifications for correct frequency, modulation or 

deviation levels and basic operation. More in-depth testing may be required for aircraft approved 

for complex missions, initial use under a contract or when interference problems are suspected. In 

these cases the following tests can be used to verify proper operation.

These tests should be performed outside and at least 20 feet from any nearby buildings. The test set 

must be at least 20 feet from the aircraft to prevent near-field antenna effects. Further separation 

may be necessary to ensure direct line of sight. When possible ensure equal distance to each Com 

antenna. 

Step 1. Measure Com receiver sensitivity. 

Receiver sensitivity determines the distance at which the radio can communicate effectively. This 

test simulates reception of a signal from 50 nautical miles (industry standard for installed radio 

verification). 

Test setup for measuring sensitivity

1. Position the test set at a distance listed in Table 1. Ensure that the test set has direct line-ofsight and approximately equal distance to each aircraft Com antenna. Record the path loss 

corresponding to the distance selected. 

2. Set frequencies of 122.925 for AM radio tests and 168.3500 for FM radio tests. 

3. Using a Nav/Com Test Set with the telescoping antenna fully extended, record the RF level 

that causes the squelch to open on each radio.

4. Add the RF level to the path loss to establish the receiver sensitivity. The radio must receive 

a signal of -73 dBm or smaller to demonstrate reception of a signal from 50 NM.

5. Repeat this test for any radios with antennas that were not at the distance in Table 1.
2 of 4 

Step 1 Example: 

-44 dBm 20 ft. Separation Signal strength at the received antenna = -74 dBm

 (From Table 1)

Path loss = 30 dB at 20 feet separation between the IFR 4000 and the aircraft

The squelch opens when the IFR 4000 is set to transmit at -44 dBm 

Result: IFR 4000 + Path Loss = Sensitivity (must be -73 dBm or smaller)

 -44 dBm + (-30 dB) = -74 dBm = PASS

20 feet apart, direct line-of-sight and 

equal distance to each COM antenna

Step 2. Measure sensitivity during simultaneous use of multiple radios.

This test verifies that communications can be maintained at a distance of at least 24 nautical miles 

during simultaneous use of multiple radios. It also verifies that transmit interlock functions are not 

incorporated. Step 2 is not required for non-fire aircraft, single-engine helicopters that are not 

approved for passengers, or single-engine airtankers. 

This is identical to Step 1 except a second aircraft radio must be transmitting while measuring 

receiver sensitivity. Table 2 provides equivalent ranges for reference. 

Test setup for measuring sensitivity during simultaneous use of multiple radios

1. Set the radio under test to receive a desired signal (such as 122.925 MHz).

2. From the copilot station, transmit on a second radio at a frequency spaced 2 MHz from the 

desired receiver frequency (such as 124.925 MHz). Verify these are not active 

frequencies in the test area.

3. Monitor the radio under test from the pilot station and record the RF level on the test set

that causes the squelch to open.

4. Add the RF level to the path loss to establish the receiver sensitivity. The radio must receive 

a signal of -66 dBm or smaller to demonstrate reception of a signal from 24 NM.

5. Repeat this test for each combination of AM and FM radios.
3 of 4 

Step 2 Example: 

-37 dBm 20 ft. Separation Signal strength at the received antenna = -67 dBm

 (From Table 1)

 

The IFR 4000 is generating a signal on 122.925 MHz and Com 1 is tuned to 122.925 MHz

Com 2 is transmitting on 124.925 MHz

Path loss = 30 dB at 20 feet separation between the IFR 4000 and the aircraft 

The squelch opens when the IFR 4000 is set to transmit at -37 dBm

Result: IFR 4000 + Path Loss = Sensitivity (must be -66 dBm or smaller)

 -37 dBm + (-30 dB) = -67 dBm = PASS

20 feet apart, direct line-of-sight and 

equal distance to each COM antenna

Step 3. Check for adjacent channel interference.

This test verifies that transmitters will not open squelch on, or interfere with, other radios on the 

aircraft which are monitoring different frequencies.

Test setup for checking adjacent channel interference

1. On the pilot’s audio panel, enable audio for all Com receivers. Transmissions are conducted 

from the copilot station to prevent audio muting from masking interference during the test.

2. Tune Com 1 to 122.925, all other AMs to 123.925 and all FMs to 138.0000.

3. Transmit on Com 1 from the copilot station while monitoring all Com receivers from the 

pilot station for squelch breaks or other noise. 

4. Repeat this test multiple times while channeling the radios to different frequencies and 

spacing throughout the AM and FM band. Interference may vary based on the frequency of 

the receiver and the spacing from the transmitter. If any interference occurs, record the 

radios affected and transmit and receive frequencies tuned during the interference.

5. Repeat Steps 3 and 4 for each AM radio on the aircraft.

6. Tune FM 1 to 168.3500, all other FMs to 169.3500 and all AMs to 136.975.
4 of 4 

7. Transmit on FM 1 from the copilot station while monitoring all Com receivers from the pilot 

station for squelch breaks or other noise.

8. Repeat this test multiple times while channeling the radios to different frequencies and 

spacing throughout the AM and FM band. If any interference occurs, record the radios 

affected and transmit and receive frequencies tuned during the interference.

9. Repeat Steps 7 and 8 for each FM radio on the aircraft.

10. Ensure the radios can be channeled to within 1 MHz of each other without squelch breaks 

or interference during transmissions.

Table 1

Free space path loss and maximum test set signal level to pass Step 1 and Step 2.

Test Set to Antenna

Separation (feet)

Equivalent 

Path Loss (dB)

Maximum Signal at Test 

Set to Pass Step 1

(dBm)

Maximum Signal at Test 

Set to Pass Step 2 (dBm)

20 -30 -43 -36

30 -34 -39 -32

40 -36 -37 -30

50 -38 -35 -28

Note 1: The difference in required signal levels between AM and FM radios is offset by an equivalent 

change in path loss. This results in the same RF level required at the test set making the values in 

Table 1 valid for both AM and FM radios.

Table 2

Received signal strength at various distances on 122.925 MHz. Assumes the transmitting aircraft is 

using the least powerful radio allowed by contract (5 Watts). 

Sensitivity (dBm) Effective Range (NM)

-64.3 20

-65.9 24

-67.8 30

-72.3 50

Table headings link to range calculator 

Pt = 5 W

Gt & Gr = 0 dBd (aviation antennas)

Pr = Sensitivity

Note 2: The effective range calculator (https://www.random-sciencetools.com/electronics/friis.htm) uses statute miles. This must be converted to nautical miles for the 

purpose of this test. 

Note 3: If using the linked effective range calculator for FM frequencies, the recorded sensitivity 

must be increased by 3 dB to account for the corresponding increase in path loss (i.e. -66 dBm to -

69 dBm). This does not apply to existing tables which take this into account.