Testing Lab #2

The Third Order Intercept Point

Impact on IMDs in an Antenna Distribution System

The third order intercept point is  when two or more transmitted frequencies become non-linear. The third order intercept point is a measure of ____. It is more important to understand when IMDs occur and how to prevent them from occurring or avoid them. Lets look at some practical examples of when IMDs occur.

One very common causes of IMD occurs when you have two of more microphones very close to each other. This commonly happens when you set out microphones on a table, or the top of a rack ready to be used. We are going to do some testing to determine the impact of this process on the generation of IMDs.

The Test: 

To measure the IMD’s Created internally in a antenna splitter. First we establish a baseline the test in XX different scenarios to capture the intermod generation in each scenario and determine a best practice going forward. For all test scenarios we will use  


1. Establish a baseline by determining  the RF Spectrum in free air 10 feet from the transmit antenna at the point that the gain of the antenna is focused. Take a snapshot of the spectrum with each of three Tx antennas to be tested (Shure PA805WB LPDA, PWS-8089 Helical, & Shure 1/2 wave whip). Capture results at each of three output power levels for the PSM1000)


2. Determine what setup has the least impact in IMD do this test at 50mW using Helical for Tx and LPDA for Rx

A. Tx in front of Rx

B. Tx Behind Rx

C. Tx and Rx side by side

D. Find lowest

Both antennas in the same place aiming at stage with x distance apart


3. Determine the distance the minimum distance between Tx and Rx antennas that the 2T3O IMD is ≤ -90dBm in the following TX RX antenna combinations in all scenarios the Tx antenna is in front of the Rx antenna at the same height focused in the same direction parallel to the floor at 3 output power also try LR in same plane

A. TX Whip RX Whip




E. TX Helix RX Whip



4. Bracket distance with two LPDA antennas


Test common Practices 

5.  PX and RX in racks next to was other both with whip antennas


6.  Helical on stand on top of rack


7. 3O2T Cross modulation on UHF R microphone




All four tests use the same PSM1000 transmitters and are tuned to the same frequencies for each test. The microphones are set to high power and the antenna for the scanner is a wide band dual dipole 1 meter above the microphones oriented in phase with the microphones.


Acceptance Criteria:

1.  the average of the 2 tone3rds is less than half the power in dBm of the average of the primary tones in relation to the noise floor.

2. Neither of the 2tone5ths are more than 10dBm above the noise floor.

Test 1: 

Establish Tx Baseline


Test 2: 

Determine Optimal Setup Arrangement


Test 3: 

two racks with whips


Test 4: 

Helical in stand on top of Rx rack

Observations: 2tone3rd IMD average is less than half the average dBm of primary tones. 2tone5ths IMD are less than 10dBm above the noise floor.


This scenario produces the most desired results and should be best practice with all handheld and  belt pack transmitters.

Conclusion of Results

The use