Transponders in the M-18

This is for you Hi-Tech Mite pilots who are flying with transponders and mode C.

If the controller tells you that he is not receiving your mode C, or it is intermittent, here is the probable cause....

Over the years, your transponder may drift off frequency. The first indication is mode C problems. The Narco, the Collins TDR-950, and also the straight King 76 transponders had a problem with drift. The King KT-76A, and the new all solid state transponders are very stable.

Do's and Don'ts

Don't remove your transponder and take it to an avionics shop to have the frequency checked.

Do have the avionics tech check your transponder with it installed in the aircraft, using a ramp test set. Here is the reason:

The coax cable from the transponder to the antenna can 'Pull' the frequency, depending on the length and the configuration. So if the ramp check shows the frequency to be 2 MHZ high, the tech can then put it on the bench and lowered it by that amount, regardless of what the frequency shows on the bench test.

Adjusting the frequency is a simple screw adjustment on the output cavity, but it must be made with the proper test equipment.

At these high frequencies, (1030 and 1090 MHZ) normal HF protocol doesn't apply. So if you are installing a transponder, make the cable to the antenna as short as possible. If you are using a stick antenna, it must be absolutely clean, no oil, dirt or paint. The blade type antenna is more tolerant. Avoid any sharp 90 degree bends in the cable. You could lose 50 percent of the power output. If the cable has to make a change of direction, make it a large radius sweeping turn.

Submitted by Ben Favrholdt, N66MX