Can I use the DF with an AM receiver?
The direction finders will work with AM receivers such as those used in the aircraft band, but the antenna needs special consideration. In mobile service in the aircraft band using a NBFM receiver, the antenna elements should be cut for resonance at the operating frequency. The system will then work at frequencies above and below resonance with only a small loss in sensitivity. This is because the relative phase between elements is not affected by the resonance condition. However, the relative amplitude (i.e., the antenna gain pattern) is very much affected by the resonance and in fact there will be a 180 degree reversal in the direction of the major lobe as the frequency is increased from below resonance to above resonance. To obtain unambiguous bearings using an AM detector, the mobile antenna elements should be resonant at a frequency that is always below the operating frequency range. If you plan to use the system in the aircraft band (108 – 136 MHz) , we suggest you order elements resonating at about 100 MHz.