ali@ukc.UUCP (A.L.Ibbetson) (11/09/84)
On 14MHz, my Ten-Tec Omni transceiver suffers from breakthrough from strong European broadcast stations in the 49M band. The spurious response is due to harmonic mixing: 3 * LO - IF, where LO = 5+MHz and IF = 9MHz. It is at a level of -75dB relative to the wanted signal. On a 100ft G5RV-type multiband dipole fed via a bandpass tuner and 600 ohm line, the unwanted broadcast stations are about 10dB above the receiver noise floor. The Omni only has a 2 pole preselector, so I guess this performance is reasonable. I could fix the problem by using an external high pass filter but this makes band changing inconvenient. Since the Omni looks as though it's been designed by direct application of the Hayward & DeMaw book "Solid State Design for the Radio Amateur", I wrote to Ten-Tec asking whether replacing the 1N4148 diodes in the ring mixer with hot carrier types would reduce the spurious resonse. They replied that in their experience there is no improvement. This conflicts with Hayward & DeMaw's findings. Do any of you electronics gurus out there have any comments, before I start hacking at my Omni and destroying its resale value? Alan Ibbetson G3XAQ. mcvax!ukc!ali