At 04:19 PM 03/21/07, you wrote: >Has anyone tried to do a 220 mhz conversion on the Mitrek?
I've been thinking about it for a while... There are two approaches - take a UHF radio and put a modified high band front end in, and bypass a doubler, or take a high band radio and move it up. The mods that move the entire radio multipliers from 12mhz x12mult=144mhz to 18.5mhz x 12mult=222Mhz just are not practical... the 18mhz rocks are not stable in the channel elements, plus they don't modulate well. The conversion of the final multipler from a x2 to a x3 is much cleaner and simpler. I'd take a high band radio and change both the receiver local oscillator multiplier and the transmitter exciter multipler so that instead of doubling from 74mhz to 148mhz you triple from 74 to 222MHz. You might need to change out the last multipler transistor to something with a little bit higher Ft. The front end work has pretty much been done by Scott and Kevin on the Micor, and the same techniques can be used both there and in the helicals between the exciter and the PA deck. The PA is hopeless as the VHF striplines won't play at 220, but a radio with a dead PA is ideal for conversion... just strip the PA down to the heat sink and put down a 220mhz power brick. Or if you have enough drive use a Wilkinson combiner backwards to split the drive to two PAs and use a second one to combine them back to a single feedline. Double the pleasure and all that... The transmitter deviation will need to be cranked down as the multipler is higher... times 18 vs times 12. If it comes to the worst case a 3:2 resistive divider can be inserted between the audio stages and the channel element. Receive audio is unchanged. The high band Mitrek is essentially an updated single-board version of the old Motran with a better front end, and a stripline PA vs an LC PA... the techniques developed by the 220Mhz guys in Chicago (CFMC ??? I forget) 30 years ago for the Motran still work fine (except for the PA mod). Mike WA6ILQ
