FM 828 repeater information

[VK4ADC]

Information in the second link indicates that U-band radios were designed for 450-470 MHz, and they need an extra half-turn turn added to the front-end helical filters, which accords much more with our experience building a repeater in Sydney. However, when we add a half turn to the helical filters, although they all peak quite convincingly at 434-435 MHz, we do not get the sensitivity that we would expect.

Lew.

Lew,

The extra half turn may have moved the frequency down but also altered the LC ratio and characteristic impedance away from the optimum design value/criteria for its environment/cavity dimensions and the coupling to either the next resonator or the in/out active device/s or terminations. It is quite some time since I looked at a UHF series FM828 so cannot recall the physical arrangements for the helical sections.

Is it possible to compress the helical resonator coil assembly slightly to increase the (1) inductance and (2) the turn-to-turn capacitance, thus lowering the frequency in lieu of the extra 1/2 turn ???
(Of course this is not possible on grooved formers....)

Also remember that receiver sensitivity is not the overriding factor in building repeater systems, effectively the duplexer separation is. A super-sensitive receiver is useless if it is being de-sensed by its own transmitter. The (common path) antenna gain can raise the effective receiver sensitivity provided the losses in the duplexer plus cables do not exceed the raw antenna gain figure. More importantly, measure the Tx-Rx port-port loss required in the duplexer to prevent de-sense and make sure your duplexer characteristic exceeds that value.

Keep in mind that it is sometimes more beneficial to run two FM828s with receiver in one active, transmitter in the other case - that provides lower RF coupling & hence better RF noise separation.

Hope my comments help

73 Doug VK4ADC