2m Interference Reduction System by Trevor S Day, G3ZYV
T
HIS CIRCU IT WtU substanlla lly reduce or completely eliminate mertereece Irom almost any l0cal source whilst leavirog the
wan ted signal relatively unaffected . even though il may be on the same frequency! Once the unit is adjusted lor a particular source 0 1 interference, the use r is tree 10 operate anywhere in me band without further adjustment. The design of the circuit linlllS operallon to a sma. band 01 frequencies, however it is sufticienl lo cover the entire lwo metre band . The principle can also be used at 50 and
70MHz and indeed this unit was hrsl constructed 10 work al SOMH z in o rde r to remove computer hash. (See iss ue 28 otme UK Silt Metre Group Ne wsJeNet) .
PRINCIPLE OF OPERATION THE BLOC K DI AGRAM (Fi g 1) shows the method of ope ration . Both the wanted and
interfering signals arrive at pre-amplifier 1 Irom the main antenna via the antenna switch. The sense antenna, by wfue of its ineHiclancy at the wa nted rrequency . picks up predominanlly the intorlaring signal and passes this to pre-amouuee 2. Both prea mp lifier out puts even tua lly a dd in the sign a l co mbiner. However, the interl e ring signa l is first changed in phase by 180°. The re sulta nt o utp ut of the co m biner is the sa me as the input to pre-amphfler 1 fro m the m ain ant enna less the inlerle ring signal.
CIRCUIT DESCRIPTION 80T H P RE-A MP LIF IERS a re standard duat gate M OSFET de signs, using the 3S K8 1
device as eese were 10 hand (Fig 2 ). Almost any common dual gat e M OSFET wiU work in this coofiguratiOn. eg the 406 73, AV3 contrl..
RYl 7>0
Fig 1: The two -'gn8l ~jft ~
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