70cm filter pre Yaesu FT-857
Each type of operation is something specific. Working via satellites has two main requirements: receiver sensitivity and transmitter cleanliness. He has both And we have it over with problems, but nevertheless the satellite device is usable. Sensitivity can be improved with preamplifiers, whereas, especially in the 70 cm band, it is very weak and most attempts to capture the signal from the satellite will thus end in failure. Fortunately, making a preamplifier is not difficult.
Second, a worse problem is the unwanted products of the transmitter from the 70cm band. These are manifested after locking (including SSB without modulation) and reception on a 2m device on one table. Only a loud noise comes out of the receiver, which reliably covers any other signal. This is probably due to the combined end step of 2m+70cm. The solution is to use an additional filter.
The described filter is the result of an experiment, it can be said, that it is a prototype. It is based on the mechanics of the helical filter, which, however, could not be tuned into the band. It will need some tweaking for the future.
The connection is based on an ordinary diplexer, more precisely, its halves - high-pass filter. Despite the concerns, that such a simple circuit cannot eliminate strong interference, the opposite is true. Losses in such a filter after tuning are minimal.
Everyone adapts the mechanical construction to their own capabilities. The condition is, that the filter is in a shielded box and if possible, so that the coils are also shielded (possibly rotated by 90 degrees). Suitable dimensions are 25 x 25 x 40mm. Remains of kuprextite will serve well as a material for making such a box. Capacitor trimmers can be ordinary foil trimmers from the nearest store, they have a capacity of 3-15pF. The coils are self-supporting, 1z on a diameter of 6mm CuL wire.
A PSV meter and a screwdriver are enough to tune the filter. By turning the trimmers, we try to achieve a minimum PSV both at the inlet and outlet (the filter is symmetrical, after exchanging the input with the output and vice versa, we must measure the same values). Since trimmers have a live rotor and stator, it is advisable to use a tuning preparation made of insulating material. and until, according to one of them, a decision is reached, we proceed step by step, so we conclude, PSV measurements, we unlock, turn the trimmer and repeat the whole procedure. It takes patience, individual elements interact with each other. The tuning is obvious, an untuned filter has a high PSV, when we meet, tuning is only minimal. C1 and C3 should have the same rotor position.
Of course, we do the tuning with minimum power, otherwise there is a risk of damaging the trimmers or the final stage. The greatest load on trimmers is precisely when they are not tuned!
The efficiency of the filter is excellent, it is possible to listen to one's own signals via the VO-52 with a device on the same table without any problems.
