Every type of operation is specific in some way. Working through satellites imposes two main requirements: receiver sensitivity and transmitter cleanliness. Both have FT-857 problems, but nevertheless the device is usable for satellites. Sensitivity can be improved with preamplifiers, especially in the 70cm band where it is very weak and most attempts to capture a signal from the satellite end in failure. Fortunately, making a preamplifier is not difficult.
The second, worse problem is unwanted products from the 70cm transmitter. These manifest after keying (including SSB without modulation) and reception on a 2m device on the same table. Only strong noise pours from the receiver, reliably masking any other signal. This is likely caused by the combined output stage of 2m+70cm. The solution is to use an additional filter.
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High-Pass 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 a helical filter, which, however, could not be tuned to the band. In the future, it will need some adjustments.
Wiring Diagram
The circuit is based on a regular diplexer, more precisely its half - high-pass filter.. Despite fears that such a simple circuit cannot remove strong interference, the opposite is true. Losses in such a filter after tuning are minimal.
Mechanical Construction and Component Selection
Each person can adapt the mechanical construction to their possibilities. The condition is that the filter is in a shielded box and, if possible, the coils should also be shielded (or rotated by 90 degrees). Suitable dimensions are 25 x 25 x 40mm. For making such a box, scraps of copper-clad laminate work well. Capacitor trimmers can be ordinary foil ones from the nearest store, with a capacity of 3-15pF. The coils are wound self-supporting, 1 turn with a diameter of 6mm CuL wire.
Tuning the filter
For tuning the filter, we only need PSV with a meter and screwdriver. By turning the trimmers, we try to achieve minimal PSV both at the input and output (the filter is symmetric, after swapping the input with the output and vice versa, we must measure the same values). Since the trimmers have a live rotor and stator, it is advisable to use a tuning tool made of insulating material. If not, we proceed step by step, that is, we key in, measure the PSV, key out, turn the trimmer, and repeat the whole process. It takes patience, individual elements influence each other. Tuning is evident, an untuned filter has high PSV, when we:we hit the target, fine-tuning is only minimal. C1 and C3 should have the same rotor position.
Tuning is of course done with minimal power; otherwise, there is a risk of damaging the trims or even the final stage. The greatest load on the trims occurs when not tuned!
Filter Effect
The filter efficiency is excellent; it is possible to listen to your own signals via VO-52 with the device on the same table without problems.