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Friday, September 12, 2014

When it Pays to Cut Corners

The antenna switch matrix was tested for its ability to maintain the proper impedance with various transmitter-to-antenna connection combinations. The transmitters ideally want to see 300 Ohms with no reactance as the RF power travels from the transmitter to the antenna. That perfect situation almost never happens, so the transmitter must tune to make up for mismatches. Our matrix is unique in that there are cases in which it allows multiple switch paths between particular transmitters and antennas. Each path must appear the same to the transmitter to avoid extra time spent in tuning.

Maintaining the proper distance between conductors helped the matrix exhibit the 300 Ohm resistive load. That was not particularly difficult. Keeping the reactive load as close to 0 as possible was more challenging. Quite a bit of work went into using 45-degree fittings to avoid 90-degree corners. Sharp corners add inductance. Too much inductance can cause problems in transmitter tuning.

The good news is that the measurements were nearly perfect for all switch combinations at all frequencies in the 6-18MHz range. Avoiding the sharp corners in the pipes really paid off. The transmitters will essentially not notice when the matrix is connected between the antennas and the transmitters.

We are currently working on verifying that the automated switch control will work. Once this is proven to work, TX7 and ANT2A/B will be connected to the matrix. Other transmitters and antennas will follow during the next several weeks.

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