Considerations for changing a tube transmitter site into a solid state site

Solid State Transmitter Installation Considerations

Source: Innes Corporation
Author: John S. Innes

Solid State Transmitter Installation Considerations

Solid state transmitters differ in certain respects from tube transmitters, and it is advisable to take this into consideration at installation time.  The checklist should include cooling, power line transient protection, load impedance matching, grounding and audio limiter settings.

Tube transmitters usually have their hot air outlet ducted outside the building, and therefore draw fresh air into the transmitter hall.  Solid state transmitters are more efficient, and so their cooling may be by fans with modest air flow volume, exhausting into the room.  Unless there are exhaust fans, or, preferably, air intake fans drawing outside air through filters and providing a slight positive pressure in the transmitter hall and providing a frequent change of room air, air temperatures may rise to an undesirable level.  Air circulation should be reviewed.

Tube transmitters can be tuned to operate into a relatively wide range of load impedances, but solid state transmitters, particularly in lower powers, expect to see an impedance close to 50 + j0 ohms, and positively require this..  Some dummy loads and some antennas are only matched as well as is needed for the tube transmitters that feed them, and will need their matching improved for the solid state transmitter.  In some cases this is a simple task, but in others, particularly directional arrays with an “Ohm’s law” power divider, achieving a close match with the existing networks may be difficult and time consuming where a directional pattern must be maintained at the same time.  A useful solution is a “fine matcher” network at the transmitter output.  Innes Corporation is able to supply such networks, on a custom basis that will cope with a mismatch of about 1.5 to 1 , for a price that is generally less than the consulting fee for an engineer to work on a directional array for a day.  In sites where the ground dries out seasonally, a fine matcher allows for quick and easy compensation for the resulting impedance changes.

Transmitter manufacturers will generally exclude from their warranty any damage caused to the transmitter by power line transients and surges, whether caused by lightning or by load switching by the supply company.  Surge reduction filters and transient suppressors are less expensive that transmitter repairs, and help avoid outages.  They work better and faster than the surge diverters of the pellet arrestor type that, with luck, the supply company may fit at the pole transformer.

Good grounding practices for a transmitter facility, which differ from studio practice, are essential both for minimizing lightning damage and for reducing the effects of stray RF energy where it is not wanted.  This may involve the use of ferrite cores to control common mode current paths, on audio and power cables as well as on the feeder coax.

Earlier tube transmitters usually have entirely different square wave tilt performance compared to solid state transmitters.  It pays to have a high pass filter somewhere in the audio chain to control this.  Some limiters have far more control over this aspect of performance than others, and it pays to review the equipment choice  and settings at transmitter installation time.

For all the above reasons, installing a new solid state transmitter involves more than taking it out of its box, connecting it and turning it on.  Support and advice from the transmitter supplier can help avoid problems later.