Entries from April 2015 ↓

Custom Control Console

This concerns antenna switching and SWR measurements. For convenience the switching and measuring is best done close to the operating position. Perhaps in a console located right next to the radio.

We have built such a console. It contains a multipurpose power supply, a 100 watt linear amplifier, an SWR/Power meter and controls, and switches allowing selection of three antennas on each of two coax feedlines.

Initially this supply was intended to provide power for a small solid state linear amp and some LED lighting. The amp was for use with the KX3 to boost its 10 watts to a level better suited to drive a larger linear close to full power. The small amp needs 24 volts at up to 3 amps. A 5 amp linear regulator was used to supply the 24 volts.

The transformer in this supply is capable of delivering at least 500 watts. A second 5 amp regulator was installed to provide 14 volts to the KX3 and LED lighting. Additional 1 amp regulators were installed for 12 and 5 volts. The 14 and 12 volt sources were brought out to the back panel through PowerX connectors.

The power amplifier is an EBY design using a pair of IRF540 fets. It covers all bands. A band switch is provided on the front panel. The amp is built into the console along with the power supply. This amp is not protected and must be used cautiously.

This is an old heathkit meter that can measure up to 2000 watts. Its sensing unit is located remotely to make antenna cabling more convenient.

There are two switches mounted to the front panel. One for each of two coaxial feed lines. The switches are double pole double throw with a center off position. The switches are connected to remotely located power and coax interface boxes through four wire shielded cable.

These switches route power to antenna relays located on the antenna tower through the coax feed lines. Plus polarity, negative polarity, and power off, select one of three antennas through the relays.

This system is patterned after a Heathkit remote switch which allowed selection of four antennas. The fourth selection was accomplished by feeding AC to the relays where two half wave rectifiers and filter capacitors activate both relays simultaneously. I could not make the AC feature work with the relays I used so I ended up with only three antenna selections.

Both feed line isolators and their power supply are located near the two amplifiers they serve. The amplifiers are located on the floor below the desk. The remote isolators are mounted to the back of the desk.

A patch panel located below the center console allows connection of any number of radios to the amps. When the amps are not powered up or set to standby, the radios are connected directly to the antennas.

Soft Key – where to install it

References to Soft Key are normally found in articles dealing with linear amplifier modifications. The Soft Key circuitry is most often installed in amplifiers whose design incorporates high voltage bias to fully cut off the tubes when the amplifier is not keyed.

Typically this bias voltage is around 120 volts. This value is conveniently chosen to provide full cut off as well as current needed to actuate the 120 vdc antenna changeover relay. By taking the relay return to ground these amps activate the relay while also removing the cut off bias. So by a single connection to ground we route the exciter input to the amp input, route the amp output to the antenna, and remove standby bias all at the same time.

The only problem here is that the 120 vdc keying line needs a device that can handle 120 vdc at 10 to 30 ma.

That is not a problem with exciters that provide a relay closure to activate the keying line. Modern solid state exciters may not provide relay closures. Those that don’t may not be able to handle 120 vdc.

Recently I bought an exciter that will not handle more than 40 vdc at 20 ma.

If I install Soft Key in the amp, that amp will be the only amp I can use with the new exciter. I do have other amps. All the amps I have work well with all the older exciters I own. I do not want to install Soft Key in every amp I own to make them all safe to use with the new exciter.

That is why I will install Soft Key in the new exciter and enable its use with any amp.

Since the new exciter is portable, its use with other amps (amps I may not own) is a very real possibility.

The Soft Key circuitry will operate off 14.8 vdc. The same 14.8 vdc that runs the exciter. The soft key output is a high voltage switching transistor. The Soft Key circuit is conveniently mounted into the same box that contains the 2 amp hour Lithium ion batteries that can be used to run the exciter as a portable.

To prevent damage to the exciter in the event of failure of the Soft Key circuitry, Soft Key circuitry uses an opto isolator between the switching circuit and the exciter.