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LNA Sequencing and Protection

A mast-mounted LNA can be very helpful, especially for EME, and in many cases for terrestrial weak-signal work on the higher UHF and microwave bands. One problem nagging most users of these sensitive little amplifiers is how to protect it from the station transmitter, and this problem is particularly severe above 100 milliwatts.

Some of the commercial self-switching LNA's available today are able to handle as much as 100 watts, and that's pretty much the upper limit. These units are expensive, mostly due to the internal circuitry that must be incorporated into the LNA enclosure. And what about power levels above 100 watts? What I'm going to describe next are my own techniques for solving the problem; they are certainly not the only ones, just the ones I have experience with and used successfully.

I did an initial write-up and prototyping on this subject a few years ago, and this material can be viewed here. If you've read this, you are probably aware that the absolute safest way to protect the LNA is to use split transmitter and receiver connections with a separate transmission line for the LNA-to-receiver connection. If this is not something you can or want to do, your solution will be less bullet-proof, but you can still have a fairly reliable setup. For the descriptions below, I'm going to assume a failsafe high-power relay resides with the LNA at the antenna, the LNA is bypassed by this relay by default (un-powered state), and that the power to operate the relay and LNA is applied using a separate connection. Some systems use the main transmission line with bias tees to also carry the DC power to the LNA, but to make things easier to describe, I'll assume a separate power wire.

When I finished developing the Amplifier Control Board (also featured in the link bar on the left), it occurred to me that even though this board was designed as a control board for high-power amplifiers, it was also a good solution as an LNA controller. More on this in a moment...

For those of us using a transverter, adequate protection can be as easy as a sequencer. A typical setup is shown to the right.

In this example, no RF is going to get up the transmission line unless the sequencer keys the transverter, which it will not do until it has the LNA powered down and safely bypassed.

Event 1 turns off the power to the LNA, placing it into it's default state (bypass mode).

Event 2 keys the transverter, making it free to generate RF.

Note that although the transceiver is generating RF before the LNA is bypassed, no RF goes up the transmission line until the transverter is active and makes the conversions.

An amplifier can follow the transverter, and can also be keyed by event 2 or any other sequencer event after event 1.

OK, now let's describe another typical setup, where you have a transceiver only (no transverter) and possibly an amplifier. Without some special controls, as soon as you key radio PTT, RF is on the transmission line and the LNA is damaged before it can get itself out of the way, even if you are using a sequencer. The problem is the soon as PTT is engaged, you have RF output, and the fastest relays in the world can't stay ahead of it (they need at least 20 milliseconds to switch over). Now we need to hold off RF from the radio while we get the LNA out of the way.

Every transceiver worthy of using has an ALC input, and this can be used to control RF output during this critical switching time. The Rev 6 amplifier control board has the necessary functions to do this. Here's how it works:

When the LNA is switched on, the control board also powers up. It immediately generates a negative ALC voltage, which is fed to the radio to prevent it from releasing RF. When the radio is first keyed, no RF is sent up the transmission line because of this ALC voltage.

At the same time (key-up), the first thing the control board does is bypass the LNA (event 1 of it's internal sequencer). In this example, it does this by shutting down the FET switch feeding power to the LNA.

At event 2, you can key up an amplifier

At event 3, the ALC is released and RF is allowed to be generated by the transceiver.

When the LNA is manually switched off, so is the control board, and the radio operates normally (there is no ALC blocking signal generated).

There are two obvious risks with this system, and the first is losing the ALC connection to the radio; if this connection is not present, or becomes unplugged, the LNA can be damaged.

The other risk is driver instability. Some radios emit a short burst of RF at key-up, whether or not the ALC is blocked. This is usually caused by an oscillation in the final stage of the radio, so it's always wise to test your system before going live with the LNA.