Solid State Exciters, Amps, and Matching

I know a diagram would help explain this subject and maybe later I will add a diagram. For now I will attempt to describe the problem using words. You know, word problems. Like the ones you used to solve in high school?

Consider the transmit problem. The antenna or antenna system is at 50 ohms. No problem there. Just connect it to the amp output. The problem comes when we connect the exciter to the amp input. The amp input is not at 50 ohms on all bands or even over the extent of a band like 80 meters. This causes us an immediate problem with solid state exciters. The mismatch reduces the output of the exciter as the exciter reduces power to protect its output transistors. If your amp needs 100 watts drive and the mismatch results in a 2:1 SWR, the exciter may only provide 50 watts max and the amp will not develop full power.

The intuitive solution is to use an antenna tuner or matching device between the exciter output and the amp input. Adjust this to get a 1:1 SWR into the amp input and you will get max output from the exciter as well as the amp.

This solves the transmit problem but what about when we switch back to receive? The antenna system is still at 50 ohms but the exciter output has been matched to whatever the amp input happens to be. We do not know what that is but we do know it is not 50 ohms. So now we have that old 2:1 SWR hurting us on receive instead of transmit.

Can’t really retune the matching because we need power to run the SWR meter to do that and that will get us back to a transmit condition.

The solution to this problem is to include the extra matching device into the amp and make it part of the amp input circuit.

That solution may be impossible to implement, particularly if the additional matching network is internal to the exciter in the form of an automatic antenna tuner. Realize that if that is indeed the case, we would not have experienced the power starving condition we attempted to correct. However, we would still be subject to the mismatch on receive.

Is the receive mismatch really significant? Or are we talking about one silly db among hundreds? A fair question but it arises from a possibly dangerous precedent. A tendency to cut corners is never good. Eventually, all those round corners will add up to become significant even if the effects of each individual compromise can be ignored.

At this point we might resign ourselves to solve the problem rather than merely treat the symptom. How about we adjust the input tuned circuit in the amp. If we are talking about 80 meters, that may not be possible. 80 meters is wide enough to allow us a match in the CW portion or the SSB portion but not both. So we are back to incorporating the additional matching device into the amp input circuit. Note that we will need an additional matching device even if the exciter has a built-in auto tuner unless we can access the input and output of the auto tuner independently of the exciter connections.

Since we probably will not be able to locate the extra matching device inside the amp, we will need to locate it outside the amp and use some creative cabling requiring two additional coax connectors be installed at the amp input.

One of these connectors becomes the internal connection to the input of the amp. This becomes the place to connect the output of the extra matching device. Label this one ‘internal amp input’.

The second connector is wired to the input portion of the antenna change over relay. That terminal to which the amp input was originally connected. This becomes the place to connect the input of the extra matching device. Label this one ‘change over amp input’.

For configurations in the future where an extra matching device is not needed or desired, these two additional connectors can be jumpered with a short length of coax and the amp is restored to its original condition.

By adding the two additional connectors we have taken the matching device out of the common T/R antenna path. We now have a receive path that is separated from the transmit path. We no longer suffer a mismatch on receive by adjusting for a match on transmit.

Explore posts in the same categories: HamRadio

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