Friday, July 23, 2010

I have a Yaesu FT-990 transceiver that I use with a Carolina Windom antenna...

Q John, KU4KZ, asks, “I have a Yaesu FT-990 transceiver that I use with a Carolina Windom antenna. Most of the time I use my antenna tuner, but the other day the tuner was accidentally in the bypass mode. I noticed that the tuner’s SWR meter was moving as I talked. It seemed to kick up as high as 2:1. I was running about 100 W output. When I brought the antenna tuner into the line, the needle did not move when I transmitted. Can you explain this?”

AYes, I believe I can. To answer your question, let’s briefly
discuss what an antenna tuner and an SWR meter do.

Part of the function of an SWR meter is to measure any power that is reflected back to your transceiver that’s caused by an impedance mismatch in the antenna system. Most modern rigs are designed to accommodate antenna impedances of 50 Ω. If the impedance at the antenna system input is anything other than 50 Ω, power will be reflected back to the radio. The reflected power needle on your SWR meter will indicate this power. If it reads zero, there is no measurable reflected power.

The job of the antenna tuner is to match the antenna system impedance to that of the transceiver. Note that an antenna tuner doesn’t “tune” anything—it matches two dissimilar impedances. The antenna tuner transforms whatever impedance exists at the end of your coax to 50 Ω for the radio. When impedances are matched there is no reflected power and, again, the reflected power needle will read zero.

So, when your antenna tuner was bypassed you were seeing
the result of having your transceiver connected directly to the
antenna system. The SWR meter indicated that reflected power was present as you spoke. (In SSB, power is generated only when you actually speak.) When you switched your tuner back in, the impedance mismatch was transformed to 50 Ω and the reflected power at the SWR meter dropped to zero.

By the way, don’t worry too much about harming your FT-990 this way. Like most transceivers, the FT-990 includes a foldback circuit that senses when there is too much reflected power getting into the radio. The foldback automatically reduces the output to a safe level.

From QST July 2000