Wednesday, June 23, 2010

I recently began using an Alinco DX-70 HF transceiver with an antenna tuner ...

Q Daryl Pate, KC5SLQ, asks, “I recently began using an Alinco DX-70 HF transceiver with an antenna tuner and a multiband wire dipole antenna (using a coaxial feed line connected directly to the antenna—no balun). For a while, everything was fine. Now, however, I suddenly find that the SWR remains extremely high on all bands. I inspected the antenna and the feed line and they appear to be okay. Do you have any suggestions?”

Figure 1—A volt-ohm meter provides a quick test for possible feed line shorts (A). Test the function of the tuner and feed line by substituting a dummy load for the antenna (B).

A Obviously something has changed in your tuner and antenna system. Try the easy steps first (see Figure 1). Get a volt-ohm meter (VOM), disconnect the antenna coax from the tuner and measure the resistance between the center conductor and the connector shell. It should be infinite. If the VOM reads zero, you have a short somewhere in the feed line or at the antenna. Inspect the antenna again. Look for loose or broken connections between the coils or traps.

If you can get your hands on a dummy load, disconnect the coax at the antenna and substitute the dummy. A dummy load is just a resistor (or several resistors) in a box or can. It acts like an antenna without radiating much RF. Your transceiver should see about a 1:1 SWR on all bands and your tuner should be able to “match” this easily. If your tuner and transceiver behave properly when the dummy is connected, you’ve just eliminated them from your list of possible suspects.

But if the antenna tuner still doesn’t work with the dummy load, it’s time to pop the cover and do a visual inspection. Rotate both tuning capacitors. If you hear a mild scraping or feel the plates rubbing each other at any points in their rotation, you must attempt to reposition the plates so that they don’t touch at any point. Look for errant blobs of solder that could be shorting a coil or capacitor. Gently tug on the wires to make sure they are firmly soldered in place.

From QST April 1999