Short Antennas & Automatic Tuners

Or, when do automatic tuners make sense over a manual tuner?

I just found a fascinating article in the May 2008 QST ... actually, it was a short response under Technical Correspondence to an article on automatic tuners. But it answered a question I've had for a long time.

One quote that caught my attention: "...typically, the [automatic] tuners have a tough time when coupling to 'short' antennas whose RR (feedline impedance, if you will) is lower than the prescribed 50 ohms."

The author continues, "...you'll note that in the general instance of the longer -- lower frequency -- antennas, where the feed point (Load) Z is below 50 ohms, the automatic tuners are reticent to couple efficiently, whereas if the feed line Z is above 50 ohms, the tuner is efficient."

A couple of things come to mind ... first, in dealing with short vertical antennas, the antenna Z can be very low, hence the need for radials and/or baluns to match closer to 50 ohms.

Second, if your antenna is "short", whether a shortened 160M dipole or a short vertical, with an impedance of less than 50 ohms, you might want to consider using a manual tuner for better matching range.

If your antenna sports a Z close to or even higher than 50 ohms, the automatic tuners can handle the match easier. The antenna design impedance is affected by a combination of antenna length and height above ground.

This does explain why my auto tuner in my KW TS-440 cranks and cranks to match my NVIS antenna (G5RV at 6 feet high) but quickly tunes on my 40 meter doublet at 20 feet high.

One of these days, I'll have to measure what the impedance is on each of these two antennas.

Something to consider ... AR