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In a recent post on the Mobile Forum was a admonition against using an automatic antenna tuner in conjunction with a mobile antenna. It was stated that this would burn up the antenna in question. I don't think burn is the correct description, and I'd like to dispel the rumor(s).
Auto antenna tuners (AATs) like Icom's AH-4, SGC's various models, and several others, have become quite popular of recent and for good reason. They're easy to install, will match just about any antenna they are attached to within their limits, and offer the ability to QSY in seconds. But their use in mobile installations still remains mysterious and evil. And this shouldn't be the case, so let's take another look at them.
The most popular tuners in current use utilize a "T" matching circuit. That is to say, a shunt, variable inductance with input and output series capacitors. Within limits their efficiency is typically over 90%, but at their limits this can drop to just a few percent. AATs on the other hand can handle just as wide of ranges as the "T" match and yet maintain efficiencies of over 90% in most cases. This is because they are switched LC circuits.
These LC circuited AATs use a computer-controlled, binary-stepped series of inductors, and a binary-stepped series of shunt capacitors which can be switched between the input or output to match loads below or above line Z (impedance) respectfully. In the case of the Icom AH-4, the values are ~24 uh and ~2,200 pf and its impedance matching range is nominally 5 to 5,000 ohms. This is enough range to match an 8.2-foot antenna from 7 to 54 MHz. If the antenna is twice this length it will cover most of 75 meters as well.
What we have then is a versatile AAT which works on 12 VDC (ideal for mobile operation), offers a wide range of matching, has a quick QSY capability, so why not for mobile? Indeed, it is an elegant solution to many old problems, but it does have a limitation which needs to be understood.
A typical mobile HF antenna (<25 MHz) is 7 to 9 feet in length and requires some sort of loading coil to cancel the capacitive reactance, a result of the less than full 1/4 wave physical length. If the coil is fixed in value, it is necessary to remove and replace coils when changing bands. On 40 and 80 meters, any large QSY also requires retuning of the coil and/or whip to maintain a low VSWR.
Of late, there seems to be a proliferation of small, short, helically wound mobile antennas. They become popular because they tend to be less expensive than other antennas, they're light and easy to mount on a variety of cheap mounts, and they match well because their inherent losses are so high. But they still have the shortcomings listed in the last paragraph.
Obviously, it would be convenient to find a simple way to QSY without having to stop. There are many ways to do this, and one of the more popular ways is the screwdriver antenna. Remotely controlled from the drivers seat, it offers large excursions of frequencies albeit slowly. One benefit of their design is they maintain a relatively high Q because the coil is not short tapped like on some bug catcher designs. In some cases, it still requires changing a tap on an impedance matching device when band changes are made. So, the AAT is looking better all the time, except for those few shortcomings I mentioned previously.
Every mobile antenna has losses and the formula Rt=Rr+Rc+Rg describes those losses. Rt=total or input resistance (more correctly input impedance), Rr the Radiation resistance, Rc the coil resistance, and Rg the ground loss resistance. To maintain efficiency, we can raise the Rr, or lower the other losses Rc and Rg. We have some control over the radiation resistance since it is a factor of the electrical length of the antenna (loading coil position is also a factor). And to a lesser extend we can control the other losses too. Just for fun, let's look at an average 8-foot, bumper mounted, center loaded 20-meter antenna. It has a radiation resistance of 11 ohms, a coil resistance (loss, not reactance) of 5 ohms, and a ground loss of 8 ohms for a total of 24 ohms. The efficiency is 45%. A similar antenna for 40 meters will measure, 3, 14, and 10 respectfully for an input of 27 ohms. The efficiency is 11%. But what if we took the 20-meter antenna and matched it to 40 with our AAT? The respective calculated figures would be 3, 5, and 10 for a total of 18. Adding in the approximate losses in the tuner and we have an efficiency of 12%. In other words, this would be slightly better than our resonant 40-meter antenna. A lot of operators would (will) dispute this, but it just means they haven't done the math.
In my personal case, I use a rather long 20-meter antenna (13.5 feet) mounted on the left quarter panel with a heavy-duty GE mobile ball-mount. The coil is 4" in diameter, 2.5 inches long, and has a Q approaching 900. With the AH-4, I have full coverage from 75 meters through all of 6 meters. Subjective and empirical testing favors this combination from 40 through 6 meters, and equal to most on 80.
Let's get back to our poster and his admonition and that one limitation. If you were to use one of the aforementioned short, small, helically wound antennas, this trick wouldn't work. The reason is that these types of antennas have a lot of coil turn to coil turn capacitance, or what is referred to as distributed capacitance. They wouldn't burn up per se, but an arc could occur between adjacent windings and might render the antenna useless.
Alan Applegate, K0BG