Remote Control Revisit

I've written about this subject before, but it seems appropriate to revisit it. After all, it is that time of year when folks start thinking of configuring their mobile stations. So, a little sound advice up front, might save some luckless amateur a few headaches.

Before I get into the real meat of the article, I need to cover a few basic facts. It really doesn't matter what HF antenna you buy, but you should consider a few basic realities before you spend your hard-earned cash.

First, any (every) HF mobile antenna is a compromise, due mainly to one simple fact; any vehicle you mount it on, including one the size of a semi, is an inadequate ground plane. Since ground plane losses constitute the largest factor from an efficiency standpoint, those losses should be minimized.

Next, length matters. Generally speaking, a 12 foot antenna is twice as efficient as an 8 foot one, all else being equal. In reality, it's somewhat dependent of the aforementioned ground plane losses, and where you choose to mount your antenna.

The size of the coil, within reason, is important too. Length, diameter, and the ratio between them, as well as the position within the antenna (base, center, or top) partly determine Q. Think of it this way; long skinny coils are lossy, short fat ones are less lossy. However, the way the coil is made (and what it is made of) is also very important, especially the size of wire it is wound with. Although there is an ideal length to diameter ratio, the ratio is partly dependent on where in the antenna it is mounted, and on the ground losses present.

It should be apparent that any comparison between short fat antennas, and long skinny ones is a difficult task, much less antennas that are in between the aforementioned extremes. But as important as all of this appears, the single biggest factor any mobile operator has to deal with, is (again) the ground losses involved.

I often get asked how to minimize ground losses, and there are several articles on my web site dealing with these issues. As redundant as it may be, the, most important factor in maximizing what limited ground plane a vehicle represents, is placing as much mass under the antenna as you can. In other words, not along side!

I will not be drawn into an argument about where, or how, you mount your HF mobile antenna. All excuses aside, the factors every mobile operator needs to understand are these; the location you choose not only has a direct relationship of how efficient your installation will be, it also has a relationship on the RFI issues you'll face. This is especially true when your antenna of choice is a remotely controlled one.

An HF mobile antenna, in the strictest of terms, is a vertical antenna. Since a vertical antenna is half of a dipole, the missing half is (in essence) the ground plane it is mounted over (the vehicle in this case). Whatever RF current is flowing in the antenna, must also flow is its ground plane. When the ground plane is inadequate (lossy), the return current seeks the path of less resistance. And that my friends, is back down the control leads (if any), and the coax feeding the antenna.

Let's address the control lead issue first. If you mounted an HF mobile antenna atop a near-perfect ground plane, there still would be RF on the control leads. The reason is, the motor operates above RF ground. Whatever current is flowing must be choked off as close to the antenna as possible. Since the motor position inside the antenna varies across the various models, it's difficult to know exactly how much RF will flow. One common rule of thumb is, the impedance of any choke must be at least 2 magnitudes higher than the circuit's impedance. Assuming 50 ohms, this means the choke must exhibit an impedance larger than 5,000 ohms. That takes about 9 turns through a nominal Mix 31 ferrite bead. Unfortunately, that isn't the whole story.

As ground plane losses increase, regardless of the cause, the amount of RF current flowing on the control leads, and the coax feeding the antenna will also increase. Thus, the requisite choke will have to be larger. There is a practical limit to the number of turns one can wind on a single bead, like the one in the photo, so larger values may require more than one bead to be used (in series).

This begs the question; just how much choke do you need? Once again, that depends on where and how you mount your antenna, with whatever ground plane losses those specifics dictate. In extreme cases, the requisite choke(s) will need an impedance of 15,000 ohms, perhaps more! An antenna mounted atop a long post, jutting out from a trailer hitch is a typical, extreme example. What's more, if the choke is inadequately sized for the application, the choke can easily suffer thermal runaway, which will cause it to fail.

The coax feeding the antenna will also have RF flowing over it (common mode currents). Most of the time it is less noticeable, because the impedance of the coax is less than that of the motor leads. Nonetheless, it is still there with all of its consequences. The problems associated with choking off the coax are compounded by its size, and this fact may require multiple chokes, rather than one large one.

You will never be able to use an automatic antenna controller without properly choking the motor leads, no matter where or how you mount your remote controlled HF mobile antenna. Whether or not you need to choke off the coax remains moot.

If you're looking for more information on chokes, and how they work, here is a wonderful white paper on the subject written by Jim Brown, K9YC.

Alan, KØBG

www.k0bg.com