Getting an effective RF ground at the higher frequencies isn't really difficult, but it sometimes takes thinking about it a little differently. The problem is that wires going to an earth ground are usually quite long in terms of wavelength. That makes them ineffective.
For example, you have a 23 foot wire running from the rig to a solid earth ground on 30 meters, you'll have no RF ground at all at the rig. Indeed, just the opposite will occur and the rig will tend to "float" at a high RF voltage while transmitting. The reason is that 23 feet of wire is about 1/4 wavelength at 30 meters. A 1/4 wavelength long wire inverts the impedance seen at each end. If the end at the earth ground is truly grounded, the other end of the wire at the rig will be at a very high impedance. In short, the copper ground wire will look like an excellent RF insulator at 30 meters. Most situations aren't that simple, but it gives a good example of how a seemingly innocent ground wire running to a good earth ground may not provide an RF ground at all. Even a 10 foot wire or less will show substantial impedance at 30 meters and hold the rig well "above" RF ground. We can put that impedance inversion that occurs at the ends of a 1/4 wave long wire to work for us to produce a simple, effective RF ground. Connect one end of the wire to the rig and insulate the other end. The insulated end is forced to be at a high RF impedance, so the rig end is, by definition, at a low RF impedance (i.e. at RF ground). It is important to insulate the far end of the 1/4 wavelength long wire, not just to avoid RF burns while transmitting, but to force it to remain at a high impedance. That technique gets the ground impedance down in the vicinity of 35 ohms, which is generally a very good RF ground (most grounds using ground stakes or a few "radials" on or buried in the ground have an impedance in the range of hundreds of ohms at RF). The value can be lowered even further though by shortening the wire and then resonating it to a 1/4 wavelength on the operating frequency. Some companies sell 'tuners' specifically for this purpose. MFJ makes one called an "Artificial Ground". They are also easy to construct. Basically, all that's needed is a loading coil. A coil and capacitor may be easier to tune because the taps on the coil become less critical. A low-power 'antenna tuner' can also be used. What we're doing is turning something that is a big problem if we're making an antenna into an advantage to make a good RF ground. When we're forced to use an electrically short antenna that we resonate to 1/4 wave long using loading coils or an antenna tuner, we have to accept relatively low efficiency because such an antenna has a very low RF impedance. The shorter we make it, the lower the feed point impedance. The lower the feed point impedance the lower the efficiency. If we use it as an RF ground, we put that low impedance to work for us. By resonating a short wire at 1/4 wavelength at the operating frequency and connecting it to the rig ground, we supply a very low RF impedance that keeps troublesome RF voltages off of the rig. The disadvantage of such a ground is that the system "Q", or sharpness of tuning, gets high. That means that for a given adjustment, it will provide an effective RF ground over a fairly narrow range of frequencies. Such systems normally must be retuned whenever you QSY around within the band more than a few kHz. That can be a pain to do and rather defeats the convenience of an automatic antenna tuner! For that reason, most operators use a 1/4 wavelength long wire, which has a low enough impedance to provide an effective RF ground for most purposes and which still has a low enough Q that it doesn't need adjustment while tuning across the band. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
