As always, Don has excellent suggestions. A very simple current meter for tuning is to put a small light bulb (flashlight bulb is often fine) in series with the counterpoise at the rig. Now apply power, adjust your tuner, if you're using one, and then start shortening the counterpoise for maximum current (brightness). You do NOT need to actually cut the counterpoise, You can temporarily fold it back on itself. The folded back section just looks like a "fat" wire to the RF: it doesn't follow tight bends. You may need to short out the bulb when moving to full power: in many situations a LOT of current flows in the counterpoise!
Another technique is to cut the counterpoise short and add a small loading coil at the rig to bring it to resonance. I've used that approach with excellent results. A good counterpoise can be as short as 1/16 wavelength with a loading coil, meaning that you can use the same counterpoise of several bands. For example a 15-foot counterpoise will work on 20, 40 and 80 meters with a suitable loading coil. You can extend that in the other direction by adding a variable capacitor in series with the coil to bring it to resonance at higher frequencies. MFJ used to make a little tuner they called an "artificial ground" that used just such a circuit and which included a simple current monitor for adjusting it. I don't know if they are still being made, but almost any QRP type ATU will work with a suitable current monitor to peak the ground current. Moxon suggests that, if possible, use at least two counterpoise wires, running in opposite directions so they don't radiate and bring them to resonance with a common loading coil where they connect to the rig. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Don Wilhelm Sent: Wednesday, February 20, 2008 1:14 PM To: Solosko, Robert B (Bob) Cc: elecraft@mailman.qth.net Subject: Re: [Elecraft] Re: Best way to secure RF ground from 2nd floor- arelated question Bob, The best way to tune the counterpoises is to use an RF current probe and tune for maximum current in the counterpoise wire. Several designs for RF current probes have been published in QST and other journals in the past. An old method (which still works) is to put an RF ammeter in series with the counterpoise wire to tune it. You are correct, cutting a wire to some formula will not necessarily yield optimum performance - the counterpoise wires are usually mounted low and close to earth and other objects - that proximity will detune them, sometimes drastically. To use the MFJ259, I would think the easiest would be to split the wire in half and measure its resonant frequency (with both ends connected to nothing) - it would be a half wave dipole on double the frequency - prune for zero reactance and reconnect the cut made in the middle of the wire. 73, Don W3FPR Solosko, Robert B (Bob) wrote: > Hi All, > > I have a related question: how do I tune the counterpoises? > > My shack is also on the 2nd floor and I had major problems with RF in > the shack on 15m, 17m and 20m. My antenna is a 100' dipole connected > with ladder line to a 4:1 current balun that's just outside the window > where my shack is. Then I have 5' of RG-8 running through the window > to my KAT100 inside. I also have the braid from some old RG-8 as a > ground wire running from the KAT100 through the window to an 8' ground > rod. I had RF in the shack problems with this arrangement that was > mostly resolved by connecting 3 counterpoises to the ground side of > the balun, one for each band. It's not perfect, but it's a major > improvement. The counterpoises were cut approximately for each band - > so the question is how do you tune the counterpoises, since the > effects of RF in the shack at this point are somewhat subtle. > > I just purchased an MFJ 259B antenna analyzer (mine actually seems to > work out of the box!), but I don't see any obvious way to use it to > tune the counterpoises, since the counterpoises are just heavy > insulated wire, and not coax. Can the antenna analyzer be used to tune > the counterpoises? If not is there some other way to do so without > needing other specialized equipment? > > Thanks. > > Bob W1SRB > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
