Discussion in 'General Technical Questions and Answers' started by AI6DX, Oct 10, 2008.
Does that device really work in place of ground radials?
Yeah, it works but you still need a wire. All it does is tune the wire to resonate on whatever band you're working. But yes, it really does work and no, it doesn't work as well as a number of resonant ground radials.
Wire lengths and numbers
The MFJ-931 certainly works. Try putting many wires onto artificial ground output terminal, all the same length and all at the same time, and spread them out. Voila! Plural radials.
Another trick is to connect wires of different lengths, all at the same time on the output terminal. That way you get multi-band tuning. I use approximate quarter waves for all of the bands I use. Exact fine tuning of the ground wire can be achieved using the MFJ-931.
Not just for Verticals. A tuned ground is a must for long wires and particularly random wires. The MFJ-931 can resonate the "other half" of the antenna. Use an ATU for the antenna and the MFJ-931 for the counterpoise, and watch the signal strength soar!
Finally, a ground tuner such as the MFJ-931 is really a cut down ATU. Make, beg or borrow and RF ammeter, put an ATU where you would put the MFJ-931, adjust it the same way for maximum current reading in the ground wire, and see if it works OK for you. Please note that the SWR reading of the ATU is to be totally disregarded for this exercise. Only the RF ammeter reading counts. Watch you do not overload any meters.
It does scoot the current distribution around, and can solve certain "R.F. in the shack" problems when you have no other options. But it is NOT a replacement for a ground radial system. Is it worth having? Certainly.
It's a band-aid.
Spend the $$ instead to erect a antenna that does not require the artificial "Ground".
Rege, I do agree with you 100% about the good antenna. BUT, when one doesn't have the ability to erect a proper antenna, the artificial ground may be the only viable (and last resort) option.
They can also be put to good use for portable operating setups where running a large ground system is not possible.
1.) It cannot move current around in the ground. That is electrically impossible.
2.) It cannot transform or decrease ground resistance.
The only thing it can do is cancel reactance in the ground lead. If you have a ground lead that has 629 -j1100 ohms impedance with a current maxima 10 feet from the radio and you tune for maximum ground current, the very best result will be some resistance over 629 ohms j0 with maximum current 10 feet from the radio.
It is electrically impossible for it to do anything other than this, because the only thing it can do is add a series reactance (along with some loss resistance) to the connection point.
629 Ohms? Sounds way too high to me. Any measurements I have made indicate more like 40 or 50 ohms, the radiation resistance of the counterpoise wire. Do not be put off!
What no one has said is that the MFJ unit is a solution for people in apartments who are too high above actual ground. It that situation, a piece of wire to the real earth may be a DC gound but is no RF ground. If you are at or near 'real' earth, use it.
i've heard apt. dwellers on the second floor (and higher) refer to the 931 as nothing short of miraculous. I am a second floor dweller myself, and just ordered one of these earlier along with a 949e....... putting together my first rig.
My understanding is that the MFJ "Artificial ground" is simply a series tuned circuit with a meter that measures the amount of current flowing in a counterpoise attached to it.
The theory is that the RF voltage will appear at the end of the counterpoise, and the tuning circuit will maximise that amount of RF voltage and current by making the random length wire become part of a resonant circuit.
Based on the amount of positive reviews and comments I have read from apartment dwellers, it would seem to me that it is quite a viable option to having the typical "RF in the Shack" problems.
A long "ground" wire radiates RF just as well as the same length antenna wire, and if passes near susceptible electronic devices, increasing RF current in it will make RFI -- if there is any -- worse.
We really need in the shack to bond each piece of station equipment to every other piece, using low inductance connections. The ideal would be everything on a single sheet of metal, all staked down to that. But getting a low impedance RF ground at any distance from actual ground (basements are good) is a problem. The best all-round solution I've seen was one pointed out some years ago by EMC expert Glen Dash; A LARGE metal object has sufficient capacitance to [free space/the universe] to be a low impedance. I once calculated I'd need sheet metal under the entire carpet and over one entire wall for my shack to have a good RF ground -- but it can be done.
It seems you totally misunderstood my example. Let me try again.
The ONLY THING that little box will do is cancel the imaginary part of the impedance of the ground, whatever that ground is. It will not, and can not, change the resistance so it cannot make silk from a sow's ear.
As for resistance values, it is VERY easy to have a ground in the several hundreds of ohms. This is because ground systems at radio frequencies have standing waves on the wire.
Let's say for example we have a standard copper rod ground that has 50-100 ohms of RF resistance, the ground lead itself to the rod is 1/4 wave long, the surge impedance of the wire we use in the environment it is in makes it about 500 ohms Z0, and we connect it as a "ground". The impedance at the radio end would be 2500 to 5000 ohms.
We are dead wrong if we think we can pull some number out of our rear sides like 50 ohms, and say every ground is 50 ohms. If the ground lead was exactly resonant the little MFJ would not do a thing. We would be stuck with 2500 to 5000 ohms or worse no matter how we twiddled the knobs.
Now if the lead to the rod was 1/8th wave, we would have a reactance of about 500 ohms and a resistance of 100 ohms or so. The MFJ could cancel the 500 ohms right out, and we would be left with only the resistance.
So we see, it is not a magical thing that defies physics. We can't just connect it to anything in the world and suddenly have magic. It does exactly what described and what I described again here.
It will change the reactance of the ground path. It cannot ( because it is electrically impossible for a two terminal device that is series connected to do so no matter what is in the box ) decrease the real part of the impedance of the ground. It can only INCREASE the real part.
So whatever resistance the counterpoise or ground wire happens to have, that is what we are stuck with. We can only change the resistance by adding wire or subtracting wire or changing the ground. Tweeking the little knobs won't do anything but change the reactance, and that certainly can help if reactance is the problem. It can't help a bit if the resistance is the problem.
No, I did not misunderstand yor example.
But to quote an unrelistically high number to put off the original questioner seems an unfair thing to do.
You again missed my point.
The common mode impedance of a ground wire can be almost ANYTHING, and is often very high at least on some bands.
This is because the wire from the station to the rod, or the length of the wire itself at some frequencies, can present a very high impedance. It is not unusual for it to be nearly an open circuit on some frequencies, so my example was not an "unfair thing to do".
Unfortunately people sometime do not like the facts, but that isn't my problem or anything I can change.
If the ground lead or ground system happens to be a length that presents a high resistance (and this is common), then there is nothing the little box will do to correct that. If it happens to have, through luck or planning, a low resistance then the little box can null out the reactance and make it work.
It may make us feel angry or sad or bad, but that's how it really works.
I think the problem between us might be that you picture using an MFJ 931 use of the MFJ 931 to tune out reactance of an earth spike or otherwise grounded connection, which has, as you point out, has its problems, whereas I view its function of the MFJ 931 as being to tune a length of wire to look like a low resistance to act as a ground wherever its end is presented. In the example given, it is to be a "ground" at the base of a vertical antenna. Lengths of wires can be modified and adjusted to get the result desired.
I suggest that anyone with an interest tunes a length of wire to eliminate reactance and see what happens. It only takes an RF ammeter, a series inductor and variable capacitor as is found in the MFJ 931. The value of the inductor and variable capacitor are adjusted to get maximum reading in the RF ammeter when a signal is applied.
For the home-brewer, An RF ammeter can be made with a ferrite ring having some turns wound upon it, a diode, a fixed capacitor, a potentiometer and a meter. The meter can be anywhere from 50 microamps to 1 milliamp FSD. One end of the coil is connected to the diode. The other end of the diode is connected to one end of the fixed capacitor (0.01uF?). The second end of the fixed capacitor is fixed to the second end of the coil so that the diode and fixed capacitor are in series across the coil.
The meter and potentiometer are connected in series across the fixed capacitor. Make sure the potentiometer (100k?) is connected to provide a variable resistance. It might be an idea to fix a first fixed resistor (10K?) in series with the potentimeter to avoid meter burnout. A second fixed resistor (100-200 Ohms?) can be connected directly across the coil to prevent the ferrite coil from presenting a series load on the wire to be measured. The wire is passed through the centre of the ferrite ring. For convenience, use a snap-on ring with the coil on just one side to avoid need to disconnect a wire to measure current through it. Measurements are relative and are estimations only. Calibration is possible.
At wire lengths of 1/2 wave or multiples the wire will present a high end fed radiation resistance, and at 1/4 wave or multipes it will present a low end fed radiation resistance. If you would insist on using an artifiial ground of 1/2 waves or mutiples, you would create problems for yourself. Just for fun, take a random length of wire, eliminate its reactance, counterpoise it against a similarly reactance eliminated random piece of wire, and find the true radiation resistance and the staggering band width of the antenna you have created. Of course, as you would expect, near 1/4 waves or muliples work best.
I have even tried reactance elimination against an earth connected system with remote into-the ground connections. It too works. If someone insists upon using wet string or resistance wire to make the connection, consequences will ensue. If this is the case, make another ground connection. If you find that the length to ground is such that resistance is high, simply add another 1/4 wave to it to make it a low resistance.
Turning to the vertical antenna, in order to get the "radiating" element to radiate, it requires to be fed with current. The net current at any point is zero. To be fed with current the "radiating" element" requires a second path for the counter flow in the opposite direction to balance the first current. The tuned counterpoise on the reactance eliminating network (MFJ 931, for example) is connected to the common point and is tuned for maximum current flow, thereby maximising the corresponding opposite current flow in the "radiating " element. Maximisation of current and selection of around the correct length to give a low radiation resistance can make the difference between night and day.
I essentialy agree with AG3Y. Artificial grounds work. Nothing can equal being in the basement, with twenty 6ft strapped earth spikes through the floor on wet deep soil. Who has that? Very few.
A tip for the originator of this thread, AI6DX, If you wish to have just one artificial ground wire at the base of the vertical, make it two instead, each the same length, each insulated, and each connected in parallel to the tuner. Make sure that the parallel wires are laid upon the ground and lead in opposite directions. You then have a ground plane antenna with the verticality of radiation restored. Proximity of the wires to the ground seems to have a beneficial addditional grounding effect. Then tune the vertical element.
By the way, we seem to have a sunspot? Anyone noticed propagation?
73 to all
just thought i would drag this old thread to the front.
A friend of mine GD4EIP had a plan printed in practical wireless in October 1990.
It seems to predate the MFJ-931. Anyway I have just built an 'Earth tuner' from the plan. It took me 4 hours (I was doing it with my 9 year old son). and I now have to all intents and purposes an (MFJ)-931 artificial ground.
Cost? no more than £20 (GBP) and thats assuming you have to buy most bits.
If anyone is interested I could scan the plans. the MFJ one retails at £112. or look at http://remeeus.eu/ham-radio-english/miscellanous/artificial-ground-tuner.html
My antenna is an end fed wire hanging over a tree. I have no other options ... period. My nearest ground is 15 feet away in the upstairs bathroom and did not solve any of my RF problems. RF was in the shack and it was getting into everything and I could only tune up on a few bands. With the help of QRZ members I tried everything with in reason. It took me weeks of trial and error and the MFJ 931 solved most of my problems. I did cut five different lengths of wire and bundle them and connected then to the artificial ground. A good connection of tying the gear together is critical. I have made contacts on 80, 40, 30, and 20 meters with minimal RF issues. It worked for me.