Ground choke for long ground wire

Hi krimles.

I think you are trying to have a ground wire that the FCC will not count as a true ground wire, but which gives you a path to ground. Convention holds that the most effective ground is as close as possible to zero ohms, so I think your high impedance would be about the same as no ground; that is, I don't think it will do anything to your signal strength.

Maybe your design would provide lightening protection, especially if your ground wire was heavy-gauge and well earthed.

Thanks for your comment Clare. I have tried this and it does act as if there was no ground connected (or at minimum a very poor one). I have noticed that the signal does drop somewhat and noise effects seem greater. I don't know if this type of solution would be of any benefit to getting around FCC legalities.

An inductor in the ground lead adds to the loading coil inductance. If the ground lead inductance is the same as the loading coil inductance, the original loading coil can be removed, and the ground lead inductance will tune the transmitter. If the transmitter uses a tuned output transformer as the loading coil, you can't, of course, remove the loading coil, because then the transmitter would not work at all. Removing the loading coil would work where it is in series between the output of the transmitter and the antenna. If the original loading coil is retained, the inductor in the ground lead would severely detune the transmitter, and reduce the output power. The inductor does not actually block the RF current in the ground lead, but detuning the transmitter has nearly the same effect, because the transmitter output is reduced greatly.

I assume that Krimes is referring to the the well-known inductor having at least 17 turns of wire wound around an Amidon FT-193-J toroid. Krimes thought that this inductor has an inductive reactance of about 10 k ohms at 1.6 MHz. Actually, the inductive reactance is likely to be lower than this, because, at 1.6 MHz, the relative permeability of the Amidon type J core material will be lower than the low-frequency relative permeability of about 5000. If the external inductance is about the same as the loading coil inductance, the loading coil can be removed, and the external inductance can tune the transmitter. However, the performance will still be poor compared to when there is no ground lead inductance. This is because the type J material is very lossy at 1.6 MHz. The RF resistance is likely to be several hundred ohms, which will add to the ground resistance, and the other loss resistances in the system.

In spite of these losses, an elevated transmitter with the lossy ground lead inductor may actually work better than a 3 m monopole at ground level, provided that it is possible to tune the transmitter to resonance. This is because an antenna on a transmitter elevated 20 feet will have over 20 times the radiation resistance of an antenna at ground level. The presence of the lossy inductor does not change the radiation resistance significantly. Even though the lossy inductance reduces the radiated power compared to when the inductor is not present, this radiated power may still be higher than for a ground-mounted transmitter.

Ermi I don't understand how you arrive at the figure of a few hundred ohms. These are my calculations:
N = 1000 x sq. root (L/AL) (where L and AL are in units of mH)
For 17 turns on a ferrite toroid of AL = 6065, I get and inductance of 1.75mH
At 1.6MHz this results in an impedance of 17.6kohms

You talk about the ferrite having an effective lower inductance at 1.6MHz, but I cannot find any reference to that effect. What gives?

The following is applicable to any soft ferrite material with a relative permeability in the vicinity of 5000, such as Amidon type J and Fair-Rite type 75:

The only RF application these materials have is as attenuators. To make high-Q coils with these materials, the maximum frequency is about 100 kHz. At 1.6 MHz, the formulas cited by Krimes are not applicable. At 1.6 MHz, the relative permeability reduces from about 5000 to about 1000, causing the inductance to be reduced by a factor of about 5. Also, the cores absorb a lot of energy at 1.6 MHz, causing the Q to be low, giving a high resistive component to the input impedance of the coil. This resistive component adds to the loss resistances of the transmitter and antenna system, causing efficiency to be reduced.

The only application high-permeability soft ferrites have to frequencies in the vicinity of 1.6 MHz is in filters, such as with "prayer beads," the ferrite beads that RF designers spread around their circuits in the hope of reducing noise or instability. The function of the Prayer Beads is to absorb RF energy.

Type J material is clearly misapplied for an inductor in the ground lead. A core material with much lower permeability is needed, which would require more turns. More turns would increase the capacitance of the winding, causing more complexity in the design. Designing with ferrites is very difficult. Once you get the design right, the design often doesn't work when using the same part from the same manufacturer, but with ferrite from a different batch.

Ermi that is very interesting information. You mention that core material with a lower permeability would be more suitable, but more turns would be required. What arrangement would you suggest for a choke that would satisfy minimizing radiation from the ground lead while allowing the transmitter to operate reasonably well? Also you mention that variations between parts could throw the whole thing off. How could a repeatable design be achieved?

Thanks

Gerry,

You asked My question is this: would the antenna signal now be worse off at a higher elevation with this choke installed or close to the ground with a short direct connection?