Loading coil calculations

In order to calculate loading coil inductance it is necessary to know the antenna capacitance. Since, for a given situation, this can only be estimated within limits it follows that inductance calculations will also be within limits.

If you search the web for "inductor calculator" you will find ways to do this yourself. That way whatever it is that you want to remain secret will be known only to you.

The approach I took to solving this problem was to overwind the loading coil by about ten turns and then remove the turns until resonance was achieved.

It is important to have a means to know when the system is in resonance. I used a home brew current transformer to measure the current into the coil and with this displayed on one channel of a two channel scope. The other channel displayed the coil input voltage. At resonance the current and the voltage are in phase which is evident on the display.

Neil

How much precision should I look for in a current meter or transformer? 

Excuse me for yelling, but I have an idea that might actually be smart.

I wound a coil and inched away until I got all the indications of a proper load, but then I was told that maybe I had brought the ground side of the circuit to resonance and not the antenna.

If this is so, then what would it take to move the resonance to the antenna and away from the ground circuit.... a different length of coil?

That and perhaps top-loading the coil.

I wound a coil and inched away until I got all the indications of a proper load, but then I was told that maybe I had brought the ground side of the circuit to resonance and not the antenna.

The only significant reactance in a short vertical antenna used in the AM broadcast band is the result of the capacitance of that antenna to the earth around it.

That capacitive reactance can be offset by use of a loading coil inserted in series with the conductor(s) leading from the output circuit of the final r-f amplifier of the transmitter to the base of the antenna conductor.

The correct adjustment of the loading coil for that installation "resonates" the antenna system -- which maximizes the amount of the power available from the transmitter that is radiated into space by that antenna (see note below).

The correct adjustment of the loading coil can be found experimentally by changing the amount of active turns of the coil that are used.

Correct coil adjustment can be found as Neil radio8z does it, by sampling the r-f current and voltage at the coil input, and adjusting the coil until voltage and current are in phase with each other.

Another approach is to sample the r-f field intensity near the antenna, and adjust the coil for a maximum indication.  That meter does not need to be calibrated to measure the true field intensity at the point of measurement. 

It doesn't matter to performance if the active length of the coil is adjusted for system resonance by shorting or removing turns at its input end, or its output end.  The only reactance being offset by the coil is the reactance of the antenna, in both cases.

____

Note: Reactance in a-c/r-f circuits causes the current and voltage to be out of phase with each other, which reduces the useful energy that the source (transmitter) can provide to the load (antenna system).  Google "power factor" for more information.

The relationship of coil to antenna as described by Rich rules out the claim made by another engineer that I might have been inadvertantly tuning the ground side to resonance rather than the antenna itself.

In fact it would seem to me that the only way to actually tune the ground to resonance would be to attach the loading coil in series between transmitter ground and the actual earth ground.

The technique for reaching resonance in my particular situation comes from instructions for modifying a SSTran AMT3000 for use with an external base-loading coil. Finding the coil tap that provides the highest output as viewed (in my case) on a spectrum analyzer, one trims the output capacitor for a reading of 13VDC. It can take a few tap/voltage adjustments to reach the exact agreement where highest output occurs at 13VDC.

I found the "ground-being-loaded" argument plausible because if the transmitter isn't well grounded, the conditions will not occur to provide the 13V reading.

Rich has convinced me that the coil/antenna circuit is resonating the antenna as intended.

Here is a device that claims to tune your ground:

MFJ GROUND TUNER: 

http://www.mfjenterprises.com/Product.php?productid=MFJ-931

However, radial wires buried in the earth have little reactance at the operating frequency compared to the reactance of short conductors located above the surface of the earth.

"Ground tuning" can be effective when the antenna is driven against a counterpoise elevated above the surface of the earth, because such counterpoises behave much differently than buried radial wires.  For best system performance, counterpoises need to be adjusted to resonance along with the conductor(s) considered to be the antenna.

Buried radial wires of any electrical length are not resonant, therefore tuning them either separately or as a set is not required.

Not a lot of precision is needed but in my case I did calibrate the transformer to quantify the current. The transformer does have some leakage inductance which causes a slight (7 degree) phase shift when connected to a resistive dummy load. This is negligible.

Here's a link http://www.part15.us/forum/part15-forums/general-discussion/rf-current-transformer-antenna-tuning to an article in which I describe my coil.

Neil

The MFJ device is a "reverse " antenna tuning unit. It cancels the reactance in ground lead.

The MFJ device is a "reverse " antenna tuning unit.  It cancels the reactance in (the) ground lead.

The total reactance of a 3-meter Part 15 AM whip plus that of a system r-f ground conductor of any length can be offset by a single loading coil placed at the bottom of the 3-m whip.

There is no need to cancel those two reactances separately -- whether the ground conductor connects to radial wires buried in the earth, or to an elevated counterpoise.

The inductance value needed for the loading coil probably will be greater when using an elevated counterpoise than when using buried radials.  So of the two configurations, the coil for the elevated counterpoise system likely will have more a-c loss resistance at the operating frequency.  OTOH there will be ~zero r-f loss in the counterpoise conductors.

Part 15 AM systems using long conducting paths to buried r-f ground radials or rods will need less inductance in the loading coil, potentially meaning less loss in the coil.  But there will be loss in the r-f ground system -- possibly several tens of ohms.  Also those long ground leads may be non-compliant with FCC §15.219(b).

Mr. Rich concluded the previous post with:

"Also those long ground leads may be non-compliant with FCC §15.219(b)."

I'm curious about the "may be," since that could also be said "may not be."

But how could the ground lead NOT be in non-compliance with 15.219(b)?

A ground lead that rises vertically into open air is apt to radiate... there have been failed discussions about ways to keep it from radiating, but so far as is known, it will radiate, making the effective length of the 3-meter antenna longer by the length of the ground lead.

My position, which has become more clarified over time based on these many discussions, is that such small nit-picking is petty, whether by commenters in these posts or an FCC inspector, but there's a defined area of law called "petty crime," so petty is no defense.

We are petty hobbiers looking for a break.

I just reviewed especially Neil's method of using two readings viewed on a dual-trace scope to achieve fine resonance... with an eye toward remoting as much of the transmitter/antenna control as possible.

Space has been cleared 100-feet behind the Internet Building for a full vertical antenna/ground radial installation with a weather-proofed AMT5000.

I already know that inserting a current transformer at the input to the internal loading coil of the AMT5000 poses a difficulty, so we need not discuss that again.

But I'm wondering how many transmitter control functions can be controlled from inside the building, 100-feet away from the transmitter/antenna.

So far it appears that final setup and tuning will necessarilly need to take place out at the antenna site.

But the idea of viewing and controlling everything from an indoor control point is appealing and if there are ways to build such features, things get interesting.