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I am trying to put together a vertical pole style antenna with a loading coil housed at the base using a plan very similar to the one described in this ARRL article. I am trying to understand how to electrically wire it though.

For convenience, the wiring schematic from that article is shown here:

Loading Coil Electrical Wiring Schematic

In this build, the author used a switch to control where he tapped the coil to tune his antenna for the different bands as shown. Here's what I don't understand about this though.

Why does the bottom of the coil bypass S1?

I understand generally that the 80 meter band needs more coil windings tapped than the 40/15 tuning, and the 10 meter band bypasses the coil altogether. But why have that connection from S1 to the first coil of L1 at all? Is that required for some reason?

What is the proper way to electrically wire in a loading coil to a vertical antenna?

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I don't think it's correct to connect the end of the coil to the common point of S1.

Shorted turns, because they're magnetically coupled to the active turns in the coil, increase the loss in the inductor. Loading coils need to be kept away from metals, and unused turns should be left open. I suggest that if you're building a new coil, you should tap it like this:

enter image description here

The extra turns on the inductor make sense for this reason: when making a loaded antenna like this, you don't know exactly how much inductance is required. You wind the coil so that it is definitely resonant below 3.5 MHz, and then on your particular ground system or vehicle find the correct tap positions for 80 m, 40 m, 20 m.

The bottom turns are wasted, but it's better to have too many than too few, so you have no choice but to build it like this.

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    $\begingroup$ Good point! It seems to me that you could rescue the design by using multiple non-coupled inductors. I've updated my answer to not claim that you should include the wire (I mistakenly generalized from resistors to inductors without thinking about the coupling). $\endgroup$ – Kevin Reid AG6YO May 17 at 14:54
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    $\begingroup$ It's a shorted turn, yes. But leaving one end open can make it arc like a Tesla coil, which is why the bandswitch almost always shorts out the unused turns in a linear amplifier's pi-network coil. The losses are low enough that the one I use in this tuner doesn't get hot even at the legal limit. That red alligator clip is connected to the far end of the coil, shorting out the unused portion. $\endgroup$ – Mike Waters May 17 at 17:11
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This is similar to a common technique applicable to potentiometers used as variable resistors, and I'm guessing that it is done for similar reasons. However, it probably isn't a good idea to implement as drawn.


Consider a potentiometer connected like so:

schematic

simulate this circuit – Schematic created using CircuitLab

Ideally, the connection between the wiper and the left end of the potentiometer would do nothing, since the portion of the potentiometer to the left of the wiper is shorted out at all times, and without that connection would be open circuit at all times.

In practice, a potentiometer may lose connection between the wiper and track momentarily as it is turned, or an extremely worn one have “dead” spots of no connection even when not being turned. Therefore, the extra connection ensures that the resistance between A and B is always no greater than the end-to-end resistance of the potentiometer. This is important if a too-high resistance or open circuit could damage the circuit (for example, if A is ground and B is connected to a voltage source would, by itself, raise point B to a voltage that would damage other components), and more generally means that the potentiometer's "output" cannot become "out of range" in the event of dead spots (unless the potentiometer's track/winding breaks entirely).


In your case, you do not have a potentiometer but a switch — therefore, unless it is a make-before-break type (which might be a good choice anyway), it will always behave like the worn potentiometer, causing J1 to become open-circuit as you make the change. Thus, the extra connection ensures that the antenna is always connected to J1, regardless of what is done with the switch. It might present a poorly-matched load, but not a pure open circuit.

But, there's a catch — instead of shorting out portions of a resistor we're shorting out portions of an inductor. All turns of an inductor are coupled together (though more separated ones less so), so there are now shorted turns, which will waste power in eddy currents rather than radiating.

Therefore, the presented design with a tapped inductor is not a good idea. You could fix it by using separate inductors for each stage, arranged to minimize coupling between them (perpendicular axes, keeping them apart).


To address your broader question of what is the proper way to electrically wire in a loading coil to a vertical antenna — I don't have any personal recommendations, but I do think that the extra wire that you were puzzled by is plausibly a good idea. Whether it's worth the complications of implementation, I can't say.

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    $\begingroup$ My exact thought when I saw that circuit was "ah, makes sense - since most switches are break-before-make", and in the case of the variable resistor it makes similar sense to present a high resistance instead of infinity. +1 $\endgroup$ – Scott Earle May 17 at 7:59
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Simulation indicates that there is an error in the diagram.

Shortened antennas exhibit capacitive reactance which can be compensated by adding series inductive reactance as shown in the diagram. However, shortened antennas also exhibit lower driving-point resistance, for which compensating reactance is no cure. The "cold" end of the coil should be connected to the radial system. The feed point is then "tapped up" the coil to form an "L" network.

Simulation shows that the driving-point impedance of a 25-ft 1" diameter vertical with four (4) 25-ft #14 radials is 6.4-j609 ohms. (These values are only significant for purposes of illustration; actual measured values may vary substantially, but those effects are outside the scope of this question.)

Entering these values into SimSmith and choosing appropriate inductance values shows that we can achieve a match on 80 meters by tapping up a ~27uH coil:

enter image description here

A different total inductance will be needed on each band and it is possible that a good match on some bands will require capacitance in the L network. A follow up item to this effect was published in QST's "Technical Correspondence" column a few months after the original article.

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    $\begingroup$ Very nice. Essentially you're not fully compensating the X of the antenna, leaving some -X, and then the parallel inductor brings it into resonance and the real part rises to 50. You can do the same with a parallel capacitor instead of L2, which might be simpler and lower loss... but you need a bit more inductance in L1 so the loss might be the same in the end. $\endgroup$ – tomnexus May 17 at 14:08
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    $\begingroup$ Also, it's only very long whips, and very good ground systems, that achieve Rant < 50. Often it's just 50 ohms, with 90% of the power lost in the ground. $\endgroup$ – tomnexus May 17 at 14:10
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    $\begingroup$ Right - ground and other losses often contribute to the false notion that good SWR = good antenna. $\endgroup$ – Brian K1LI May 17 at 17:50

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