Setting Resonance

A Typical or is it an Atypical Antenna

I have published several articles here on eham.net primarily covering HF mobile antennas and noise abatement (Article numbers: 4407, 4424, 4425, 4623, and 5299). The articles generated a fair amount of threads and a ton of e-mails. Based on the number of e-mails I received about the UNUN matching transformer, there seems to be a misunderstanding about how to properly set the resonance point of an antenna. This article will address this topic.

For those who understand complex impedances and how to calculate, measure, and adjust them, this article isn't for you. In fact, I have purposely left out all the esoteric formulas and charts (including Smith® Charts), in leau of basic Ohm's law and SWR charts. Anyone wishing more complex theory should enlist the aid of the ARRL Handbook or Antenna Book. That said, the reader must understand some basic theory and part of that is knowing the difference between resistance, reactance, and impedance.

Resistance is the opposition to current by conversion into another form of energy, typically heat. It is measured in ohms and in the treatise will be referred to as R.

Reactance is the opposition of alternating current by storage in a magnetic field by an inductor (L) or in an electrical field by a capacitor (C). It is measured in ohms and in this treatise will be referred to as X and expressed as +j (inductance) and -j (capacitance). Please note the sign of these expressions.

Impedance is the complex combination of resistance and reactance. It is measured in ohms and in this treatise is referred to a Z and expressed as R+j or R-j.

We need to know a few basics about short (less than 1/4 wave) mobile antennas too. Below is a schematic diagram of a short mobile antenna. The various components are as follows.

XA represents the capacitive reactance of any short vertical mobile antenna. Depending on the resonant frequency and length of the antenna, this reactance is between -j100 to -j7,000 ohms.

XL represents the inductive reactance necessary to cancel out the capacitive reactance and bring our antenna into resonance. In our discussion this is the ubiquitous loading coil which can take many forms.

The three resistance values, RL, RG, and RR, represent the losses in the antenna. RL is the coil losses which should be kept as low as possible by using a high Q inductor. RG is the ground loss which is unavoidable, but can be minimized (see previous articles). RR is the radiation loss. This desirable loss needs to be kept as high as possible while minimizing the other losses particularly RL.

CS represents the stray coupling losses and was covered in article 5299. It should be kept as low as possible as it directly effects antenna efficiency and is actually the worst of the losses. It is exacerbated by poor mounting positions and techniques.

Now comes the hard part. No matter what brand of HF antenna you use or how you mount it, it will always have some reactance even at its resonant point. And it will never be exactly 50 ohms and a perfect 1:1 SWR match. Even a dummy load has some reactance, so don't believe the hype!

Sans any matching device or scheme, a short HF mobile antenna will exhibit an input impedance of between 10 and 36 at resonance. This impedance (Z) consists of a combination of all of the values shown in the antenna schematic above. In all fairness, at resonance the Z is mostly resistive (R) with a few ohms of reactance (± j). So, at resonance our SWR will measure between 5:1 and 1.4:1 So how is it when you use an SWR bridge to resonate your antenna the SWR is less than 1.4:1? Well I did say this was the hard part didn't I?

Lets look at the SWR chart below. The red line represents the reactive component and the blue line the SWR.

To clarify the point, let's assume a few parameters of a typical 40 meter HF mobile antenna. (One might say atypical hence the subtitle of this treatise.) It will have an input Z of 19 ohms. This would represent an SWR of 2.6:1 when fed from a 50 ohms source. But is we make the antenna too long with respect to our desired resonant point it becomes inductively reactive. To a point, when this reactance is added to the input Z (remember, inductive reactance has a + sign) and our SWR bridge is fooled into seeing a Z closer to 50 ohms. Assume for a moment this was +j20, then our SWR would appear to be 1.3:1. This is because an SWR bridge only measures the voltage component of the complex impedance present at this off-resonant point.

In all fairness, the process is much more complex in the real world than I make it sound here. Remember, I used the word atypical which means aberrant, abnormal, artificial, contrived, and a few more negative things. Which just means nothing is typical when it comes to HF mobile antennas.

Nowadays thanks to MFJ (and a few other companies) we amateurs have an inexpensive way of checking impedances fairly accurately. Using the MFJ 259B, not only can we measure the resonant point (least reactance) we can also measure the complex SWR, not just the voltage SWR of the typical (there's that word again) SWR bridge. This allows us to select the best tap on our aforementioned UNUN, or select the best shunt (L or C) matching value. And it'll prove to you the effectiveness (or lack of it) of any built-in matching device your antenna might incorporate.

Alan Applegate, KØBG