Load Curves for the Single Coil Z Match

The curves show the Resistance Load Range of VK5BR Single Coil Z Match design at the various amateur frequencies. Curves are also included for reactive loads combined with 50 ohms resistance.

by Lloyd Butler VK5BR

(The curves were originally included as part of the article published in Amateur Radio, April and May 1993)


Taking up an idea by Tom Seed from New Zealand, the first experimental models of the Single Coil Z Match were assembled by an Australian Amateur who prefers to remain anonymous. Because of my previous interest in testing the performance of Two Coil units, he sent them to me for evaluation and alteration for best performance. Of course there was always the problem of making them work over a wide range of load conditions and after a lot of experimentation, I settled on the design included in this internet site with 14 primary turns tapped at 10 and 7 turns and 4 turns secondary. The load curves given in this article are the original test results for that design.

My circuit was published in the Random Radiators column written by the two Rons in February 1993 issue of Amateur Radio. Sample experimental units by our anonymous amateur were also included. I followed up with more detailed information on the design and performance of the circuit in the April and May issues of Amateur Radio. The curves which follow were part of the material included with the May issue.

I note that this particular design has been taken up by numerous writers in their own articles overseas. In some cases credit has been given to Random Radiators but it really gets back to the assembly work carried out by our unnamed amateur and the experimental work I carried out to find the design which really worked over a wide load range. I also don't overlook the fact that Graham Thornton identified a drop out region I had previously missed and that he helped me understand why it occurred, leading to a simple solution which I introduced to fix it.

What follows is the load curves and information on how they were obtained.

Load Curves (HF Bands)

Tests on the single coil Z match units have been carried out at 3.5, 7, 14, 21 and 28 MHz. Initial load impedance tests were carried out by loading the output with incremental values of resistance between the range of 10 and 2000 ohms and adjusting the Z match tuner for correct match. To set up for a match, a noise bridge with its output connected to a receiver was loaded with 50 ohms resistance. With the receiver set to the required frequency, the noise bridge was adjusted for a balance indicated by the noise null. The 50 ohm resistance was then removed and replaced with the input of the Z match tuner. With the controls of the noise bridge unchanged, tests on each selected load resistance were carried out by adjusting the two variable capacitors for a match as indicated by the noise null. The variable capacitor dial readings were logged against each load resistance tested. Calibration of capacitance measured against dial readings later allowed conversion of dial readings to direct capacitance.

Using the above procedure to obtain the readings, calibration curves for the unit of figure 2 have been compiled. Figure 1 plots the series capacitance required for each frequency measured over the resistance load range of 10 to 2000 ohms. Figure 2 plots the capacitance required in each half of the split stator capacitor for each frequency over the same load range. From these we see that the input capacitor tuning range is around 20 to 350 pF and the split stator capacitor tuning range is around 20 to 250 pF per side. A very interesting observation in figure 2 is that we require much more capacity across the coil for 14 MHz than f or 7 MHz, the lower of the two frequencies. This clearly shows that at 14 MHz we make use of the upper frequency resonant range with less inductance, whereas at 7 MHz, we make use of the lower frequency resonant range with the full inductance.

Figure 1 Input capacitor matching.
Figure 2 Shunt capacitor matching.

1.8 MHz

The Z match unit under discussion was never meant for operation at 1.8MHz. However, it can be made to work on that band by adding capacity to the two tuning components. Similar tests to those descibed above were carried out for 1.8 MHz. Figure 3 shows the total input and total shunt capacitances required to make it work for various values of load resistance at this frequency. As far as shunt capacitance is concerned, this is only added to the value across the full coil and nothing is added at the coil centre. The curves are plotted for the complete load range of 10 to 2000 ohms although for most of us, with electrically short antennas on this band, resistance above 50 ohms is probably irrelevant.

Figure 3 Matching capacitances for 1.8MHz

Figure 3 shows that the normal HF version of of the single coil Z Match circuit with a maximum capacity of 350pf in the series variable capacitor and a maximum capacity of 250 pf in the shunt variable capacitor is inadequate to tune 1.8 MHz. However fixed parallel capacitors can be switched in as described in an attached article on this internet site.

Reactive Loads

The idea of the Z match tuner is to interface with a wide range of complex impedances as exhibited by all sorts of odd lengths of antenna wire. This means it must match loads which include a considerable reactive component. Possible combinations of reactance and resistance are numerous but some sort of check is needed to assess the performance of the tuner with reactive loads. For my tests I used a fixed 50 ohm resistance in series with various reactances in the range of minus 1000 ohms to plus 1000 ohms. At frequencies of 3.5, 7, 14 and 21 MHz, I was able to match the load for the complete test range. At 28 MHz, I was able to match for around minus 250 ohms to plus 800 ohms. The capacitance settings over the test range for the input capacitor and the split stator capacitor have been plotted in figures 4 and 5 respectively. These particular curves have been joined up from a limited number of plot points and hence are not guaranteed to be too precise. (As it became apparant later, in joining up the limited number of points, I initally missed the drop out regions for capacitive loads which have been described by Graham Thornton and myself in another article attached to this internet site. This led to the need for the addition of S1 and L3 circuit). The regions in figures 4 and 5 where drop out might be experienced (with a need to switch in L3), are now identified in red. I did not include 28 MHz in the curves as I had some doubt about the validity of the readings considering the small capacitance values I had to use in the load for testing this band.

Figure 4 Input capacitor matching
for different values of reactance in
series with a 50 ohm resistive load.
(Drop-Out regions in Red - See text)

Figure 5 Shunt capacitor matching
for different values of reactance in series
with a 50 ohm resistive load
(Drop-Out regions in Red - See text)

In introducing reactance into the load, the matching system must correct for this and obviously something must be retuned as shown in figures 4 and 5. What is interesting in figure 4 is the dramatic fall in the value of series input capacity when reactance is introduced. This is particularly noticeable at the low frequencies. As part of the 'L' matching network, the input capacitance must decrease when load resistance is increased. Hence, adding reactance to the load also increases the resistive component reflected across the network and possibly makes it easier for a match when the load resistance is low.


1. Lloyd Butler VK5BR - The Single Core Z Match Simplified

2. Modifications for 1.8 MHz

3. The AR Single Coil Z Match - Random Radiators -February 1993

4. The AR Single Coil Z Match - Lloyd Butler -April and May 1993

5. The Z Match and its Matching Load Range - An inherent Drop-out with Certain Capacitive Loads - Lloyd Butler and Graham Thornton - March 1997

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