An Treatment to Antenna Tuning and Matching)

As a preliminary exercise to designing a fresh tuner, the writer set out to find out what tuning components would be needed and how they might best be connected. What goes after is essentially a paper exercise making use of a computer program to simulate a broad range of tuning conditions. From the results, some interesting forms have evolved leading to a few ideas on tuner application.

Using the principles described, the tuner as shown in Figure Four is evolved.

The value of phasing capacitance C2, or phasing inductance L2, can be read off as a function of Xa for each band from Figure 7. The very large value of C2 is also illustrated for low values of antenna inductive reactance (Xa).

PARALLEL ANTENNA PHASE CORRECTION

As an alternative to phase correction by series tuning, as shown in Figure Four, the reactive component can be cancelled out by a parallel reactance equal but opposite in sign to the equivalent parallel reactance. This method of phase correction has a number of attractive features as goes after:

Three. Referring back to Figure Five, we see that the capacitance required in the matching circuit decreases as the antenna blast resistance (Ra) is enlargened. The effect of parallel tuning is to present a fresh value of geyser resistance (Ra I ) higher than the value of Ra and hence the size of the capacitor in the matching section can be diminished.

Figure 11 illustrates the application of parallel phase correction. Capacitor C2 combines the function of matching capacitor for Ra I >, Rs with the function of phase correction for an inductive antenna. Capacitor C1 provides matching for Ra I <, Rs and is set to minimum for Ra I >, Rs.

out Xa I for inductive antenna.

C1 – Matches for Ra (or Ra I ) I ) > Rs and also

balances out Xa I for inductive antenna.

Antenna Phase Correction

The kinks of Figures Five and 6 can be used to calculate the matching sections components using parallel antenna phasing with the series antenna resistance (Ra) substituted by equivalent shunt resistance (Ra I ).

PRACTICAL VARIABLE INDUCTORS AND CAPACITORS

a = radius in inches

P = 100W, R = 1000 ohms, E PEAK = 442 V

P = 100W, R = 5000 ohms, E PEAK = 990 V

P = 400W, R = 100 ohms, E PEAK = 280 V

P = 400W, R = 1000 ohms, E PEAK = 885 V

P = 400W, R = 5000 ohms, E PEAK = 1980 V

REDUCTION OF MATCHING CAPACITANCE

Range of capacitance required for C1:

Trio.Five MHz — 80 to 450 pf

7.0 MHz — 25 to 225 pf

14 MHz — 13 to 115 pf

28 MHz — 11 to 57 pf

(Capacitors are incremented on steps of 1.Four:1).

Three.5MHz — 30 to 2200 pf

7.0MHz — 20 to 1500 pf

14.MHz — Ten to 1200 pf

28.MHz — Ten to 820 pf

Capacitance range of C1 plus motionless C:

Three.Five MHz — 150 to 750 pf

7.0 MHz — 40 – 300 pf

14. MHz — 20 – 120 pf

28. MHz — Ten to 50 pf

Range of Inductance and Capacitance Required for

Circuit of Figure 15.

Of course, table 1 is a theoretical conclusion based on simulated conditions using flawless inductors and capacitors. No allowance is made for the effect of loss resistance in the components themselves. (For example, an antenna resistive fountain of Two ohms might be considerably enhanced by the loss resistance of the tuning inductor in series with the antenna).

coupling inbetween the balanced inductor halves, as shown in Figure 16. In this arrangement, the same combined inductance with the same number of combined turns, is achieved as with the single inductance in the unbalanced circuit. The idea is to cut the coil at its centre and connect one half in each line gam, making sure that the sense of connection gives additive and not subtractive combined inductance (refer to Figure 17).

A balun transformer is required to interface the unbalanced transmitter output circuit to the balanced circuit. The primary reactance of the balun should be at least four times the circuit impedance at the lowest operating frequency, (i.e. four times 50 ohms or 200 ohms). At 1.8 MHz, this means a minimum inductance of 17.6 microhenries. A shunt reactance of 200 ohms across the 50 ohm circuit reflects an equivalent series reactance of about 17 ohms (i.e. one microhenry) to the matching circuit. This will affect the matching circuit but only to a minor degree.

The minimum number of tri-filar turns is calculated as goes after:

L = minimum primary inductance in microhenries

f = frequency (Hz)

N = number of turns

Ac = cross-section area of core in square cm.

At the time of writing this article, the writer had not been introduced to the now popular Z Match Tuners which also make use of the L matching principle as described in the article. (Refer to later articles by the writer on the Z Match and in particular the single coil version. A feature of the Z Match circuitry is that it enables adjustment of the inductive reactance element in the L match network by using a variable shunt capacitor connected across a motionless inductance.

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