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Last edited - 9th May 2017
Inspired by AA5TB and M0UKD, I built the following impedance transformer used to fed wire antennas with impedance between 1000-5000 ohms. Basically, this transformer convert the input impedance of 50 ohms to a much bigger one. A variable capacitor is used to tune the transformer to a given frequency.
As it can be seen, the schematic is very simple. An ordinary transfomer that transforms voltage-current ratio and a variable capacitor which cancels out the inductive reactance of the secondary winding for given frequency, a.k.a tuning.
In order to design entire circuit, we should suppose what impedance our antenna will have and the desired frequency. End fed halfwave antennas have an impedance between 1000-5000 ohms, depending of the height, counterpoise length, soil, air moisture, etc.
Let's suppose our antenna have 4000 ohms, we need to calculate transformer ratios. Transforming 50 ohms to 4000 means an impedance ratio about 1:80. We also know, that the turns ratio = √ impedance ratio. We already know the impedance ratio, so according to that formula, √1:80 = 1:8.9. Of course, 8.9 can be approximated to 9, therefore the turn ratio will be 1:9 and impedance ratio 1:81. The inductance of the secondary winding and desired frequency will be used to calculate the capacitor value.
As my SSB HF transceiver only has 80m band, the desired frequency is about 3700 KHz. My plan was to use an old Polyvaricon variable capacitor. After combining its internal gangs and trimmers together it will provide a range of 40pF to 300pF. So, for 3700 KHz I want to tune somewhere in the middle, about 150pF. According to LC resonance formula, f = 1 / (2 * PI * √(LC)), for 150pF we need an inductance about 12uH to resonate on 3700 KHz.
Let's recap:
Transformer
Using 1mm tick wire I built the secondary winding on a Fair-Rite 2643540002 cable core ferrite. Empirically I found that 18 turns gave me an inductance approx. 12uH. In order to obtain 1:9 ratio from our transformer, two turns will be used as a primary winding.
We have all info in place so far, let's build it!!
Pieces like hooks, SO-239, bolts, nuts, washers and plastic box were used, nothing special. Transformer was fixed in place with glue gun. The tuning knob is actually a bottle cap glued with epoxi onto polyvaricon shaft.
In order to check the correct operation of the balun, 4k2 dummy load resistor was connected to its output. Then the balun was connected together with SWR Bridge and HF transceiver. While applying 4W CW of 3700 KHz and turning the tuning knob, somewhere in the middle I found the perfect match, the SWR Bridge turned off its LED. The balun worked excellent at the very first attempt as it was designed!
I used two resistors of 3k3 and 1k in series, rated at 0.25W as a dummy load. The total power over them is about 1W (for short tests will be fine) since the transceiver delivers 4W of total power, the SWR Bridge has an attenuation about 4 times, -6dB. Using simple math we can find voltage across dummy load resistor. Combining Ohm's Law and Power we can deduce P = U2 / R, resulting voltage across dummy load would be U = 65.5 VRMS or U = 92.7 Vpeak or U = 185.5Vpp for 1W. Also, for 4W the voltage would be: 131 Vrms; 185 Vp; 370 Vpp, so care must be taken regarding variable capacitor voltage rating.
Using the scope I measured 175Vpp across the 4k3 resistor, transforming this to power results into 900mW, meaning -0.5dB loss. So, this balun has a total loss of approx. 0.5dB, efficiency is 90%
At very first design of this balun I used a ferrite rod recovered from an old medium wave radio, but after some measurements I've found it was very lossy, the balun had about 1.8dB loss. So, changing the ferrite rod to a bulky cable core of Fair-Rite 43 material the losses were improved. I'm wondering if I can further reduce these losses, using a bigger toroidal core, or an air coil...
This video demonstrate the power transfer between a transceiver and an antenna using some tools like SWR Wheatstone Bridge and EMF Strength Meter. Basically, when SWR is at minimum the maximum power is transferred to dummy load or antenna.
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