QRP EFHW SOTA TUNER

             This is a mini QRP tuner for EFHW antenna tuner. End Fed Half Wave is a type of dipole antenna of which the feeder point is at one end instead of the traditional dipoles where feeder line is connected to the middle point where the impedance is very low. 

    

When RF energy is applied to a conductor of half wave length a standing wave is formed on it with voltage and current of 90 Deg. Phase shift. The Voltage will be maximum at both the ends where as the current will be maximum at the center. Thus the center point will have low impedance which is near to 72 Ohms which makes it easy to feed the dipole with a 50 Ohm feeder. But at the same time the impedance at the end points of the same dipole will be very high ranging from 1800 to 5000 Ohms. 50 Ohm feeder cables cannot be directly connected to the dipole in such cases. So we need a matching transformer which will match the impedance difference.

The important element which need to be taken under consideration while using EFHW antenna is the counterpoise. Based on experimental observation the ideal minimum length of the counterpoise is 0.05 times wave length. Again we can still find many opinions and disputes regarding the actual ideal length of counterpoise. 

    Another important aspect for the design is the coil winding details of the impedance transformer. The number of turns and ratio of primary and secondary can be varied depending upon various factors like the AL value of the toroid, the inductance value requirement for secondary coil to tune the particular frequency and the value of the Variable capacitor used for tuning. Depending upon the resistive impedance of the antenna it varies in which 10:1 is considered to be ideal for impedance Z ant. near 5000 Ohm.

Inspired from the projects and documentation from AA5TB , G3WQW   

Now talking about the circuit details the mail advantage is the added VSWR indicator bridge which is added at the back end of the impedance transformer. SWR bridge build around few 50 Ohms resistors and a diode, An LED is used to indicate SWR indicator by the scale of LED brightness proportional to actual SWR. The impedance transformer and tuning part consists of  2J PVC variable capacitor of value 260pFx2 connected in parallel recycled from old SW broadcast radio. The transformer is continuous winding of 30 turns of 28 SWG wire on a T50-6 toroid with center tap on 3rd turn making first 3 turns as primary and remaining 27 turns as secondary. A DPDT mini push type switch is used to connect and bypass the SWR bridge in the transmission RF path. Banana plug connectors are used to extend the connectivity to long wire and counterpoise.

Components used for construction

Bottom view of partially completed tuner

Top view of tuner placed inside the plastic case

    I have used two 2 Watt 100 Ohm resistor in parallel to form an equivalent 50 Ohm resistor. The SWR indicator will work fine up to 15watt for short spam of time and the tuner can handle upto 20 watt max as PVC capacitors will fail above the limit due to high voltage surge. Its always recommended to work only at QRP level with this tuner with input power less than 15 Watt.

    Depending upon the inductance of the secondary winding and the PVC variable capacitor value the tuner can tune from 80m to 17m. In my setup I have only highlighted the 40 M and 20 M band tuning points in the dial. 

    The components are soldered in dead bug style on a manual itched copper clad and the small housing is made using the pieces of copper clad for better mechanical strength. The unit is placed inside a plastic box and the cable with PL259 connector assembly is extended out to connect to the radio RF output port. The tuner is very compact and easy to carry anywhere including SOTA operations.