OARC Transmission Line Project 2013 Session 4 Home TCA Index TCA Files Antennas Presentations Oarcproject  2013 Oarcproject 2015 Oarcproject 2017 Gallery Contact
4:1 Transformer (144 MHz)  In this session, we will experiment with the classic half wave 4:1 transmission line Balun/Transformer.   See photos at bottom of page. This is a Voltage type Balun in that it attempts to supply a constant voltage to the load while other Baluns force a current into the load and are called Current Baluns. The 4:1 Voltage Balun is often used to transform the high impedance of a folded dipole antenna so it can couple nicely to a coaxial cable. These Baluns and derivatives of them are discussed in QST, September and December 2012, with further discussion going back to issues of QST that can be found in their archives.  QST articles also show how to convert the classic circuit to a current Balun. It is called a Q3Q Balun. Extensions of the ideas used in this experiment lead to all sorts of couplers, power combiners, power splitters and many microwave devices such as the rat-race two way power splitter. It even relates to optical coating on camera lenses. The Experiment 1) Use the cables that you used in the stub experiment during session 3. Use one piece for the main feed line. The other piece will be used for the half wave section. 2) Use TLDetails to calculate a cable length that is one halfwave length long at 146 MHz. Wireman CQ118 model can be used. Cut the cable so it is 5 cm longer than one half wave length so it can be connected as shown in the photographs. Mark the cable at each end with an ink marker 2.5 cm from each end. 3) Strip the cable back to  one half wavelength and connect it as shown below to a 200 Ohm load. You can see the unit that I made to get an idea on how to do this.  The length of the dielectric should be one half wavelength long. 4) Measure the SWR and Return Loss with the Aim 4170 impedance meter over a frequency range from 100 to 180 MHz.  If you achieve a return loss of 25 dB over the complete 2 metre band, you have met the specification. The first unit that I made, was tuned high in frequency but just met a Return Loss of 25 dB over the complete two metre band. Reason for tuning high not yet understood.  The first guess is to lengthen the cable a bit. Other reasons: connector reflections, resistor not quite equal to 200 Ohms, measurement errors.  Remember, a 30 dB Return Loss translates to 0.1 % reflected power which is extremely hard to achieve. An LTSpice simulation might shed some light on the problem.  My first attempt will be to lengthen the loop. A Note on Return Loss The AIM impedance meter displays an item called Return Loss. What is that? This is a term that is similar to the gain of an amplifier. If an amplifier has a gain of 10 dB, then a 1 Watt signal (0 dBw) will be amplified to 10 Watts (10 dBw) at its output.  Then the gain of the amplifier is 10 dB. Note that dB referred to a Watt carries the subscript “w” to tell us that we dealing with Watts and not milliwatts or some other unit of power. The gain of the amplifier does not carry the subscript since it refers only to a ratio. That means the 10 dB amplifier in this example will produce 10 mW of power at the output for a 1 mW input signal. Now to Return Loss. We can use the term Return Gain to make an analogy to amplifiers. In this case the Return Gain is defined as the reflected power from the input of an amplifier or  transmission line with respect to the input power. This is expressed in dB. Consider the simple example where the input power to a cable is 10 Watts and 1 Watt is reflected back to the source (this is seen on SWR meters that have two needles). Here the Return Gain is -10 dB. The minus sign means that the reflected power is less than the input power.  By definition, Return Loss = - Return Gain and the term Return Gain is hardly ever used. Return Loss is used incorrectly by many software programs including the AIM 4170. TLDetails uses the term correctly.  Microwave engineers are very careful with the proper use of these terms since they often deal with reflection amplifiers where the reflected power is greater than the incident power to an amplifier. A good design goal for Return Loss is to achieve between 20 (1.23 SWR) and 25 dB (1.11 SWR) over the frequency range of interest. Most commercial transceivers will accept a Return Loss of 14 dB (1.5 SWR) before they start  reducing power levels and require the use of an antenna tuner.
200 Ohm Load   Half wave length @ 146 MHz
Power in Load = (2V1)^2/200 Watts   Power Supplied = V1^2/50 Hence: Power in Load = Power Supplied
RG8-X 50 Ohms
Half Wave Loop at 146 MHz
Main Feed Line Any Length
4:1 Balun
200 Ohms