Last Update: January 5, 2010
One of my favorite single band portable antennas is the end fed half wave antenna . Such an antenna requires a special coupling network to properly couple my 50 ohm coax to the antenna's high impedance (~2500 ohms). For a number of years I just used a circuit that I threw together with large components that I had in my junk box. For portable use however I was in need of something much smaller. In my junk box I also had several Mitsumi Polyvaricons. This are the small encapsulated variable capacitors that helped revolutionize the pocket radio industry and can be found in almost all analog transistor portable radios. They are not intended for transmitting applications and I wasn't sure if one would hold up to the high RF voltages that would be across it when used in the parallel tuned circuit that is used to couple energy into the end fed half wave antenna. Since all of my operation would be at QRP power levels I thought I would give one a try. The circuit works great at the 5W level and I've successfully used it at 20W with no problems. In fact, during a test transmission at 20W for several minutes nothing failed and there was no detectable heating of any of the components. This indicates high efficiency of the circuit. Of course, the LED SWR detector circuit described below will require larger resistors (R1-R3) to handle 20W. The schematic is available below.
The unit described here also contains a common resistive SWR bridge with a LED SWR indicator. The advantage of this circuit when it is switched in is that it always presents a relatively good match to the transmitter no matter what the SWR of the antenna is. Many of the QRP rigs do not have any type of SWR protection circuits in them and can be easily damaged by a high SWR. The procedure is to flip the switch to the "SWR" position and then transmit a carrier. The capacitor is tuned for a dip in the brilliance of the LED. Usually the LED will go completely out. Some designs I have seen incorporate a potentiometer to adjust the maximum brilliance of the LED but I had found that even at the 5W power level the pot would be turned all of the way up, therefore I just eliminated it from the circuit. The only precaution is to make sure that the resistors (R1-R3) in total can dissipate the full transmitter power. 5W resistors will be more then enough for a maximum QRP power of 5W.
The coupling network itself uses a parallel tuned circuit consisting of the coil L2 and capacitor (polyvaricon) C3. The full RF voltage of nearly 112V peak may be across the capacitor when 5W is used and when the antenna is the around 2500 ohms. The coupling link L1 is used to convert the 50 ohm impedance of the transmission line to the high impedance of the antenna. I usually try for a 10:1 turns ratio but due to constraints of resonating with the capacitor in the center of its tuning range and by experimentation the best ratio in my circuit came out to be 7:1. This should result in a 50 transformation to 2450 ohms and it worked fine here. A tap could be used here just as well but I found that the link coil was much easier to adjust in this design. The main coil is constructed of 22 AWG enameled wire wound on a FT-82-63 toroid. The optimum number of turns for the coupling link was found experimentally to be 2, which in this circuit created a transformation up to 2450 ohms. This was done by loading the coupler with a 2.4k ohm resistor and adjusting for maximum power dissipation - a true smoke test. During this experiment it was found that when the impedance ratio wasn't correct, the toroid core would dissipate the energy instead of the load resistor. Once the correct transformation was found, only the resistor would get warm. Also, the input VSWR when checking it (in operate position) with my MFJ-259B antenna analyzer would be 1:1 when the ratio was correct for a given output impedance. Here is the best method to adjust this coupler for the first time: How to Make An End Fed half wave Antenna Work
This circuit has been optimized for 20m although without any changes it tunes 30m through 17m. You may have noticed that I have not provided for a ground connection. You may opt to do so if you wish but I have not with this portable setup for the reasons mentioned on my End Fed half wave Antenna page.
Any kind of enclosure can be used but since this type of antenna and coupler always draws interest from other hams I thought that I would make the enclosure transparent so I wouldn't have to keep opening it up to reveal its mysteries. The box (item # 44080) was purchased from The Container Store here in Fort Worth and was originally designed to store your hockey puck in. It comes apart in two pieces and goes together with a tight fit. Once it is together you cannot tell where the seams are.
You can easily see in the photographs how I mounted the coil and capacitor but the rest of the components may not be so visible. I soldered together two small pieces of tinned G10 double-sided circuit board together and constructed the SWR bridge and associated switch using classical "ugly style" construction. This allows the components to all be constructed over a common ground plane. I made sure first that the circuit board pieces fit properly into the transparent case. The toggle switch and the input BNC connector provide not only an electrical function, but they also hold the boards in place. The LED is the green component that you can see inside the box in the right photograph.
The front face label was made out of an inkjet transparency and white paper. First, I drew up my front face using Microsoft Power Point, although any drawing program should work. I then printed a reverse image of the drawing onto the rough (print) side of the transparency. I then sprayed the ink side of the transparency with a clear spray adhesive. The white paper was then applied to the sticky side of the transparency. I trimmed the transparency/paper to fit the front side of the box and cut out the necessary holes with a sharp knife. Once I was sure everything lined up and fit properly (it took me a few tries) I sprayed the paper side with the clear adhesive and carefully applied it to the face of my project. I have done this labeling procedure with several projects and it seems to work very well. Time will tell for sure though since I have had some inkjet photos fade due to ultraviolet light exposure in the past so my projects may erase themselves over time. I hope that the transparency outer surface may help in this regard.