Having a good match between your antenna feed point and coax is all about looking at antenna as a system. The system being all the losses and gains from the connector on the back of your transmitter until it is RF. When you account for all these factors you can try to reduce the biggest losses first. It is not difficult to reduce the coax mismatch losses and Common Mode Currents often have significant negative consequences.Core Material Selection
Toroid cores come in sizes and I needed a common size sufficient for 12 turns (24 single turns) to accommodate the two BALUNs I had in mind with #14 AWG windings. Later on I determined #20 AWG was more than I needed for a 100W. I knew ferrite cores can permanently change their permeability after being subjected to relatively high power so I selected the largest core which will handily fit into my enclosures. I found permeability of 125 (125 u) is better than 850 u for 1 to 60 MHz 4:1 BALUNs. The 125 u is equal to 850 u for 1 to 30 MHz 1:1 BALUNs. The best high power (~1KW) 4:1 BALUN 1 to 30 MHz I constructed was a dual 2.4 Inch 125 u core (2.40 OD, 1.40 ID, 0.40 HT) with12 turns (24 single turns) #14 AWG per core. The best 1:1 BALUN 1 to 60 MHz I constructed was a dual 2 Inch 850 u core (1.40 OD, 0.80 ID, 0.40 HT) with12 turns (24 single turns) #14 AWG per core. I define best as close to purely resistive at the BALUN input. In more moderate power (300 to 500W) levels I found the best 4:1 BALUN 1 to 60 MHz built was a dual 1.50 OD 125 u Teflon coated core and #20 AWG. The best 1:1 BALUN can use the same wire as the 4:1 and a 125 u or 850 u core as I built both and measured little difference.
Power Handling and Winding Dielectric Selection
My power handling estimates are based on the statements in the discussion paragraphs labeled Photo 8F and Photo 8G on page 51 of “Understanding, Building, and Using Baluns and Ununs, Theory and Practical Designs for the Experimenter” by Jerry Sevick,W2FMI that strongly implied #20 AWG (PVC 300V & 90 Deg C) should handle 150 Watts continuously and 300 Watts Peak. To ensure full 1500W power handling capability he suggested #14 AWG enameled wire inside Teflon tubing a 2.5 Inch cores. The toroids (permeability of 125) I have used should be better than the core permeability of 250 used in his tests for 160M operation and equal at 10M based on the core material discussion paragraphs labeled Table 5-1, Figure 5-2 and Figure 5-3 on pages 18-19.
I found using four different color wires for winding it saves on wiring mistakes and keeping windings from crossing accidently. I found the 0.75 Inch ID is sufficient for 12 turns (24 single turns) for #20 AWG and 1.25 Inch ID is required for 12 turns (24 single turns) of #14 AWG. Select your wire and core based on your power. I noticed some variation in dielectric thickness between wire companies which will impact the number of turns and spacing. The spacing is will change the characteristic impedance. I found a combination that worked by experimenting. I did not measure any difference between solid and stranded wire for 1-30 MHz
Construction of a 4:1 Dual Core Guanella BALUN
Key points in construction of a 4:1 Dual Core Guanella BALUN in a square four inch box.
The construction used three stainless steel eye screws and a water tight box similar to most designs. One feature are the separate brass bolts for better RF connections.
Above are the two 125 u (1.50 OD, 0.90 ID, 0.40 HT) Teflon coated cores to support 12 turns (24 single turns) for #20 AWG at 100W so there is plenty of power margin. The wire has 300 volt PVC dielectric.
The winding wire is always too short or too long and never just right. Some catalogs have charts that suggest the proper wire gauge versus the required number of coil turns for a given core size. They also tell you to use the largest wire you can fit to ensure minimum losses and highest Q. Just the wire gauge equal to the power handling needed. Once you decide on the wire for your windings you must determine the length. One way is to wrap five turns, remove then, then measure the length. Multiply the length by the required number of turns, and allow four extra inches of wire for the leads at the end of the winding. My solution is guest-estimate a little long for the first try and measure carefully. Cut the wire length for the second core adjusted based on the excess wire length on the first core winding. In this case I used 36 Inches of #20 AWG.
Above and below are the wound cores with 12 turns (24 single turns) for #20 AWG on each. Take a look at the different color wires for winding. Color wire is cheap and it saves on wiring mistakes and keeping windings from crossing accidently. Try to keep your inside windings together and the outside windings spaced at the natural curve of the outside to align with the next inside wrap. Most recommended methods for winding suggest leaving a 30° space between the ends of the winding. This minimizes unwanted capacitive effects and inductance cancellation. Compressing the turns increases the inductance. Spreading the turns decreases the inductance. The gap can be clearly seen on the core below. I have not tested this winding advice, I just followed instructions. I have found keeping leads short is important for a low Q and SWR.
As shown above line up your wound cores in the same direction as the windings and label the wires to match the schematic below.
Carefully compare the wired cores and the schematic noting both are facing with UN left and BAL to the right. First nest the cores by placing wires 1 and 3 inside the red/black wound core. Pull out wire 6 for connection to wire 4 as highlighted by the red arrow in the photo below.
Connect 3 wires 3 and 7 to the SO-239 shield and wires 1 and 5 to the center conductor. Wires 2 and 8 connect to the large lugs for the dipole elements. Crimp and solder each connection.
Keep the wires short but not tight. Route the wires clear of the case and metal edges. A small amount of RTV silicone or epoxy is needed to keep the cores secured in the wind. Less is better. DO NOT get any glue on the RF connections.
Clear off your workbench and now build an indoor 4:1 Dual Core Guanella BALUN.
The cores, winding and wiring are the same but the box and antenna connection are different. The antenna is connected through a Panel Mount (red/black) Dual Binding Post commonly used on test equipment. The price is reasonable and the dielectric is 1000V rated.
The box must be selected to fit the core size used for this case 6 X 3 X 2 Inches. Using the lid for all connections makes construction simpler by providing better access.
Above and below is a 1:1 Dual Core Guanella BALUN. Take a look at the different color wires for winding. The wire color I used is different for 1:1 vs. 4:1 BALUNs. I did this to prevent mistakes and a quick way to identify the ratio. I soon learn to permanently label the interior and exterior of each BALUN when I sweep test each one.
A perfect BALUN and terminated in a purely resistive load would reflect nothing and have zero insertion loss. In the real world perfect does not happen. Even a wire has resistance, inductance and capacitance. So all BALUNs have some non-linear contribution to your total antenna system. A good BALUN will have a small amount of reflected energy and insertion loss. My testing is very basic using an antenna analyzer to measure R, X and SWR as seen at the BALUN SO-239 with a carbon resistor for a load. Therefore I can measure the ratio of the power into the BALUN and the power reflected back as SWR. In the professional BALUN world this same reflected energy is return loss which is measured in dB's. My meter reads in SWR so my test data is in SWR but for comparison the 1.1 SWR is ~26dB, 1.2 SWR is ~21dB and 1.3 SWR is ~18dB. I do not have a dual port meter so my insert loss test is 100W of CW for five minutes and check for hot spots. Everyone pass the 100W smoke test.
Looking at the results my under $10 are equal or a little better than the $50 commercial 300W BALUNs, but not as good as the $180 commercial 5000W units. Just for contrast the last chart is an air-core 4:1 BILFAR BALUN I construction!