Power Splitter, Power Divider, baluns, va3diw

Power splitter combines or divides power. It works in bi-directional way. You can split, you can divide. You can use it for combining antenna signals, or use it as a two-port output device. I recommend to start with the article from w8ji about power splitters, baluns, and w6pql.

Test it at low power, than at full power. You would be suprized how many mistakes are in handbooks. Two channel scope is good. These devices can have many physical forms dictated by frequency, bandwidth, and used power. Everything works on paper. Simulation programs work always. It is a different story running the device at full power. These circuits work also on 2.4 and 5GHz. The theory behind is solid.

The HF wire splitter can be wound on heavy high permeability 43 or 73 ferrite material, few turns. Core 3.5 by 3.5 mm hollow cylinder. The wire is about 0.25 to 0.125 mm diameter twisted pair, wound as bifilar. Measure the characteristic impedance. Z = sqrt(L/C). The start wire and the tailing wire shall be connected together. The broadband isolation of the first antenna to the second antenna is not extraordinary in this design. The mechanical design significantly influences the mutual isolation between the ports, symmetry, and overall performance. Digging into details, it is not simple as expected.
Try multiple designs and ferrite cores. Material 61 is not the right one for HF. The "43" (lower bands) and "73" material seems to work best for HF.
Try materials with u_r=850 to u_r=2000. Fair-Rite, NY product. Two hole ferrites size "73-2402" or larger 73-202 from the Houston, Texas store. They are nice. You can make the purchase on phone, it is a customer oriented service with nice chat and instant feedback. As it should be.

Then there is the power splitter with three resistor. It is not worth of trying, considering inherent signal loss.

The picture depicts two core wire power splitter with tap by ARRL. It has low port isolation, significant loss, and impedance mismatch. The picture was many times published without any performance notes.
For wire or twisted wire version throw away the T1 core on the left. Use only the two sections on the right side T2. It looks like Wilkinson.
Running the splitter at high power can cause ferrite core saturation. You can think about the wire impedances, as a transmission line based device. It is useful to have a L-C meter and an oscilloscope on hand to check reactances. A spectrum analyzer will reveal the IMD₃ distortion in transistors and ferrites. The IMD₃ numbers are useful to know.
Motorola power splitters work with transmission line on the core. Hand made transmission line with strips of copper on Teflon tape gives better performance. The coaxial cable version is power bingo, requiring larger cores. Start winding the culprit again. Looking at the spectrum analyzer to manage 5 MHz bandwidth coupling hundreds of miliwatts, the C1 made no measurable difference in my case. Experiment with C1 for wider bandwidth to compensate for leakage inductances.

Practical Example
The point of interest was to split two power signals and provide adequate isolation between the two input ports. Minimize the IMD₃. More inductance per turn helps the transformer symmetry and lowers the magnetic flux in the core. More turns move it away from saturation. The saturation starts slowly. Each ferrite material has different properties. Motorola recommends to keep the magnetic flux under 100 Gauss = 0.01 Tesla. The structure can be resistively balanced with sharp minimized crosstalk between the two input ports. That's the 100 ohm resitor, replaced by trimmer.

The signal leakage and resulting nonlinearity between the two branches, used as power splitter, can hit -52dBc to -55dBc IMD₃ distortion. That means the device is working right. It can work better. Load all ports. Connect one resistive trimmer 220 ohm (set to 50R) to the split power output. Second trimmer 1k ohm between the two ports. A signal generator has rarely 50 ohm output. Small attenuator between the signal source and the device can fix it. The isolation between the two antenna ports for 10% bandwith can reach >40dB at 10MHz.
The T2 inductor reactance shall be more than 18uH/6t with 43 ferrite material @10MHz. jX = 1.13k ohm. The rule of thumb and (rule of 20's) = 20*50 ohm = 1000 jX. The balance and isolation of Ant1 and Ant2 ports is quite sharp when tweaking the resistors.

Harmonics and signal shape distortion will limit the symmetry and balance floor.
Use sine wave.
While working on-site in office, I had only $10 B&K signal source made with OPAmps.. The performance sucks. Thinking why this multi-billion corporation does not have few bucks for a regular signal generator. Dreaming about my own signal generator sitting on the bench.
Use 10dB 50 ohm attenuator for the signal input, if available. The input/output impedances are never identical and never pure real impedance. The input impedance shall be 50 ohm (100 ohm input trimmer) to comply with the transmission line theory. A Spectrum Analyzer has 50 ohm attenuator on the input. Load the second output port with two 100 ohm resistors in parallel, make 50 ohm. Watch the output on spectrum analyzer.

This Agilent EMI Spectrum Analyzer with LCD screen is weird.
The replacable CMOS battery unit in the rear carries also clock generator and configuration memory in one pack. If the replacable battery unit is flat, the spectrum analyzer won't load right calibration numbers during boot. Forget replacing the CMOS battery. You need the actual calibration numbers that might be lost. Check the present day/year/time on the screen. If you see weird date and time, it needs battery unit replacement. It means without working battery unit, you can measure numbers large as a barn.
If you buy used similar spectrum analyzer, after few years it becomes useless.
Thinking what's going on in the Agilent's Santa Rosa, CA snake nest. Is it made by rookie engineers, or by Chinese engineers in Santa Rosa? I never saw similar anomalies with old HP CRT tube screen spectrum analyzer.

Where the IMD₃ comes from?
It comes partially from the ferrite transformer. For curiosity I compared ultralinear transistors for CATV distribution network, 2N5109, and 2N2222 at 10MHz. Two Norton type of amplifiers with ferrite power splitter. There is not significant difference between 2N5109 and 2N2222. The difference was about 3 to 3.5 dB in IMD₃.
To cover the generic limits:
The circuit needs larger cores with form factor related to delivered power, better isolation between the two power amps to be split, collector output nonlinearity shall be isolated. Signal amplitude on the collector can be a limiting factor. Lowering transistor gain might help.
The performance in common emitter configuration is weak for all above mentioned transistors. It is about the circuit architecture.

The IMD₃ numbers were confirmed on pocket-size spectrum analyzer of my own design.
Every 1dB drop in input amplitude resulted in 3dB drop of IMD₃ products. That's how it works. Correct. For power level of one hundred milliwatts, the IMD₃ is -80dBc or hidden in the noise floor.

The configuration with micro-coaxial cable or transmission line described in Motorola appnotes is another way, how to make power splitters, dividers, and baluns. It can split 0.5 Watt HF signal. The core loss is hardly measurable. 43 material. A band-pass output filter cleans power splitter output and adds IMD₃ measurement consistency with repeatability.

Resources
[1] Fair-Rite ferrite materials, NJ
[2] muRata ferrite materials
[3] w8ji notes
[4] va3diw notes
[5] w6pql notes
[6] Motorola Appnotes
[7] balun notes