Analyze My Homebrew Diplexer

Question

I'm in need of a diplexer that'll separate RF signals from a single feedline to two antennas. What I designed and built was a diplexer that would allow ~38 Mhz and below out port 1, with ~38 MHz and above out port 2. This is primarily to use a separate antenna on the 6M ham band and a separate antenna for the remaining HF frequencies with an output power of 100 watts.

I followed these directions: http://vk3atl.org/technical/Diplexer_1cc.pdf (PDF) using the student version of Elsie. Layed out a PCB, etched it, populated with the parts calculated by Elsie. Here's the design:

As built:

I soldered a 51 ohm resistor across each output port, then fed the input with an MFJ analyser. Below the crossover, SWR is ~1.7:1. Above the crossover, the SWR ~2.1:1. I was thinking the SWR should have been lower on both ports - around 1.5:1 or better. 1.7:1 isn't all that bad for the HF port but >2.0:1 on the VHF port isn't. My initial thoughts are the coils are too close together and need to be spread out. That's all I can think of at this point. I would redesign the PCB to have more distance between coils and make them orthogonal. What other modifications to this design should I do?

UPDATED

Pulled one of the 51 ohm resistors off the board and tested it with my LC meter. Sure enough, it looks like these resistors are wirewound.

EDITED to reduce image sizes

Answer 1

What your trying to do here is build two filters, one high pass, and one low pass. Horizontal inductors shunted to ground with caps, create a low pass filter. When the caps are horizontal and the inductors got to ground, you have a high pass filter.

If you want to analyize this in detail you need to learn how to use a technique called ABCD matricies.

http://en.wikipedia.org/wiki/ABCD_matrix_analysis

Each series or shunt impedance is entered into the 2x2 matrix, and then they are multiplied and you get a final relationship that describes the circuit.

For a simple circuit as you have here. there is no reason to go to the trouble, as it is already well understood. The circuits are equavilent to a transmission line, below a critical frequency, and a low or high pass filter above. There are simple formulae that give the cutoff freq and the characteristic impedance of the circuit.

These circuits need to be terminated in their characteristic impedance, or they will not work properly, and in that case you will get standing waves.

Your antenna has an impedance, so the filters need to be designed to work into that load impedance.