u/CoastalRadio

I wanted to buy one of K6ARK’s mini transformer kits to build a 9:1 random wire antenna for my Elecraft KH1, but they were sold out.

But I can buy toroids, magnet wire, and BNC connectors! I 3D printed a spacer to lift the toroid just off the BNC pins, and I printed a little strain relief plate for the wire.

Soldered the ground side of the BNC first, inserted the spacer (notice the notch on one side), soldered the center pin with the spacer in place, stack the toroid on the spacer, solder a little PolyStealth to the magnet wire, run it through the strain relief plate, stuff a bunch of hot glue inside and heat-shrink it all together.

With 35.5’ of wire, a bit of 2mm rope, and tiny little weight, if I happen to have a tree at a park or summit, I can get up a better radiator than the 4’ whip. No need for an extra counterpoise, as the KH1 already has one.

Followup to this post:

https://www.reddit.com/r/amateurradio/comments/1spe6bi/some_dumb_questions_about_lc_circuits

Story time:

A while back, I wanted to make a diplexer for a homebrew, dual band Yagi-Uda antenna for working amateur satellites with an HT (Wouxun KG-UV9D Mate). I came across a schematic for a 5th Order Chebyshev Low-Pass and High-Pass Filter diplexer designed by KI0AG.

Design Schematic found at https://kw4fb.com/micro-diplexer

This design used 50 VDC rated SMD components, and the diplexer was rated to 10W, which was perfect for an HT. I did not buy the little board he sells, and I did not buy the SMD components. I instead purchased through-hole capacitors because I was more comfortable working with them. I don’t remember what my plan was for the inductors. I don’t recall why I chose the specific capacitors I did other than they had the proper capacitance value. I ordered enough capacitors from Mouser to build 5 diplexers, because they are so cheap you are mostly paying for shipping, and it gave me several chances to mess up. 

Before I got around to building the diplexer, I got an Arrow antenna, and I just got theirs because it fits in the handle. 

Now several years later, I want to play with a more base-station type satellite station using eggbeater antennas, and a more powerful radio (Yaesu FTM-150RASP), so I’m back to needing a diplexer. As I was doing some research, I remembered I had a bag of parts laying around somewhere, and I found the capacitors I bought. I wasn’t sure how much power they could take, and I ultimately decided to just build it to see what happened. I decided to make my own inductors using a 3D printed core from Printables and some magnet wire. 

Of course I don’t currently have a working L/C meter (I ordered one, and it gets here tomorrow), but I wasn’t going to let that slow me down! There is a fair amount of room between 2m and 70cm, so I decided to try it with estimated inductance values and see what happens. 

Below are the parts I used along with a few SMA connectors and some perfboard. 

Inductors (hand wound):

1x 4 turns, 1mm magnet wire, 7mm PLA core, 1.3mm pitch - supposedly 100nH and 

2x 3 turns, 1mm magnet wire, 7mm PLA core, 1.3mm pitch - supposedly  68nH 

2x 1 turn, 1mm magnet wire, 7mm PLA core, 1.3mm pitch - supposedly  15nH 

Capacitors (purchased from Mouser):

2x TDK CC45SL3FD180JYNNA - Ceramic Disc Capacitors D: 5.5mm 3kV 18pF SL 5% LS:7.5mm

2x Vishay / Cera-Mite 564RC0GAJ602EF4R7D - Ceramic Disc Capacitors 4.7pF 6000volts

1x Vishay / Cera-Mite 561R10TCCV27 - Ceramic Disc Capacitors 1Kvolts 2.7pF 5% .25LS NPO

I got it together, and somehow it actually works!

The 2m Low-Pass Filter has 0.725dB insertion loss at 146.2MHz and 31.363dB rejection at 437.6MHz.

The 70cm High-Pass Filter has 1.825dB insertion loss at 437.6MHz and 54.601dB rejection at 146.2MHz.

People who made the SMD version reported 0.3dB and 0.6dB insertion loss, so mine is a bit worse. 

I did some redneck power-handing testing: key down for a period of time, key up, immediately touch all the components to see if they are hot, starting at 5W for 1 second, and working up to 55/50W for 20 seconds on each band, because satellite transmission are usually around 5 seconds. Even at full power FM for 20 seconds, nothing on the board was even warm at all (the dummy load warmed up a bit, though), so I am certifying the circuit for 55W.  

I might re-make it with inductors that actually have a measured value. I could probably tighten up my construction a little too. I suspect I’d get back some of my insertion loss, but if I’m doing that I should probably use capacitors that are better suited to this application and less acceptable to thermal drift. At that point, I have to question if I should re-build this or choose a different design.

Overall, I’m pretty happy with the result considering a medium amount of jankiness in my inductors and overall construction technique.  In any case, I’m going to put it on the air and see what happens. This project has been about getting out of analysis paralysis and just sending it.

I’ll probably just buy a decent diplexer at some point.

Nano VNA screenshots attached in the comments.