Optimising a low frequency Wilkinson power combiner? - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 03, 2019, 11:49:14 am
content: Thanks to forum member Thomas I have the means to build a Wilkinson combiner for use combining 2 or 3 LF Class D amps running at 137 kHz. I have a few questions re optimizing them. If I use an on line calculator to wind an air cored inductor of 81.85uH on a former of outside diameter 75mm it tells me to wind 55 turns. But if I measure using an in calibration Fluke LCR meter that excites at 1kHz, it shows 82uH. From past experience using an AIM4170 antenna analyzer set to excite at the operating frequency of 137.5kHz I will get another reading again.The on line calculator for a 3 port combiner is here:https://leleivre.com/rf_wilkinson.htmlSo, to my questions. Do I attempt to wind the inductors with their design impedance at 1kHz according to the Fluke LCR, or according to 137kHz injected via y AIM4170 antenna analyzer?Once wound can I optimize like this? Inject a sine wave of 0dBm at 137.5 kHz into one output port, put my SA on the other outlet port, and a 50 Ohm load on the input jack. Then add / remove turns looking or the minimum level on the SA port in combination with minimum insertion loss when injecting the signal into the input jack, andgetting the least loss at each output port, with the other output port terminated in 50 Ohms? Or do I just wind according to the various air cored inductor calculators and not worry about how good or bad it is if it seems to work OK (which it does, but much better it could be is nagging a bit....)Thanks!

author: SpencerTC
author_rank: Contributor
timestamp: July 03, 2019, 01:38:50 pm
content: In my experience, calculators and simulation software gets you in the ballpark, they can even get you really close if you feed them with adequate accurate data. You then must tune to desired performance.If I were going to tune a passive combiner(or divider) I would inject a 0dBm CW signal at the desired center frequency into the (assumeing this is a combiner) output, terminate input 2, and measure input 1 while tuning to lowest loss (-3dB should be an ideal number) all the while keeping return loss to a minimum. I would then swap the termination to input 1 and measurement to input 2 and do the same. You might have to split the difference, as tuning one might adversely affect the other, use your best judgement.you might also measure the isolation between the two inputs, but if that's an issue it can be easily overcome with a pair of isolators if tuning for isolation hurts your IL/RL too much.Good luck, I hope it turns out well.I'm curious to see a Wilkinson for that low of frequency, when you are done, if you don't mind I would love to see a photo.Best regards,Trent Spencer - KM4YTU

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 09, 2019, 09:50:13 pm
content: Thanks for your help, I have now built it and it's working overnight on 137.550kHz. I forgot the photos until it was jammed on my bench, but they should give you an idea. The alloy boxes contain the low pass filters with forced air cooling (although now I use 2 I am not using the fans), the other alloy box is a "Scopematch", seehttp://njdtechnologies.net/want-a-little-more-info-than-an-swr-meter-alone-can-provide-meet-the-scopematch/for info, one of the most useful things I have built in years.

author: SpencerTC
author_rank: Contributor
timestamp: July 10, 2019, 12:19:31 pm
content: Thanks for posting, this is the first time I've seen (at least knowingly) a wilkinson build of lumped elements. I work with them regularly at higher frequencies (1.2-5.8GHz) built with distributed elements and I -naively- thought you were going to give it a shot with distributed elements at sub MHz band. Hindsight being 20/20 I can now see how silly that thought was. Nonetheless, it looks to be (at least from what I can tell) a carefully built project, and I hope it serves you well.Thanks again for the photos.Best regards,Trent Spencer - KM4YTU

author: Chris Wilson
author_rank: Super Contributor
timestamp: July 10, 2019, 12:31:42 pm
content: No worries, I see why you were curious if you thought I was going to have a go with striplineIt got through the night with no noticeable heating and no smoke, so it passes my QC testI got into the Canary Islands from here in the UK on a static noise infested band, so all in all, a success, thanks again for your input.