Ultralight SWR indicator? (Part 1)

Thanks Martin!

(there’s a “/” missing in the URL - it should be http://www.sp5jnw.sem.pl/konstrukcje/atudf3oshtm/atudf3oseng.pdf )

I’m interested in perhaps ordering some of your PCBs here in the US once you post the files (or maybe laying out a leaded parts version of the PCB). I also will wait for more info on the best LED/zeners and other information you will post.

Thanks & 73,
Barry N1EU

Hi Barry,
thanks - you will be able to order the PCBs directly from


once I will have set the project to public. I need to replace a part footprint and apply a few minor improvements before releasing it, but the work is almost done. For the parts, I plan to release a Mouser or Digikey shopping list so you could order those directly from there.

As a future direction, one could replace the array of diodes by an ATtiny 85 and control the LEDs digitally. That would be more reproducible but require a 1.8 - 5 V power supply.


Many years ago I solved this problem by ordering a couple of replacement (…) cross-needle meters for the MFJ-971 QRP Portable ATU (a problem-free transaction without any question from MFJ).
My SWR/PWR meter was then built for 30/6 watts according to the original.


Here is the promised schematic for the TinySWR meter. Note that the schematic is from Dieter Engels (DJ6TE) and Hannes Hiller (DL9SCO), based on the earlier indicator design by Hans Steinort (DF3OS). I just created it in KiCad and designed a respective PCB.

Regarding the PCB, I will have to fix a few minor issues (e.g. I used a footprint for the trimmer that is no longer available). Also, RF-wise, the layout can be improved (but the SWR and losses are okay in my prototype).

Of course, you can also assemble this on a breadboard if you do not want to wait for the PCBs. The types of the LEDs are critical; I will post recommendations once I will have finalized my experiments.

If the LEDs cannot be turned off even when transmitting into a perfect 50 ohm dummy load then you have swapped the wires from the transformer.

More to follow!
73 de Martin, DK3IT


FYI: Another design is described here:

The QRP version uses a Stockton bridge on a tiny BN43-2402 two-hole ferrite core. He also suggests to add a DC voltage of 1.5 - 1.8V in order to make the circuit suitable for lower power levels.

As indicated, I will first finalize the TinySWR circuit shown above and later turn to a tiny digital version.


That looks nice, although again we’re dealing with judging the dimness of a single LED (in sunlight) to ascertain relative SWR. They’re also shut down for the season.

Barry N1EU

Thanks Martin for all the details. Excellent and deep investigation!

I’ll try to build one here when you provide final info of the leds.
Stay tuned, 73 de Ignacio

I have finalized the choice of components and the schematic diagram. Please find it in the attachment.

The choice of LEDs is critical. The following types work well in my tests:
SWR1-3: KINGBRIGHT L-934LID (Reichelt: LED 3MM 2MA RT), Forward Voltage typ 1.7, max 2.0
RF1: KINGBRIGHT L-934LYD (Reichelt: LED 3MM 2MA GE) Forward Voltage typ 1.8, max 2.1 V
D1: OSRAM OPTO LS M67K-H2L1-1-Z (Reichelt: LS M67K), Forward Voltage min: 1.7, max 2.2, typ 1.8 V

With 5 W, the first LED lights up for an SWR of ca. 1:1.3 - 1.5, the second joins at ca. 1:1.6 and beyond ca. 1:1.9 the third starts to glow. You can calibrate the baseline with the trimmer (but the steps are determined by the forward voltages of the LEDs and Zener diode).

I will finalize the PCB next week or so. If you are interested in building one yourself, I recommend waiting for the PCBs, because a compact wired version is difficult due to stray RF.


There is another neat design:

Martin, currently SV9/DK3IT/P

I checked out http://www.qrpproject.de/Media/pdf/MicroSWR.pdf and it appears that they offer neither a kit or blank pcb’s. The green/red LED pair it uses is a very cool design to indicate SWR.

Barry N1EU


!!! ACHTUNG: die ROT-GRÜN-LED Anzeige Platine gehört nicht zum Lieferumfang !!!
!!! Caution: The red-green-LED pcb is not part of the kit → out of stock, new red-green-LED pcb under construction !!!


Thanks, so it looks like you can get half of the MicroSWR - it also says “RED-Green-LED unit kit is not included and not available!”

Another interesting source as start-up aid for those who want to build something individual - either with a IN/BYP switch or inline (which does not require any power supply) - though not specifically tiny:
The Portable QRP SWR Meter – “Magic Bridge”

Blank PCBs available here:

Looks nice but it can’t stay in-line while operating.

If it is not urgent, you may want to my nee digital verdion of the TinySWR.

Hello Martin,
how could I get one of your PCB and parts here in Europe?

Thanks for feedback, 73 de Ignacio

Martin, what do you recommend using for the 1K pot R2? Is there any chance of a shared project at Mouser for ordering a kit of parts (other than the PCB)? Perhaps I could help put that together, although I’m fairly clueless on SMD parts.

Hi Barry, Ignacio:

You will be able to order the PCBs directly from OSH Park for very fair prices, always in batches of 3.
Three (!) PCBs will be USD 2.30 including (!) postage to Europe.

As for the parts, I will set up a Mouser part list, so you can order there directly. Due to minimal order constraints and surcharges, this makes more sense for quantities of 10 or greater, but any club or group could handle that locally (it does not sense for me to order the parts in 100s and then have the logistics to set up billing etc. for individual parts). Also, in Germany it is pretty much complicated to sell electronic kits due to legal and regulatory constraints - doable, but no business you would start for small stuff.

So this will be really open hardware and all files and documentation will be on the Web on Github.

As for the trimmer, this is the only critical component, because the footprints of SMD trimpots vary a lot. I am still searching for a popular brand and will include that in the updated PCB design.

If you build a THT version with wiring by your own, you can use any trimpot you like. Just keep in mind to keep the directional coupler part as symmetrical as possible.

If you have a bit more time, then I would advise you to wait for my digital version, which will be more accurate and not more expensive.


Hi all:

I finalized the TinySWR PCB and project page. Please find all details here:

You can order the new PCB directly from OSH Park at a very fair price (as of writing: USD 2.30 for three boards including shipment to Germany):


Warning: The current release reflects a new, improved PCB design that is NOT YET TESTED. There might still be problems with the PCB. Try at your own risk.

I have not yet had the time to order and try the new layout. If anyone of you is willing to take the risk in terms of time and money, I would be happy to receive your reports.

73 de Martin, DK3IT