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Ultralight SWR indicator?

Dear Ignacio:
Thanks for your great report! I am glad to see that the Open Hardware way of sharing ham radio designs seems to work well!

BTW, the yellow or orange LED for RF1 is not as critical as the red ones.

73 de Martin

PS: When adjusting the trimpot, I recommend to use a position at which the first LED at least glows a tiny bit a a SWR of 1:1.2 or so; this gives you confidence that the circuit works.

Dear all:

Yesterday, I got the boards for the latest revision of the layout (2017-10-24), as now available via OSH Park. I just assembled a new prototype and can confirm it works like a charm. It works best with 2.5 - 5 W, but can be calibrated to also work with 1 W. The only downside of the simple design is that the calibration is accurate only for the same power level (or at least within a narrow power range).

As for winding the toroid, use enameled wire no thicker than 0.22 mm for the secondary winding, otherwise the 25 turns won’t fit well. Make sure you only occupy ca 270 degrees of the core, otherwise directivity might suffer.

When ordering the parts, it may make sense to add a few 100 Ohm / 2 W resistors, for they make for nice dummy loads for various SWR levels.

I hope this tiny SWR indicator will turn out useful for many of you and will save lots of final transistors.

As for the ease of assembly: The board is pretty small and tightly packed with components on both sides. It should not be your first SMD soldering project. But then again, all parts are 0805 sizes and the pads optimized for hand-soldering. Take your time and a good magnifying glass :wink:

73 de Martin, DK3IT

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Dear Colin, all:

I updated the GitHub project files at

and tried to implement the following issues:

  1. Removed links to unnecessary KiCad libraries.
  2. Improved the assembly instructions.
  3. Added an illustration showing the correct direction of the toroid windings.

The project should now be ready for use.

Any feedback will be very welcome!

73 de Martin, DK3IT

Another (slightly customized) TinySWR is ready and works as expected - not as a SWR Meter but as a SWR and RF Indicator - excactly as promised by the circuit developer.
Many thanks again, Martin@DK3IT.

My TinySWR should especially draw my attention if there is a real problem with the home made resonant antenna (trapped/linked EFHW) used together with the Mountain Topper Transceivers. That’s why I used different colored LEDs - the red LED starts to glow at antenna loads of: infinity, short circuit, <= 20R and >= 125R (SWR of approx. 2.5). The yellow LED starts to glow at <= 33R and >= 75R (SWR of approx. 1.5).

Modifications made (all the LEDs are low current types from Knightbright):
R4: 680R
RF1, SWR1: green LED (1.9V)
SWR2: yellow LED (1.85V)
SWR3: super red LED (1.7V)

The filter housing FG1B 37x20x20 mm used is very tight …
… the next larger FG2B 50x20x20 mm may be better suited for less experienced builders


Dear Heinz,
thanks for the report and pictures! I am always impressed by the level of craftsmanship in your constructions!
73 de Martin, DK3IT

Hi Martin,
Here is your tiny SWR meter installed in one of my MTR-5B SOTA radios. The LEDs are a little dim but bright enough. All that is needed are assurances that the radio and the antenna are working properly, and this does it!
Thanks & 73,
John K1JD
Santa Fe, NM


Hi John,
thanks for the feedback and picture! As for the brightness of the LEDs: It will be better if you operate full 5 W from 12V. You can use my switchable 6-9-12 V DC-DC boost converter with input and output filters for that purpose:

Also, I am working on a fully digital version of the SWR meter with a tiny Stockton coupler and an ATTiny in SMD. It will be only slightly bigger than the current version but actually measure the SWR independent of the power level. But this is work in progress, will likely take until 2018 to complete the firmware etc.

hpe 2 cu on the bands one day!

73 de Martin, DK3IT

PS: I saw on QRZ that you are a fan of the MTR-5B - maybe this all-in-one GoBox project is interesting for you: New SOTA Station Design: Mountain Topper Go Box


Dear Fred, sorry for my slow reaction: I do not have the exact types from Barry’s list at hand, but basically most 2 mA versions should work fine, and even some standard bulk 3mm LEDs will do. Really critical is the OSRAM SMD LED on the PCB, because most other SMD LEDs in red have a significantly higher forward voltage.


Barry kindly sent me a board to play with…

? What gauge magnet wire should I use on the torriod ??

Richard // N2GBR

Hi Richard,

for the secondary winding, use ca. 30 cm of 0.2 mm enameled wire 0.2 mm. For the primary, I now recommend using simple insulated wire, because burning of the insulation from such a short piece of magnet wire with precision is a bit challenging.

0.2 mm is ca AWG 32 (see https://www.rapidtables.com/calc/wire/awg-to-mm.html).

The exact diameter is not very critical, but thicker wire may not fit evenly on the tiny toroid.

Hope that helps!

73 de Martin, DK3IT

Thanks… Martin… visiting the company R+D lab this week (I work from home)… so its a good time to pick up a couple of items for home electronics projects… and that’s something encouraged by the company I work for… they like us to have home electronics projects!

Richard // N2GBR

FYI: I found another page with simple QRP SWR indicators, maintained by JA1XRQ:


He partly uses the circuit that is the basis of TinySWR and has some hints on ideal resistor values for varying power levels.

The page is in Japanese, but Google translate is your friend :wink:

73 de Martin, DK3IT

Hi all,
Ondra, OK1CDJ, is now offering a complete kit of TinySWR:

For EUR 5, you get the PCB and all parts, which is a really fair price.

Thanks to Ondra @OK1CDJ for this great service to the community!

73 de Martin, DK3IT


Well done Martin @DK3IT and Ondra @OK1CDJ.

I bought 4 and very happy with testing so far.

73 de Pedro, CT1DBS/CU3HF

Hi Martin

I ordered the kit from Hamshop for about three weeks. It is the perfect accessory for the Monutain Topper MTR3!

Many thanks to Martin and Hamshop!!!

vy 73 de Matt HB9FVF


Can you please post the link?

Hi Barry
is already above but:

vy 73 de Matt HB9FVF

Thanks Matt!

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Actually that link also comes up in Czech until you click on the Union Jack flag :uk: at the top right of the screen.

73 Ed.

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Mounted a tinySWR in my MTR3 the same way…

I couple this with an EFHW tuner built into a dental floss case. Also visible are the two posts which serve as the paddle of a built-in touch keyer. I log using a small card clipped to the back of the radio. In this way sitting down (usually not a good idea) isn’t necessary. A tethered spacepen (stores in an attached soda straw) serves as writing instrument.

  • Fred KT5X (aka WS0TA)