The Portable QRP SWR Meter – “Magic Bridge”


KIT PRICE – $32 with Free Shipping USA only.
ASSEMBLED PCB – $30 with Free Shipping USA only.
BLANK PCB – $12 with Free Shipping USA only.
ASSEMBLED and TESTED – $55.00 with Free Shipping USA only.

New PCB v 7.0 is arrived. Better Quality, easy to assemble and Lead Free.

New PCB v 7.0 for Magic Bridge
New PCB v 7.0 for Magic Bridge

Hello everyone. I want to introduce the new project from Ivan RA3WDK which is a Portable QRP SWR Meter. We call it a “Magic Bridge” (MB). The MB can measure SWR from 1.0 up to 4.0 and holds 10 Watts. So it’s ideal for the QRP radios like an Elecraft KX-3 or Yaesu FT-817ND. You can take it with you to the camping, hiking or expedition.

It pocked sized and works without any batteries (NO external or internal DC power source) and can be used everywhere and with any QRP transceivers. The MB has a 4 color LED’s and indicates the certain SWR when transceiver is on TX. As you can see on the photo there is the two BNC connectors, one is male (to TRX side) and female (to ANT side).

Portable QRP SWR Meter
The Portable QRP SWR Meter

So, what’re the benefits of this particular SWR meter?
– pocket size
– no batteries required
– LED (good in dark or at night)
– light weight
– you don’t have to be present at the meter to check a SWR

Sounds good, doesn’t? Let’s see what we have inside in that “magic” black box.

The "Magic Bridge" with open lid
The “Magic Bridge” with open lid

As you can see there is only one small PCB with the SMD components and a couple film metal resistors. You’ll be surprised but the board hasn’t special or expensive components. Basically it’s the simple RF bridge and voltage comparator. Let’s take a look the schematic diagram of our “Magic Bridge”.

"Magic Bridge" schematic diagram
“Magic Bridge” schematic diagram

You can download the schematic diagram and part list as a PDF file here.

It’s really easy to assemble and will take just an hour of you time. No tuning required after assembling it’s should work without any issues. I’ll give to you advise how to assemble, test and use. We’re offering to you a kit which is an enclosure, blank PCB board, all components, two BNC connectors with a cable and hardware screws. That’s all that you need.

"Magic Bridge" kit
“Magic Bridge” kit

The price for the basic kit is $29.99. The “Magic Bridge” comes only with BNC connectors and holes are nor drilled. You can use the drilling template.

Assembly Instruction :

PCB board
Click to enlarge

The first, we have to solder all SMD components and then the through hole components.

1 – solder the LM339
2 – resistors 3.9KOhms and 2KOhms
3 – diodes VD2-VD5
4 – resistors 5.1KOhms
5 – resistors 100KOhms and 56KOms
6 – caps C1-C4
7 – caps C5-C6, C8-C11
8 – resistor R14

Now the through hole components. The resistors all through hole resistors should be mounted 1 or 2 mm above the PCB. Don’t forget they will be hot during operation.

9 – zeiner VD6, diode VD1
10 – resistors R1, R4, R6
11 – resistors R3, R6
12 – cap C7
13 – all four LED’s

You also can download the assembly instruction from Howard W1ZKK. Thanks Howard!

Now let’s check all components and soldered points. I recommend to use a NON clean flux but anyway after soldering please clean the board.
Here is some tips about the components.

Some tips.
Some tips.

Check again. This is how the board should look with the all installed components.

The PCB board with the all components
The PCB board with the all components

After that our board is ready to use and now it’s time to get ready the case (enclosure). We have to drill the 4 hole for LED’s and other 2 for the cable and connector.
Please use this template to drill a holes. Diameter of all holes for the LED’s is 3 mm.
Here is a datasheet for the 1591XXM enclosure.

Drilling template
Drilling template

The holes for the RG-58(.195″ or 4.95 mm) and BNC connector depends on your uses and can be not centered. When you finish drilling please verify that all holes are correct and have a right diameter. If they’re fine now the next step is put the board and screw it down.
Now we have to solder a cable with the male BNC connector and single female BNC to the board. Look on photo.

Male BNC to TRX and Female BNC to ANT
Male BNC to TRX and Female BNC to ANT

As you can see for the female BNC I used a short solid wire (20 AWG) between the pin and PCB. Also use a same wire for the ground connection.
Please check again all connections and PCB board with the components as well. Now is time to test our SWR meter.
– HF transceiver
– 50 ohms load or just 2 W 50 Ohms resistor (it’s better to have at least a couple with different values)

The first, set the output power of TRX to 1 or 2 Watts. Connect the SWR meter to the TRX and leave open the ANT input. Select a CW mode and press the key or set to FM mode and press PTT. All LEDs (red, two yellow and green) should light. Perfect. Release the key or PTT. Now hook up the 50 Ohms resistor to the ANT input of the SWR meter and repeat the previews procedure. Should be NO light. This is equivalent to the SWR 1.0. Great, everything is work!!! My congratulations!

If you have a couple more resistors (25, 75, 100, 150, 300 Ohms) and free time you can try to do more experiments to find out the right SWR measurements.
Here is a table, look at it. Questions?

SWR measurements
SWR measurements

Worked fine? Since that our SWR meter is ready to use. If the “MB” didn’t worked don’t give up, we’ll try to find the problem and fix it. Let’s start a troubleshooting.
1 – visual check for shorts, solder bridges or missing components
2 – voltage check

Voltage Diagram
Voltage Diagram
Click to enlarge

Check this out. Here is a voltage diagram. To check the voltage in those points you need to hook up the “MB” to the transceiver, set the output power to 2-5 Wstts and push the PTT (in AM or FM mode on any HF bands), another words we have to transmit the RF signal. Take the multimeter and check a voltage in all points.
That’s it, if there still no voltage, please check the circuit, diodes and LM339 (could be you soldered them backwards). That is the most typical issue. There is no more possible issues. The circuit it’s easy and should work if all components were soldered properly.

Ver 4 current one
– resistors R3,R6 now rated 2W
– capacitor C9 now 1206 size
– capacitor C7 in new position
– capacitor C8 moved to new place
Ver 3 
– needs a jumper or 0 Ohms resistor in the joint point VD2, VD3
– second generation with the one single PCB
Ver 2
– first generation of the MB on two boards

Thanks for you patience!
Don’t forget – DO NOT EXCEED POWER MORE THAN 10 Watts.

Stay in touch!
Our technical support is 24/7.

KIT PRICE IS – $29.99 with free shipping USA only.
International customers please contact me before buying.

You can buy it on eBay in my store.

Alex K6VHF

44 thoughts on “The Portable QRP SWR Meter – “Magic Bridge”

      1. Alex,
        Thank you for the prompt reply !
        I would like to purchase an assembled and tested unit for the price you quoted $50. I use pay-pal.
        Let me know the next step….

        Don W7SSB

        1. Hi Donald. Thanks.
          I’ll make one for you as soon as possible and start work on it today.
          You can find my email on, I use the same for the paypal. In the comments please put “Payment for the Magic Bridge”. Thanks.

          Alex K6VHF

  1. The Portable QRP SWR Meter.
    Саша питания без Интересно, как это работает.

    Amiran (4L1UN)

    1. Привет Амиран. Очень просто работает. Посмотри на схему – ВЧ напряжение детектируется и выпрямляется тем самым питает устройство.

  2. Nicely done Alex. Number of lit LEDs decreases as you tune toward a good SWR – very quick, visual way to tune a QRP tuner. Take care, 73, John K0FTC

  3. I have a 40m Rockmite transceiver that is rated at less than a watt. Would your magic bridge work with that low power of a rig?

  4. The lowest power that needs to feed this device is 1 W. With 1 W I guarantee 100% functionality of the Magic Bridge.


  5. Hi Alex, I tried to find your store on ebay, but couldn’t, so how do I order a kit? I have PayPal… Thx and 73, Roger, KC6NKF

  6. Hello Alex

    I’m new to SMD soldering and have your kit. As I am sure you have heard numerous times before I sent one of the resistors off into space and need to get some spares. its the 3.9 kOhm resistor specifically position R10. Any recommendations for sources would be greatly appreciated

    Thank you

    Ralph Perreca

    1. As I mentioned free shipping only to USA. International buyers must pay actual shipping cost to their country.
      Package weight is 6 oz. Thanks.

    1. Frequency range is : 1.8 MHz up to 50 MHz with maximum power of 25 Watts.
      They are available in my online store and on eBay (just look for K6VHF)

  7. From circuit diagram, I assume once your antenna matcher is tuned for minimum VSWR, you must disconnct the Magic Box from circuit or you will have a loss from the series resistor in the bridge, unless there is a bypass switch to take out the resistor in the signal path?

    1. Absolutely correct. You can work thru SWR meter but you have to consider about 25-30% of power losses. The best solution would be use a bypass switch.

  8. Would like to purchase qrp SWR kit. I live in Scotland and would pay via Paypal. Are you willing to post this to my relative’s US address in Annapolis MD. They will be coming across the pond soon. What is your delivery timescale? How do I give you their address?

  9. Concerning a bypass switch to compensate the power loss, is it OK to place the switch parallel to the series resistor R1 or would it be better to place the switch before the TRX entry, so that the whole circuit is not disrupted and sees no RF?

  10. Hi Alex. I’d like to purchase an assembled and test magic bridge please. Please advise shipping costs to NZ. I have a PayPal account. 73, Mike

  11. IN one place I saw max power of 25 watts and in another 10 watts max. Would like to use it to tune mag loop at low power then operate 25-50 watts USB max. What is the max USB power that I can use? Also, what is the power loss in your unit? Thank you,

  12. Alex,
    I have now 4 of your “Magic Bridge”
    products and use them with my go-pack
    with my 817, my SGC 2020 at the shop,
    my KX3 at home and my Ten-tec in the car! Thank you for a great product.I use
    loop antennas, Alex Loop,Alpha Loop,Chameleon Loop and some portable wire antennas.Have you thought of putting your “Magic Bridge” in a QRP tuner with a in / out switch ? Tnx again for a great product!
    Don W7SSB

  13. any plans to make one for 2m and 70cm?

    or something like the spark 100 and have an android hookup?

    will get one when i start going on the lower frequencies


  14. Hello,

    seems a nice addon to my tentec 1340. Is this product still available ? Would be interested in the kit version or just the pcb kit (pcb + components, without enclosure and connectors).

    Shipping to switzerland possible ?

    Martin HB9FXX

  15. I see that you’ve stopped making these kits. I could use the schematic and part list to put one together for myself, but that would require laying out he PCB. Do you have gerbers available for the PCB layout that you would be willing to share?

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