4:1 current balun – identifying bad ones

Correspondents have informed me that the balun dealt with in article 4:1 current balun – review and fix and variants are very common. This article gives a checklist of common issues and some basic measurements using an antenna analyser that should reveal some issues without breaking into a sealed assembly.

Introduction

Baluns are commonly employed to obtain nearly balanced feed line currents (ie negligible common mode current) in two wire lines or negligible common mode current on coaxial feed lines. This article discusses baluns for that application.

A very common 4:1 current balun is Guanella’s 4:1 current balun, but there are others including pretenders.

Three common 4:1 current baluns

Guanella 4:1 current balun

(Guanella 1944) described a 4:1 current balun in his 1944 article, he did not show the winding pairs coupled by a magnetic core as shown above.

Guanella421

Above is Guanella’s circuit, and he does not show coupling between the two winding pairs.

Properly implemented, this device is known to work very well.

Sevick’s single core 4:1 current balun

Let us look at Sevick’s device because it underlies so many failures.

If you look at it very carefully, you will see that the two output wires emerge from opposite sides of the core, the left hand wire exiting under the core was wound from front to back of the core and the right hand wire exiting on top of the core was wound from back to front of the core.

(Sevick 2001) describes a balun that he states works only on Isolated Loads, but Isolated Loads MUST have perfect current balance, they do not require a balun.

You might ask whether such a balun which works only on an Isolated Load which itself MUST be current balanced, works due only to the load isolation.

Most HF wire antennas are not well represented as a Isolated Load. Whilst you might at first thing there are only two terminals, there is a third terminal in most cases, ground.

Nevertheless the ‘net abounds with instructions on how to build this and there are plenty of commercial sellers who apparently endorse the maxim the customer is always right... but it does speak to their technical expertise.

The VK2DQ 4:1 current balun

The following diagram is from WIA 4:1 current balun, it is a design made very popular by the work of VK2DQ (though he may not have originated it).

4-101Note that the output wires at the top both emerge from the top side of the core, both were wound from back to front.

Common implementation issues

To be brief, the common issues are:

  • Sevick’s single core 4:1 current balun on a single core will not work well for generalised three terminal loads;
  • VK2DQ’s balun as has series opposed windings which cause the device to almost short circuit the input;
  • poor choice of ferrite core (size, material);
  • wrong number of turns for application;
  • poor transmission line characteristics for the application;
  • mis-wiring which may prevent the device working as intended.

Measurement tools

To make these measurements, you need an instrument that can measure impedance at the balun coax socket. The shorter the connections the better, a single coax adapter from balun to instrument will give acceptable results.

The instrument could be a VNA (including a one port analyser such as a MFJ-259B, FG-01 etc), a calibrated noise bridge, a high end RF impedance or admittance bridge.

Real world measurement of baluns has uncertainty that will introduce error into the measurements, so don’t expect extreme accuracy. Nevertheless, defects will usually be easily recognised by a large departure from ideal.

You will also need a load of 200+j0Ω. This could be made with two 100Ω carbon composition resistors in series, or similar SMD resistors (100 x 100Ω 1% SMD resistors can be bought for about $3). Low power / small metalfilm resistors with pigtails trimmed may measure up just fine at 3.6MHz.

Connecting wires MUST be short and direct.

Measurements

If your instrument does not directly measure the sign of X but measures the magnitude of X, label your measurements |X|.

These are all tests than any good 200Ω:50Ω current balun should pass, not just the balun types discussed above.

Perform these tests @ 3.6MHz , writing down the results at each step in a table:

  1. measure the 200Ω resistor with your instrument to verify both;
  2. measure input impedance (R and X) with nothing connected to the output terminals; and
  3. measure input impedance (R and X) with the 200+j0Ω load connected to the output terminals;
  4. measure input impedance (R and X) with the 200+j0Ω load connected to the output terminals and the shield of the coax connector bonded to one output terminal; and
  5. measure input impedance (R and X) with the 200+j0Ω load connected to the output terminals and the shield of the coax connector bonded to the other output terminal;

Think about what you should expect:

  1. very close to 200+j0, VSWR≈4;
  2. a quite high impedance, |Z|>1000 ohms;
  3. very close to 50+j0, VSWR<1.2;
  4. very close to 50+j0, VSWR<1.2; and
  5. very close to 50+j0, VSWR<1.2.

A good Guanella 4:1 current balun with appropriate core material and turns will pass all of the tests.

Sevick’s single core 4:1 current balun will fail 4 and 5.

VK2DQ’s single core 4:1 current balun will fail 2, 4, 5 and probably 3.

Examples

Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω

Below are the results of performing the above tests on a 4:1 balun at hand.

The yellow cells are my actual measurements (using a nanoVNA), the other values are calculated but they may be displayed directly on some instruments.

In this case, all tests are passed easily.

It is not a current balun you say? Sure behaves like a good current balun at 3.6MHz.

VK2DQ / WIA single core 4:1 current balun

FT140-43 core with 9t for each winding (per WIA instructions).

Ft14061balunAbove, the DUT.

Above are the test results. The DUT fails tests 2,3,4, and 5. This does not work on the Isolated Load (Test 3), and even worse on the asymmetric loads (Tests 4 and 5).

Why? Look at the Test 2 result, to a first approximation, that low ‘magnetising impedance’ impedance shunts the transformation spoiling Test 3, a result of the series opposing sense of the two windings. Further, the common mode impedance is low which results in significant change between Test3 and Tests 4 and 5.

But my analyser does not display R and X

Some analysers may not display R and X, but they may display magnitude of X (|X|), use that and label your result column |X|.

Some analysers may display may not display R or X but may display VSWR and |Z|, or other combinations from which the others can be derived. Try the calculator at Find |Z|,|X| from VSWR,|Z|,R,Ro to find the needed data.

References