Voltage symmetry of practical Ruthroff 4:1 baluns – finding TLT Vout/Vin

I have been asked to expand on the calculation of voltage magnitude and phase set out in Voltage symmetry of practical Ruthroff 4:1 baluns.

Above is Ruthroff’s equivalent circuit, Fig 3 from his paper (Ruthroff 1959). Focusing on the left hand circuit which explains the balun as a transmission line transformer (TLT), and taking the node 1 as the reference, the loaded source voltage appears at the bottom end of the combined 4R load, and transformed by the transmission line  formed by the two wires of the winding, and inverted, at the top end of the combined 4R load.

It is the transformation on this transmission line that gives rise to loss of symmetry.

The complex ratio Vout/Vin is dependent on the complex reflection coefficient Gamma at both ends of the line and the line propagation constant gamma, all of which are frequency dependent complex quantities. Continue reading Voltage symmetry of practical Ruthroff 4:1 baluns – finding TLT Vout/Vin

De-embedding transmission line to allow remote measurement

Recent articles An interesting case study of measurement of a balun’s Insertion VSWR and Another measurement of a balun’s Insertion VSWR made measurements through a transmission line of the load on a balun.

In the first article, the measurements at the input of around 7m of 50Ω line were adjusted to move the reference plane to the load end of the coax using the add/subtract cable feature of Antscope to de-embed the transmission line.

The second article used a FA-VA5 analyser and VNWA software to make the measurements and to some extent, de-embed the transmission line. In this case the transmission line was quite short at 370mm, and whilst the facility adjusted for propagation time, it did not adjust for attenuation though that was very small in this case and of little consequence. The FA-VA5 analyser and VNWA software combination would not suit the scenario in the first article as will be demonstrated.

This article examines the response to a 6m length of RG58 with O/C load at 30MHz.

We can see that although the phase of Gamma (phase of 0.85+j0.01) is close to zero the magnitude is 0.85 when the magnitude should be 1.00 for an O/C load. Continue reading De-embedding transmission line to allow remote measurement

Another measurement of a balun’s Insertion VSWR

Further to An interesting case study of measurement of a balun’s Insertion VSWR, this article presents similar measurements of a small DIY balun.

Balun404

Above is the top view of the balun, and the test termination comprised two 100Ω 1% resistors clamped between the screw terminals, so pigtails were just 3mm in length.

Balun402

Above is a view of the interior.The coax pigtails are quite short, they exist at the input and output. Continue reading Another measurement of a balun’s Insertion VSWR

Voltage symmetry of practical Ruthroff 1:1 baluns

Well, I guess Voltage symmetry of practical Ruthroff 4:1 baluns begs the question, what about Ruthroff 1:1 voltage baluns?

The Ruthroff 1:1 voltage balun can be seen as two back to back Ruthroff 4:1 voltage baluns with the redundant winding removed… and that prompts the thinking that the cascade of two baluns back to front might cancel the phase delay.

Let’s measure a popular Ruthroff 1:1 voltage balun.

RAK BL-50A

Above, the RAK BL50-A was a quite popular balun, and probably the balun of choice for half wave dipoles… well until the message about current baluns escaped. Continue reading Voltage symmetry of practical Ruthroff 1:1 baluns

Voltage symmetry of practical Ruthroff 4:1 baluns

Much is written about antenna system balance, this article looks at balance issues with the very common ATU configuration that uses a Ruthroff 4:1 voltage balun to adapt coax transmitter output to two wire open feed line. This type of balun is employed in most ham market ATUs that contain an integral balun.

Above is Ruthroff’s equivalent circuit, Fig 3 from his paper (Ruthroff 1959).

If one looks carefully at the transmission line form, there is effectively a two wire line wound into a helix (usually on a magnetic core) and connected from the unbalanced source to one half of the load inverting the connection for the necessary phase reversal.

Ideally, Vout of this line is equal to Vin, ie Vout/Vin should be 1∠0°. That is unlikely as it implies a zero length transmission line which provides the decoupling of the phase inverting line.

This article looks at the Ruthroff 4:1 balun balance using the very popular MFJ-949E as an example.

Above is a pic of the MFJ-949E Ruthroff 4:1 balun. The transmission line is not uniform, but let’s make an approximation to predict its behavior with a centre tapped 100Ω load, the centre of which is connected to the ground terminal. Continue reading Voltage symmetry of practical Ruthroff 4:1 baluns

Equivalent circuit of an antenna system

Common practice is to treat antenna systems as a two terminal device in free space.

Pickup most handbooks, and even text books, and antennas and often antenna systems are described in this way.

That model is quite inadequate for many or most antenna systems installed in proximity of natural ground. For example, a two terminal dipole and feed line system representation cannot have feed line common mode current, and it follows that thinking in terms of two terminal models denies a full understanding of the antenna system.

A three terminal model of an antenna system

(Schmidt nd) sets out a three terminal model of an antenna system in presence of ground using quite conventional linear circuit theory.

Above is Schmidt’s Y network based on values of three intermediate impedances, ZD, ZU, and ZC. These are found from measured values Za, Zb and ZC as explained by Schmidt: Continue reading Equivalent circuit of an antenna system

VU3SQM directional wattmeter build – #4

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

At long last, some PTFE rod arrived to permit assembly of the transformers.

For reasons discussed in an earlier article, the transformers use a larger core than the original VU3SQM. They need to stand above the board, and whilst that compromises the mechanical strength of the assembly, it should have better performance. Continue reading VU3SQM directional wattmeter build – #4

Measuring common mode current with a scope

I wrote recently of a flawed test of balance performance of an antenna system and an ATU, and some readers have taken up the issue, basically asking the question “then, how do you measure balance of a two wire line with a scope?”

The first step is that you must define what you mean by “balance”.

For most wire HF antennas, the balance objective should be equal but opposite currents in the adjacent wires at all locations along the line (recalling the currents may vary along the line). This reduces radiation from the feed line (which can cause EMC problems with nearby appliances / systems), and reduces very local noise pickup on receive (from those same appliances / systems).

Let’s take KA0KA’s scope display from the reference article, but assume that they were taken from current probes so that we are directly measuring feed line currents rather than voltage. Current probes allow the scope to measure current on a conductor placed through the probe, an RF current probe (or current transformer) can be as simple as a suitable ferrite toroid with the primary conductor passing once through the center of the core, and a secondary winding of 10-30 turns loaded with a low value resistor, and the scope input connected across the resistor.

The obvious measurement method

Above, the first measurement shows both channels, and the currents appear almost equal in magnitude and almost opposite in phase, but it does appear that there is a slight phase difference, perhaps 5-15° from exactly opposite phase. Each channel is almost 2div peak to peak, and let’s assume the calibration factor is 1A/div. Continue reading Measuring common mode current with a scope

ATUs and periodic maintenance

This article is about PM of a traditional manual T match ATU with two variable capacitors, and an air cored variable ‘roller’ inductor.

Above, internals of the stock ATR-30.

It has been about 5 years since the last PM on my ATR-30 ATU, so time for covers off, thorough inspection for signs of heat damage (particularly coil support insulation, more so if it is thermoplastic like the very popular polystyrene), contact and other arcing, cleaning and lubrication of mechanical parts as needed (including the fan in this case).
Continue reading ATUs and periodic maintenance

VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

The parts have arrived and construction commenced.

Above, the PCB populated with the SM parts and soldered. It was soldered in an IR reflow oven. Continue reading VU3SQM directional wattmeter build – #3