RF transformer design with ferrite cores – initial steps

A review of transformer design

In a process of designing a transformer, we often start with an approximate low frequency equivalent circuit. “Low frequency” is a relative term, it means at frequencies where each winding current phase is uniform, and the effects of distributed capacitance are insignificant.

Above is a commonly used low frequency equivalent of a transformer. Z1 and Z2 represent leakage impedances (ie the effect of magnetic flux leakage) and winding conductor loss. Zm is the magnetising impedance and represents the self inductance of the primary winding and core losses (hysteresis and eddy current losses). Continue reading RF transformer design with ferrite cores – initial steps

An online expert on the unsuitability of #43 for HF UNUNs

An online expert recently advised:

…The spec for type 43 makes it clear that it should never be used for HF unun construction. It is specifically engineered with a complex permeability that makes the core lossy on most HF frequencies. Since an unun is not a TLT (transmission line transformer) but rather an autotransformer, a low loss core is essential for efficient operation….

Now it contains the very common FUD (fear, uncertainty and doubt) that masquerades as science in ham radio, but without being specific enough to prove it categorically wrong. To a certain extent, the discussion goes to the meaning of efficient operation. Continue reading An online expert on the unsuitability of #43 for HF UNUNs

High end VSWR compensation in a ferrite cored RF transformer

The article Estimating the Insertion VSWR in a ferrite cored RF transformer discussed the importance of sufficient magnetising impedance to InsertionVSWR at low frequencies.

Above is a low frequency equivalent circuit of a transformer. Although most accurate at low frequencies, it is still useful for RF transformers but realise that it does not include the effects of distributed capacitance which have greater effect with increasing frequency.

The elements r1,x1 and r2,x2 model winding resistance and flux leakage as an equivalent impedance. Whilst for low loss cores at power frequencies, flux leakage is thought of as an equivalent inductance, purely reactive and proportional to frequency, the case of lossy ferrite cores at RF is more complicated. Winding resistance with well developed skin effect increases proportional to the square of frequency, but with lossy ferrite cores will often be dwarfed by the loss element of leakage impedance. Continue reading High end VSWR compensation in a ferrite cored RF transformer

Estimating the Insertion VSWR in a ferrite cored RF transformer

The article Estimating the magnetising or core loss in a ferrite cored RF transformer discussed a first cut approach to determining the minimum magnetising impedance from a core loss viewpoint.

This article considers the effect of magnetising impedance on VSWR.

For medium to high µ cored RF transformers, flux leakage should be fairly low and the transformer can be considered to be an ideal transformer of nominal turns ratio shunted at the input by the magnetising impedance observed at that input winding.

A good indication of the nominal impedance transformation of the combination is to find the VSWR of the magnetising impedance in shunt with the nominal load (eg 50+j0Ω in many cases), and to express this as InsertionVSWR when the transformer is loaded with a resistance equal to n^2*that nominal load (eg 50+j0Ω in many cases). This model is better for low values of n than higher, but it can still provide useful indication for n as high as 8 if flux leakage is low.

Magnetising impedance can be estimated using one of the following calculators, but keep in mind that there are quite wide tolerances on ferrite cores.

Magnetising impedance can be measured (eg with an analyser), but it should be measured with only the measured winding on the core. Did I mention the wide tolerance of ferrites?

Example – FT240-43 3t @ 3.6MHz

You might ask the question is 3t sufficient for the primary of an EFHW transformer that delivers a 50+j0Ω load to a transmitter. Continue reading Estimating the Insertion VSWR in a ferrite cored RF transformer

Estimating the magnetising or core loss in a ferrite cored RF transformer

The article End fed matching – design review and many later ones set out a method of estimating the magnetising or core loss in a ferrite cored RF transformer (such as often used with EFHW antennas).

There are two elements that are critical to efficient near ideal impedance transformation over a wide frequency range, low flux leakage and sufficiently high magnetising impedance. While low magnetising loss is essential for efficiency, it does not guarantee sufficiently high magnetising impedance for near ideal impedance transformation.

Magnetising impedance can be estimated using one of the following calculators, but keep in mind that there are quite wide tolerances on ferrite cores.

Magnetising impedance can be measured (eg with an analyser), but it should be measured with only the measured winding on the core. Did I mention the wide tolerance of ferrites?

Example – FT240-43 3t @ 3.6MHz

You might ask the question is 3t sufficient for the primary of an EFHW transformer that delivers a 50+j0Ω load to a transmitter. Continue reading Estimating the magnetising or core loss in a ferrite cored RF transformer

Exploiting your antenna analyser #30

Quality of termination used for calibration

Some of us use a resistor as a load for testing a transmitter or other RF source. In this application they are often rated for quite high power and commonly called a dummy load. In that role, they usually do not need to be of highly accurate impedance, and commercial dummy loads will often be specified to have maximum VSWR in the range 1.1 to 1.5 (Return Loss (RL) from 26 to 14dB) over a specified frequency range.

We also use a known value resistor for measurement purposes, and often relatively low power rating but higher impedance accuracy. They are commonly caused terminations, and will often be specified to have maximum VSWR in the range 1.01 to 1.1 (RL from 46 to 26dB) over a specified frequency range.

Return Loss

It is more logical to discuss this subject in terms of Return Loss rather than VSWR.

Return Loss is defined as the ratio of incident to reflected power at a reference plane of a network. It is expressed in dB as 20*log(Vfwd/Vref). Continue reading Exploiting your antenna analyser #30

Pawsey Balun – what is it good for?

The Pawsey Balun (or Pawsey Stub) is described as a device for connecting an unbalanced feed to a balanced antenna.

Above is a diagram of a Pawsey Balun used with a half wave dipole (ARRL).

Pawsey Balun on an asymmetric load reported model results in an asymetric dipole antenna, and showed very high common mode feed line current.

Pawsey Balun on an asymmetric load – bench load simulation showed that although the Pawsey balun is not of itself an effective voltage balun or current balun, it can be augmented to be one or the other.

So, you might ask what they do, what they are good for, and why they are used. Continue reading Pawsey Balun – what is it good for?

Pawsey Balun on an asymmetric load – bench load simulation

The Pawsey Balun (or Pawsey Stub) is described as a device for connecting an unbalanced feed to a balanced antenna.

Pawsey Balun on an asymmetric load reported model results in an asymetric dipole antenna, and showed very high common mode feed line current.

This article looks at two test bench configurations modelled in NEC.

The configurations are of a horizontal Pawsey balun for 7MHz constructed 0.1m over a perfect ground plane. The ‘balanced’ terminals are attached to the ground plan by two short 0.1m vertical conductors which are loaded with 33 and 66Ω resistances. At the other end, the horizontal transmission line is extended by two different lengths and connected to the ground plane using a 0.1m vertical conductor. The two extension lengths are almost zero and a quarter wavelength.

Zero extension

The total horizontal length from the ‘balanced terminals’ to the grounded end of the transmission line is a quarter wavelength for the Pawsey balun and a further 20mm making approximately a quarter wavelength in total.

Above is a plot of current magnitude and phase from 4NEC2. The current on the two vertical conductors containing the 33 and 66Ω loads is quite different, and the product gives load voltages that are approximately equal in magnitude and opposite in phase. Continue reading Pawsey Balun on an asymmetric load – bench load simulation

Pawsey Balun on an asymmetric load

The Pawsey Balun (or Pawsey Stub) is described as a device for connecting an unbalanced feed to a balanced antenna.

Above is a diagram of a Pawsey Balun used with a half wave dipole (ARRL).

Whilst these have been quite popular with VHF/UHF antennas, the question arises as to how they work, and whether they are effective in reducing common mode current IIcm) for a wide range of load scenarios. Continue reading Pawsey Balun on an asymmetric load

Nagoya NA-771 2m/70cm antenna

Around 10 years ago, a friend gave me a Nagoya NA-771 2m/70cm antenna to suit hand held radios for the purpose of testing it. He had bought two of them on eBay for around $10 each.

These are often sold without specifications, but where specifications are given, VSWR is given as 1.5, though not stated as maximum so should perhaps be read as typical.

This article looks at 2m performance alone.

2008 purchase

Above is a VSWR sweep around the 2m band.
Continue reading Nagoya NA-771 2m/70cm antenna