A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use.

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a 4t primary and 12t secondary for a nominal 1:9 50:450Ω transformer.

Lets consider a couple of simple starting points for low end and high end rolloff.

Low end roll off

A simple model for these devices with low flux leakage is an ideal transformer with primary shunted by the magnetising impedance. To obtain low InsertionVSWR, we want the magnetising impedance in shunt with 50+j0Ω to have a low equivalent VSWR.

Typically complex permeability changes in-band, and although it tends to decrease, increasing frequency means that the critical point for magnetising impedance is the low end.

High end roll off

At the high end, transformation departs from ideal usually when the length of wire in a winding exceeds about 15°.

Going forward

A small core makes for short windings to obtain high frequency performance, and sufficient turns are needed for low end… but not too many as it restricts the high end.

There are lots of rules of thumb for minimum magnetising impedance, most treat the inductor as an ideal inductor and these ferrites are not that.

A quick analysis using the method in BN43-2402 balun example hints that a 4t primary is probably good enough down to 1.8MHz, depending on one’s limit for InsertionVSWR. We are not being too fussy here… this is not an application that demands InsertionVSWR < 1.2.

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Above is a plot of expected R and X for a 4t winding using my common mode choke design tool. Z at 1.8MHz is 49+j199Ω, or Y=0.00117-j0.00474S. (If your design tools are not giving you similar values, you might consider validating them.) Adding the shunt 50Ω (Y=0.02), we get Yt=0.02117-j0.00474S, and plugging that in to calculate VSWR… Continue reading A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

Turning 1kW into QRP

Effective Isotropically Radiated Power (EIRP) is one means of comparing the performance of a transmitting station.

An inefficient antenna can lead to very low EIRP, perhaps surprisingly low. Consider these four examples at 3.6MHz,

The following NEC-4.2 models give some insight.

QW vertical with 120 buried radials

Considered by so many experts to be the benchmark for a grounded monopole, here is a quarter wave vertical with 120 buried radials.

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Above, 120 buried radials: GAIN=-1.8dBi, radiation efficiency=20.7%.

At 1kW RF input, EIRP=661W. Continue reading Turning 1kW into QRP

Adapting a 5V TTL GPS to RS232

5V TTL interface GPSs can be had for about $10 on eBay. They are aimed at the RC market and although the GPS chip is a 3.3V chip, they have a 5V regulator on board and level shifters or 5V tolerant IO.

To use them with APRS you usually need to get RS232 output and to support the common 12V feed on the DE9 connector, the RS-232 / TTL adapter needs to incorporate a 12V/5V regulator.

This article describes a couple of modules that are available on eBay for a dollar or so, and suitable for modification by adding the regulator.

max3232powerAbove left and bottom are two MAX3232 modules that have had a LM78L05 regulator and decoupling cap added to provide for 12V in and 5V to power the MAX3232 and GPS (top right is a Ublox NEO6M).

In the case of the lower module, a track has to be cut to disconnect the pad for 12V in from the existing circuitry.

 

BN-43-2402 balun example

An online poster recently sought to design a broadband 9:1 transformer for HF.

Choosing a BN43-2402 balun core, he planned to use a 2t primary and 6t secondary for a nominal 50Ω input. He subsequently posted measurements of the prototype.

What might we expect… is it a good starting point.

A first approximation at the low frequency end with a medium µ core is that it is like an ideal transformer withe the magnetising impedance in shunt with the primary. Continue reading BN-43-2402 balun example

iinet – pre-connect experience

The NBN is coming….

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Our NBN street cabinet, or Node in FTTN. The pic is not crooked, the cabinet is out of level.

We received advice that NBN access is now available in our street… about 10 months after the street cabinet was installed.

This is the best hope we have had of escaping Telstra’s appalling service that we have endured for 6 years.

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Above is a plot of the median download speed of the Telstra 8Mb/s ADSL service.

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Above, the distribution for last week show that although HTTP speeds would appear to be capped at 2Mb/s on an 8Mb/s access pipe, the inevitable slide to poorer performance is shown where around 10% of the measurements are below 0.25Mb/s. Continue reading iinet – pre-connect experience

OCF short vertical dipole for HF

The OCF short vertical dipole for HF has become popular, particularly disguised as a flag pole for low impact installations and encouraged by claims of outstanding performance.

The rationale for the design is that it is a short dipole, not requiring radials, and feed point offset downwards by 30% as an optimal value for performance (driven by often unsound assessments of coax loss).

Claims include:

Off-Center Fed Vertical Dipole design means no radials, 90% efficient or better across 80m – 10m

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Above is the promising gain plot for one of the commercial implementations, it is only one S point (6dB) behind a quarter wave vertical with 4 buried radials. Continue reading OCF short vertical dipole for HF

Cadweld Plus Control Unit

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The CADWELDPLUSCU is the igniter for the Cadweld Plus line of electrically initiated thermite weld capsules. The plug seen on the end of the cable slides over the flat connection strip to the capsule.

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The environment around the weld crucible is subject to a sudden ejection of hot gases and smoke, much of which is emitted out of the slot in a Oneshot crucible intended for the connection strip, and this is only a 45g capsule. There is probably less gas emitted towards the connector on reusable moulds as the gap for the connection strip is very narrow.

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Despite dressing the plug away from the stream of hot gasses emitted, it can be seen above that the plug has suffered heat damage. Of particular concern is deformation of the parts at the cable clamp rendering it less effective. The plugs come on a new lead set that is relatively expensive.

Since one wants to be able to yank the cord away from the crucible as soon as possible to minimise heat damage, the cord grip needs to be effective.

This article describes two measures taken to protect the plug and ensure the cord grip works. Continue reading Cadweld Plus Control Unit