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Registered Member #326
Joined: Sat Mar 18 2006, 01:12PM
Location: Glasgow, Scotland
Posts: 66
I'm attempting to build a standing-wave ration (SWR) meter for use in the 50 ohm feedline of my amateur radio HF antenna. Frequency range will be 1-30 MHz and power up to 150W. Many commercial manual and automatic antenna tuners use a SWR measurement circuit similar to this:
A short section of thick solid wire carrying the RF signal passes through the centre of a ferrite toroid. It's clear that the seccondary windings sample a small amount of the RF current, and the forward and reverse currents are half-wave rectified, smoothed/filtered and then driving the microammeter.
What I'm not clear about is toroid selection. The usual rule as far as I understand is that the turns ratio and core material are chosen such the the winding inductance has around 4x the impedance of the circuitry which drives/loads it. I'm not clear how to apply this to the line carrying the RF to my antenna. I'm also not totally clear about the function of the capacitors, inductor and resistor connected to the centre tap.
Frustratingly the various textbooks and websites give examples of specific SWR meter circuits, rather than explaining how to design one.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
This circuit was invented by Warren Bruene of Collins Radio in 1959. I remember it well, I was complaining about the weather on 80m at the time. Here's his original article...
It recommends "60 turns of No. 30 wire on a carbonyl E core". Carbonyl is the low permeability, low loss ferrite grade intended for RF work, not the stuff we all use in our switchmode power transformers.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Carbonyl may refer to Iron-Carbonyl which is a grade of iron-powder. For example micrometals iron-powder core type-2 (red/clear) is known as a carbonyl core. These show lower high-frequency losses (and lower permeability as Steve said) than the more common Hydrogen reduced iron powder cores.
Carbonyl iron powder is typically used in RF power transformers, matching networks and RFCs where the AC flux excursion is large and you don't need that much inductance.
Hydrogen reduced iron powder is typically used for buck and boost chokes in Switched-mode power supplies in the 10's and low 100's of kHz. Inductance is typically larger and AC flux excursion is lower because ripple current is limited by design in SMPSUs.
I suspect that the differences between the two grades of material is that the grain size of iron-powder produced by the carbonyl iron process is smaller than that produced by the hydrogen reduced method? Can anyone confirm this?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There's no need to use a toroid at all if you don't want to.
You can adjust your antenna system at low power using a simple wideband resistive bridge, as there is no need to monitor SWR constantly during transmission.
Using low inductance carbon resistors, the resistive bridge has a far greater frequency range than any toroidal or 'monimatch' SWR metering system.
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SWR Meter Toroid and Component Selection
