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Impedance measuring devices: simple and notso

The VK5JST Aerial Analyser is an excellent kit project to add to the antenna building support collection. This device runs a bit under $150 for a kit of parts. W5BIG has an article in November 2006 QST, An Antenna Impedance Meter for the High Frequency Band (W5BIG site link) that describes another approach. It is interesting comparing and contrasting. W5BIG sells his item for about twice (and up) the price of the VK5JST.

Both use a bridge to determine voltage across the load and current through it at a given frequency. The VK5 uses an analog oscillator with a bridge that provides rectified and filtered voltages for the 8 bit ADC in a Picaxe microcontroller that then handles all the arithmetic and display functions. The W5 device use DDS (direct digital synthesized) signals for the oscillator and mixes two of them so the load measures are down converted to 1 kHz signals fed to a 12 bit ADC (except his commercial offering for hams only use 8 or 10). The microcontroller feeds off both the voltage and current amplitude and phase measures to a PC to do the number crunching and display management. The RigExpert line includes a display.

The VK5 uses a 28 pin Pic microcontroller while the W5 RigExpert uses a 64 pin Amtel with a lot of support circuitry. Both have 8/10 bit ADC and are very similar in terms of capabilities. The Amtel has a lot more internal memory for programming and this shows in the W5 device capabilities.

The use of DDS rather than analog oscillator and a graphic LCD rather than a 2×16 character display means the W5 can scan frequency ranges and create graphs of SWR versus frequency or whatnot.

The VK5 is much simpler at least as far as basic functions and software go. That gives it an edge in terms of learning and potential for modification, especially when compared and contrasted to other approaches such as that W5BIG describes.

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