This page last updated: 19 Jul 9

A Modern Thermocouple Ammeter

By David A. Reid (PA3HBB / G0BZF GQRP member 3677)

(based on work by Yardley Beers W0JF)

Click for bigger version of the picture

Thermocouple ammeters are very rare these days, but the job they were perfect for - measuring antenna currents - is still a modern requirement especially in respect to groundplane currents. When it comes to ground systems, the famous QRP phrase 'Less is More' is unfortunately not true. However, by optimising a small ground system more of your precious power gets radiated.

The thermocouple ammeter is useful for measuring antenna currents and finding resonances in antenna systems. I made one for measuring the ground currents in my groundplane for a short vertical I have been working on. It has proved an invaluable aid in creating an effective ground system. Basically this piece of test equipment allows relative measurements of currents running in the ground wires of my system. I used two crocodile clip leads to make the coupling transformer. The two wires of the two clip leads are tape together to make a parallel line, which is then coiled into a suitable size and taped together. During a test of a ground wire, one lead is inserted into the ground wire, in series, and the other, with a small diode (e.g.1N4001,1N914) inserted in one lead, is connected to a digital multimeter, or small meter. The transmitter is operated and a reading taken on the meter, adjusting the power to get a reading which is within the range of the meter. The ground lead is then altered in length and the reading recorded. The highest reading on the meter indicates the highest currents in the ground wires, and therefore the higher effectiveness of the ground system.

An interesting fact, which I discovered is that ground radials of 1/8th electrical wavelength produced the highest readings. I researched this phenomenon and discovered that some work supporting this claim has been published in the past ( by John Stanley, K4ERO/HC1 and Roger Hostenback, W5EGS in QST 1976).

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David Reid PA3HBB / G0BZF

Leenderweg 46

5591 JE Heeze

The Netherlands

 

Email: DRCP@compuserve.com

 

Click the colour photo for a full size view of the circuit and meter