“Wooden construction”

In the hastily rearranged talk to the club on Tuesday, 21/11/06, Brian, G3YKB talked about many different methods of home construction of experimental or “one off” circuits.  These included: conventional printed circuit board, strip board, “paddy board”, ugly bug, and others, and also how to make screened enclosures by soldering sheets of single sided PCB board together, and he showed several examples of each.

There is one method which I have used on several occasions for audio and low RF frequencies which many people might be reluctant to use because of the suspicion that undue losses might be incurred.  This involves the use of wood, either as the base or “bread-board”, or in small strips as in “tag” or terminal boards.  This method takes us right back to the 1920s and 30s when home construction of “wireless sets” could be undertaken on the kitchen table with virtually no tools or test equipment.  It was always known that wood was not as good an insulator as bakelite or ebonite, (in the 20s and 30s), or the resin bonded laminates available today, (early versions of which were known in the 1950s to 1980s as Tufnell and Paxolin).  The question I set out to answer was; How good or poor was wood as an insulator when used in the way I had sometimes used it?

I had used wood as the base board in several experimental circuits for the amateur LF Band, (around 136 kHz), where, (except for parts of the circuit where I needed a high Q), I considered the frequency was near enough to DC for most practical purposes.  The method of construction I used was to tap small pins like nails into the wood and to solder the components between their heads.  (One of the advantages of a flat wooden bread-board is that you can lay out the circuit almost exactly as in the circuit diagram and every component is easily accessible to a voltage or ’scope probe).

I therefore decided to measure the DC resistance between small nails, similar to those which I used in the circuits, spaced at various distances in different samples of wood.  Partly because I suspected that there might be polarisation effects which would distort the measurements of resistance at low voltages, and partly because I suspected that the resistances would be high, I chose to use a fairly high voltage, (1400 V), and directly measure the current which flowed.  I will not bore you with all the details of the measurements, but in summary, my findings were as follows.

All measurements were made between pairs of 5 eighths inch nails spaced between 3mm and 27mm apart inserted into augered holes in the wood samples.

Softwood sample 1.     3mm spacing, 38 MW  (Sample stored for >5 years in garage).

Softwood sample 2.   27mm Spacing, 60 MW (Sample stored for >5 years in garage).

Softwood sample 3   25mm spacing, >150 MW (Sample stored for >5 years in house).

Hardwood sample 1.    3mm spacing, 12 MW  (Sample stored for >5 years in garage).

Hardwood sample 2.  24mm spacing, 38 MW  (Sample stored for >5 years in garage).

As only a small number of experiments were done it is difficult to generalise, but it seems that softwood has a higher resistance than hardwood, that drying for a long period indoors further raised the resistance, the resistance increases in a non linear manner with pin spacing, and that for DC and almost certainly for lowish frequency AC, the insulation resistance between the pins in this breadboard form of construction, is high enough not to worry ordinary circuits of low or medium impedance.

                              John,  G0NVZ


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