Saturday, August 27, 2011

Surplus RF-Transformers for Random Wire Antennas

In some of my last posts I was sharing thoughts about broadband aerials using transformers to match the high impedance of the non-resonant "random" wire. Well, the leghth of those wires is not that random at the end. Some typical lengths that come to my mind 6m, 7m, 7.5m, 11.5m, etc. you may know some more...

Now, let's have a look at some options provided by surplus (the junk box respectively).

CFL bulb transformer  vs.  surplus iron powder toroid
On the left side we have a transformer that was in a (defective) CFL bulb, aka as energy-saving light. The thing on the right side is one of the is a surplus transformer based on a T68-2, which came in bags of 5pc for just 0.80 €-cents.

So, what do we have, the CFL transformer countd 3:3:12 windings. Using the black and the white in series, that would be 6:12. This would be, in other words, be a fine 9:1 UnUn.

The T68-2 is provided with 12 bifilar windings. This would therefore make you average 4:1 UnUn. Obviously, those can also be 1:1 isolation transformer.

Both junk-box items are proven in ham-radio designs. I figure, I will experiment with both out of the box options. Additionally, I may possibly try a 6:1 UnUn, as in the BB6W/BB7V design. Staying at QRPP levels would even offer the change of trying the termination resistor used in the BB6W/BB7Vs.
Oh, I forgot, to get closer to the BB7V, I will add some additional Al-pipe to top of the 5.5m  long 27MHz vertical.