[Lowfer] Isolation transformer, Computer noise, etc.

[Bill Ashlock]

"I'd like to say someone said years ago that you want 4x the feedline impedance "for the primary & secondary of the transformer"

Correct, Mike, but the 4X relates to the inductive reactance of the primary and secondary of that transformer and this value of reactance is proportional to the frequency of use. The formula is: XL = 2 x pi x F x  L    So set up the inducance (L) for the lowest frequency of use. At higher frequencies the 'X' factor will be proportionally larger and this is OK.

One of the advantages of an isolation transformer back at the shack is that it prevents most of the powerline hash present on the house ground system from being sent out to the antenna site. To prove out this theory and determine how much the power line noise is, connect a wire of the same length as the lead-in to the AC ground in your house and place the other end near the base of your antenna. Listen to a weak signal at a frequency close to where you are operating and 'make' and 'break' the connection to AC ground. You should hear a considerable increase in noise when the wire is connected.

Another way to determine the effectiveness of your isolation transformer is to temporarily jumper the return lines on both sides of the transformer. The noise should also increase by quite a bit.

Bill

> Date: Thu, 21 Jan 2010 13:34:34 -0600
> From: we0h at gmx.com
> To: lowfer at mailman.qth.net
> Subject: Re: [Lowfer] Isolation transformer, Computer noise, etc.
> 
> Hey Andy,
> 
> Turn the computer off and unplug the power supply. Is the noise gone 
> now? Build a 1:1 broadband transformer for your feedline. That will stop 
> the common mode noise you have. I did years ago, no noise now. I'd like 
> to say someone said years ago that you want 4x the feedline impedance 
> for the primary & secondary of the transformer.
> 
> How about this easy idea, wind 10 turns each side interweaving the turns 
> on a FT240-77 core. That should be enough inductance on each side to 
> work for your LF setup. You could use a much smaller core if you like 
> but use -77 material. On your HF antennas, use a -43 core. Stick with a 
> FT240-43 core for the typical 100-200w rig. It should work fine.
> 
> I found one other possible noise source hear many years ago and that was 
> the negative terminal on my bench power supply was tied to the A/C mains 
> ground. It isn't anymore.
> 
> On those audio 1:1 isolation transformers, dig inside a old computer 
> power supply and find a 1:1 transformer with a whole lot of inductance 
> on each side. Use that. Problem solved. My audio transformer is about 5 
> millihenry each side. Been working great for 7 plus years.
> 
> Mike
> WE0H
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