[Lowfer] Isolation Transformers

Tue, 20 Aug 2002 21:12:11 -0500

I have an easier approach for you Les. Sell the house and move out far away
from the power lines and the city. You could move out to Wyoming and run
everything off of solar power.

Mike>WE0H
http://www.we0h.us/lf.html

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Lyle Koehler
Sent: Tuesday, August 20, 2002 10:11 AM
To: [email protected]
Subject: Re: [Lowfer] Isolation Transformers

As long as the remote preamps are battery powered, and both the + and -
leads of any battery chargers are disconnected while you're receiving, the
line is isolated, and there should not be any need for chokes at that end.
If chokes are used, they would be inserted in series with both the + and -
lines. The tuning line is usually very high impedance, so a 100k resistor in
series with that line would be cheaper and better than a choke. Since the
shack end of the line is isolated, you could put the chokes out near the
preamps, but it wouldn't do much good unless you also put RF isolation
transformers in the coax lines at that end.

Mouser Electronics has a variety of chokes. Their catalog usually shows the
Q, self-resonant frequency and DC resistance. You want the DC resistance to
be low enough so that chokes do not introduce an excessive voltage drop.
Usually that's not a problem because the preamp current drain is fairly low.
For example, if the preamp draws 30 mA and you don't want more than 1 volt
of drop from your 12-volt supply, the total DC resistance of the chokes
should not exceed 33 ohms. The inductance should be as high as possible, as
long as the self-resonant frequency is above the operating frequency of the
preamp, and the DC resistance is not excessive.

The rotator hash is probably from ground currents flowing through an
intermittent contact between the rotating and stationary portions of the
rotator. I had that same situation on my receiving loop, which also uses a
3-wire TV rotator. To cure it, I inserted an insulating section of PVC pipe
in the metal mast that supports the rotator. This brings up another point:
If you want complete isolation, you also need to isolate or choke the
rotator control lines. That's a problem, because the rotator needs quite a
bit of current and the DC resistance of small series chokes would be
excessive. A 120 volt AC isolation transformer probably wouldn't help -- the
capacitance between windings may be enough so that it still looks like a low
impedance path for RF. You can isolate at the shack end by unplugging or
switching out both sides of the rotator power line after it gets to the
desired position.

I presently use no RF isolation at either end of the lines; no virgin
grounds at the loop/preamp. With that comes the disclaimer that any advice I
give on isolation and grounding is speculation; I have very little personal
experience to back it up!

Lyle, K0LR

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