Topband: Transformer isolation vs common mode choke was: Re: T Vertical feed

[Guy Olinger K2AV]

The actual ratio, or incidental winding to winding capacitance,
particularly on 160m, is pretty immaterial.  What is accomplished by the
isolation transformer is simple removal of all possible current paths along
a conductor.  The various bead choke, wound balun like devices, etc, all
have a physical conductor path which has a reactive/resistive "buck"
imposed on it as a current reducing device.  This bucking effect is at its
very least on 160 unless it is specifically "tuned" in some way to 160, a
procedure not found in ANY of the commercially available devices.

Even if the buck is TUNED to 160, if the buck on the line sets up a voltage
node from the viewpoint of the unwanted signal, the voltage at the bucking
point can become very high unless there are TWO such devices spaced roughly
a quarterwave. Thus a single common mode buck can paradoxically make the
unwanted signal ingress WORSE.  In my case, the block between my TX
vertical wire and the window line feedline COMMON MODE, was having to
handle 900 volts running QRO !!  Any available bucking device was either
spectacularly ineffective or was destroyed by dissipated power.  The
isolation transformer canned all such problems AND reduced the level of
local broadcast stations some three or four S units.

One issue with an isolation transformer is its possibly needing incidental
inductance to be taken care of in some fashion, rendering the device
properly set up for a given band a single band device.  But when one is
talking about 160m devices which aren't needed anywhere else, that doesn't
matter. The incidental inductance on the RX side probably doesn't matter.

73, Guy

On Fri, Jan 27, 2012 at 2:33 PM, Pete Smith N4ZR <n4zr at contesting.com>wrote:

> Someone recently commented in favor of using transformer isolation in
> lieu of a common mode choke in a receiving application.  I presume this
> would be a 1:1 transformer using a binocular core, or at least
> completely separate primary and secondary windings.  What are the pros
> and cons of this idea?  Does capacitive coupling between primary and
> secondary, or some other factor, limit how much isolation can be
> achieved this way?
>
> 73, Pete N4ZR
> The World Contest Station Database, updated daily at
> www.conteststations.com
> The Reverse Beacon Network at http://reversebeacon.net, blog at
> reversebeacon.blogspot.com,
> spots at telnet.reversebeacon.net, port 7000 and
> arcluster.reversebeacon.net, port 7000
>
>
> On 1/27/2012 1:07 PM, Jim Brown wrote:
> > On 1/27/2012 8:40 AM, Charles Moizeau wrote:
> >> I am willing to insert a common-mode choke, but don't know what to
> measure beforehand to learn if one is needed.
> > There is NO DOWNSIDE to using a good common mode choke other than cost
> > and weight, and as W4TV has noted, there are downsides to NOT using one.
> >
> > As it turns out, there was a typo in the link I posted to my RFI
> > tutorial, which includes Cookbook guidelines for winding effective
> > ferrite chokes.  The correct link is
> > http://audiosystemsgroup.com/RFI-Ham.pdf
> >
> > 73, Jim K9YC
> > _______________________________________________
> > UR RST IS ... ... ..9 QSB QSB - hw? BK
> >
> >
> _______________________________________________
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