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

[ZR]

That might have been me as Ive been a bit vocal about it.
A typical balun is wound as an autotransformer with all leads having a 
common DC point and wound together which provides little to no isolation.

When wound as a true transformer with seperate primary and secondary 
windings the isolation can easily be 30dB or more with a BN73-202 binocular 
core. Ultimate isolation is attained when the distributed C between windings 
is only a few pf and you do this by running each side thru its own sleeve 
which gives the furthest seperstion between primary and secondary. Its also 
easier to do an impedance match when they do not conform to the standard 
balun ratios. A well matched Beverage will also perform better and not have 
SWR varying a lot over say 1.8 to 10MHz.

Carl
KM1H



----- Original Message ----- 
From: "Pete Smith N4ZR" <n4zr at contesting.com>
To: <topband at contesting.com>
Sent: Friday, January 27, 2012 2:33 PM
Subject: Topband: Transformer isolation vs common mode choke was: Re: T 
Vertical feed


> 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 
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>
>
> 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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>>
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