Topband: Share of a common mode current experience

[Jim Brown]

On Sun, 10 Jun 2007 08:38:41 -0700 (PDT), Eduardo Araujo wrote:

>With a couple of chokes with more than 2K, the
>broadcast signal is reduced by more than 30 db now
>because it is being forced to pass thru the filter in
>differential mode. 

Sort of. The two currents are more or less independent of each 
other. See my previous email. 

>Now I am working in all cables coming down because as
>soon as I connect each other cable not being treated,
>the problem reappears.

Yes. That implies that the common mode RFI is getting into 
equipment that has pin 1 problems. The chokes are probably the 
easiest way to solve it. 

>I have a few questions: I understand why it is
>necessary to use a low loss material like #61 at the
>antenna feedpoint (QRO) where current could be high
>and if so the R part of Z will need to dissipate a lot
>of heat, but what confuse me a little bit with #61
>material is:

NO, NO, NO!  Low loss in a ferrite core is important ONLY when 
the full transmitter power is in the core. That is ONLY when 
the ferrite is the core of a TRANSFORMER. When you wind coax 
around a ferrite, you are forming a CHOKE, NOT a TRANSFORMER. 
With chokes, you want HIGH losses. 

>1  Using a high Z mostly inductive is ok if the
>combination of line length, antenna and ground
>connection dont result in a capacity that combined
>with that inductance may make things worse.

Yes. BUT -- the feedline will be a different electrical length 
at every frequency. AND, with a high inductance, the choke will 
resonate at some low frequency, and be useless above that 
frequency. So in a choke balun, RESISTANCE is GOOD, INDUCTANCE 
and CAPACITANCE are BAD. 

>2  What could be ok for one band ie: Z combined in
>the right direction, it could be bad in another band
>where this system resonates and capture signals from
>our others antennas or other sources.

YES. 

>3  Living at 35 mts from ground level as it is my
>case, is it possible to estimate/simulate in advance
>if it will be better or worse on different bands?

HIGH RESISTANCE is ALWAYS GOOD. 

>4  It is very difficult to get high Z with #43
>material al low fcies. If I understood correctly your
>measurements Jim, we will need 14 turns on 8 cores to
>get aprox 2K at 1 Mhz and 4K at 2Mhz. 

Several problems. You cannot get more than 7 turns of RG8 
through a FT240 core, and about 12 turns of RG8X or RG6. AND, 
that larger cable will have more stray capacitance. This means 
that the turns MUST be widely spaced. It also means that the 
stray capacitance, even with wide spacing, will move the 
resonant frequency lower with fewer turns. Bottom line -- 7 or 
8 turns through 7 #31 cores is a pretty good choke for 160-15M, 
but you'll need nearly twice as many #43 cores. And with twice 
as many cores, the resonant frequency will be lower, so the 
choke will probably only be good 160M-30M.  

As I have time, I've been measuring some coaxial chokes, and 
when I have enough data to present a complete picture, I'll 
revise my cookbook to reflect that data.  

>5  I thought to use two chokes at the antenna
>feedpoint one with #61 and the other with #78 but if
>due to combination of Z #61 does things worse, then
>the one with #78 will need to dissipate all the heat
>by the current (if any of course) 

Use ONLY the #78 (or #31 if you can get it). #61 is a waste of 
money for this purpose. 

>I am doing some measurements on the cores extracted
>from old TV to have an idea of its characteristics,
>and when done, I will append it here.

Don't waste your time. These materials are likely to be very 
inconsistent from one part to another. 

73,

Jim Brown K9YC