[TowerTalk] Balun question

[Jim Brown]

On Thu, 14 Jun 2007 07:46:39 -0400 (EDT), donovanf at starpower.net wrote:

>Jim,

>Thanks for providing the additional links showing the details of your
>measurements.  ScalarChokeMeasurements1.pdf shows six traces, but only
>five are labelled.  What is the unlabelled trace?

It's an optical illusion. There are only five curves -- the traces cross 
each other.  I went back to the Quattro spreadsheet to confirm it. :)

>Were the chokes also tested with power levels actually used in the
>intended application on transmitting antennas?  Overheating and core
>saturation are common problems that don't show up with the low power
>bench tests you documented.  It's important to know this information
>before a choke is used at 100 watts or 1500 watts (or whatever power
>level is intended).  Teflon coaxial cable is typically used on chokes
>at the 1500 watt level to avoid damage from overheating.

Hi Frank,

Thanks for the questions. 

Lots of things have typically been done, but one of the points I'm 
making in this tutorial and my subsequent work is that there are better 
ways of doing what we have typically done. W1HIS correctly observed the 
value of using high impedance chokes to reduce coupling of noise 
received on the feedline to the antenna, but his method of winding 
chokes wasn't very good. The two recent pdf's document a better way. 

The measurement method is documented in the pdf's. These are small 
signal measurements. HOWEVER: the matter of dissipation and heating is 
addressed in the tutorial. The executive summary is that in coax, the 
magnetic field related to transmit power is confined almost entirely 
within the dielectric. A CHOKE "balun" sees only the UNBALANCED current 
(the common mode current) flowing on the outside of the coax shield. If 
the RESISTANCE of the choke is on the order of 5K at the frequency of 
the transmitter, the current (I2R) losses are small because I is small. 
The key is to insure that R is large. A somewhat more detailed 
discussion is in the tutorial. 

My engineering judgement is that a PROPERLY APPLIED choke like those 
described will not have heating issues. Heating problems arise when 
something goes badly awry -- that is, we are forcing lots of common mode 
current through it. That implies an antenna that is BADLY broken. High 
VSWR alone does not force common mode current -- UNBALANCE does. 

If you doubt this, I suggest you wind some of these chokes and transmit 
through them QRO into high VSWR, but not terribly unbalanced, loads, and 
observe the heating (or lack thereof). Then, if you wish, force some 
imbalance (at the feedpoint of an OCF antenna like a Carolina windom) 
and see what happens. 

I don't have a problem with frying a choke when it's up in the air on an 
antenna that I'm going to have to lower to fix because the ANTENNA is 
broken (i.e., wind damage, etc.). And that's the sort of problem that 
can create enough imbalance to fry one of the chokes I'm recommending.   

73,

Jim Brown K9YC