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?
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.
73
Frank
W3LPL
---- Original message ----
>Date: Wed, 13 Jun 2007 20:54:32 -0700
>From: "Jim Brown" <jim@audiosystemsgroup.com>
>Subject: Re: [TowerTalk] Balun question
>To: "towertalk@contesting.com" <towertalk@contesting.com>
>
>On Wed, 13 Jun 2007 20:22:48 EDT, K7LXC@aol.com wrote:
>
>> Anyway, all comments appreciated.
>
>At the risk of sounding like a broken record, I will point you to the
>tutorial on my website, supplemented by subsequent measurements of some
>coaxial chokes.
>
>http://audiosystemsgroup.com/RFI-Ham.pdf
>
>The following are some internal communications I shared with other guys who
>were doing their own choke research. I haven't had time to finish the
>measurements (I've been on the road a lot) and revise the tutorial to
>reflect the new data.
>
>http://audiosystemsgroup.com/ChokeMeasurementsK9YC-K6TD042207.pdf
>
>http://audiosystemsgroup.com/ScalarChokeMeasurements1.pdf
>
>When Kevin (K6TD, who worked with me on the first series of measurements
>documented above) put up a 40M beam last weekend (10 ft above the StepIR
>that you installed for him), he wound 4 turns through a stack of 7 #31's.
>He only did 4 turns because that's the best he could do without taking the
>connector off the coax. Two more turns would have been better.
>
>For 6M, I suggest 2-3 turns of RG8 through about 5 #31 toroids. The turns
>MUST be wide spaced outside the toroids. See the photos in the links above.
>
>To understand my recommendations, consider that R comes from the short
>length of the coax passing through the ferrite cores, not the coil. The
>coax outside the cores adds inductance and stray capacitance, both of which
>lower the resonant frequency and raise the Q. An ideal choke has lots of R
>and a very low Q (which makes it more broadband).
>
>So it is important to minimize the contribution of the coax that is outside
>the cores. That contribution will be least if the diameter is smallest and
>the turns are widely spaced. Increasing the diameter of the turns lowers
>the resonant frequency of the choke a lot if you allow the turns to be
>closely spaced outside the toroid stack, but not a lot if you do not. The
>MOST important thing about winding these chokes is to keep the turns widely
>spaced outside the stack. I recommend the smallest diameter that does not
>compromise the recommended bend radius of the coax.
>
>73,
>
>Jim Brown K9YC
>
>
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