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Author Topic: What Ferride toroid to use.  (Read 2946 times)
KM1H
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Posts: 2633




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« Reply #15 on: November 01, 2017, 03:08:47 PM »

Quote
Fair-rite does make a line of suppressor cores, not to be confused with the toroids.  Mainly since their dimensions, not the materials, are suitable for slip-over-the-cable applications.

The 31 mix and a few others are considered RFI suppressors by F-R and include the large 2.4" cores which most of us still refer to as toroids.
I'll check out the latest F-R catalog when I get a chance, after dinner or tomorrow.

Carl
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WB6BYU
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Posts: 17190




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« Reply #16 on: November 02, 2017, 08:45:49 PM »

Quote from: K8AXW

How do you know if you have the minimum 20db of attenuation across the HF spectrum without a spectrum analyzer?




Spectrum analyzer really doesn't help much.

And it is difficult to know when you have 20 dB of attenuation anyway, because that totally
depends on what the common mode current was before you added the choke, and how the
choke changes it.

Some antenna configurations (such as a single wire with no ground connection) will act as a
current generator, and will force current onto the shield regardless of the impedance of the
choke.  (That just raises the feedpoint impedance.)  In other cases, such as a simple dipole
with the coax shield grounded 1/4 wavelength from the antenna, the common mode current
is likely to be low anyway, and adding the choke won't make much difference in the actual
numbers.

Fortunately it isn't difficult to measure the common mode current.  You can make a simple
clip-on RF current meter and calibrate it with a known power into a 50 ohm load.  Then snap
it around your coax and measure the common mode current. 

Here is an example of such a meter.
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KM1H
Member

Posts: 2633




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« Reply #17 on: November 04, 2017, 06:23:38 AM »

Many have proven by testing that the crossover method of balun winding does nothing.
The core that smoked is an unknown, what are its specs? Is it really ferrite or powdered iron?
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K0WA
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Posts: 102




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« Reply #18 on: November 12, 2017, 06:15:11 PM »

K9YC recommends two 1 to 1 rf chokes in a feed line, one at the antenna feed point and the other at the bottom of the line if you want to run high power.  He uses one ft240 31 using 12 gauge THHN solid copper wire at 12 to 14 turns.  The balun runs well at 700 watts but he says a second balun  at the  output of the amp will increase the power to legal limit.  See his power point presentation on his web page.

Word of warning, on a Force 12 type antenna, one of these baluns realy screws up the feed point and the swr goes wonky.  I think this is due to the feed point scheme that is used in these antennas.  W ould stick with the recommended balun.

Lee
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NK7Z
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Posts: 1846


WWW

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« Reply #19 on: November 14, 2017, 04:18:46 AM »

Word of warning, on a Force 12 type antenna, one of these baluns realy [sic] screws up the feed point and the swr goes wonky.
Hi Lee,
That surprises me!  I think that means that means the coax is radiating, and is part of the antenna system, which in some cases is not what you want.
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
N4NDX
Member

Posts: 217




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« Reply #20 on: November 14, 2017, 08:13:30 AM »

I thought about it and I think I will keep the one that comes from factory instead
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KM1H
Member

Posts: 2633




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« Reply #21 on: November 14, 2017, 03:30:01 PM »

Quote
I'm puzzled when you say that we're not discussing transformers here.  Clearly, a closed-circuit core with windings amounts to a transformer.  Whether a current or voltage transformer balun depends on how it/they  are designed and connected to the transmission line.

Umm, read Post #1 which clearly states a Dual Core Isolation Balun.. I take that to mean a Common Mode choke or any other name for the same thing.
Plus the number of cores and the number of coax turns is well described in K9YC's papers which have been referenced on here many, many times. I have yet to find enough deviation in the dozens of chokes Ive wound to even waste the time discussing it.
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