Radio Shack ferrites measured!

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May 28, 2009
I use a number of RS ferrite snap-on chokes as either feedpoint baluns / chokes, or as a way to tune the common-mode of the feedline as a 1/4 wave element.

I measured them with an MFJ 259B antenna analyzer with a very small length of RG-58 with the braids and center conductor twisted together on each end, and attached to the shield and antenna posts of the analyzer with the bead snapped on. I only measured up from about 30 mhz, where there was no reactive component (X) present when the bead was snapped on.

This is the best I could do, and on some of the chokes, I did not have an absolutely tight fit. I'm sure someone can do a better job than I did here as I don't have any fatter jumpers available - and I'm not cutting up my existing feedline. :)

Problem is, how may of these do you need? Most references to the choking impedance needed to make a good feedpoint choke on symetrically-fed dipoles vary from 500 - 1000 Z, with 1000 ohms preferable.

For my quarter-wave common-mode elements where I purposely use this "feature" of coax, you need around 3000 ohms or more! (See my other antenna notes about the half-sloper about how to get around this and use less chokes.)

So how many do you need, ie what is the value of the individual chokes and at what frequencies?

RADIO SHACK 273-105 (or 273-0105)
Type-43 material

------- ----
30 150
45 195
55 245
70 260
88 270
108 236
118 221
125 210
136 189
146 175
160 165
175 141

(I'm not sure what mix this is)

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88 245
108 375
118 400
137 377
146 337
160 288
170 250

This 273-069 was a very tight fit for the RG-58. Could that tightness mean that my earlier measurements are a bit on the low side for the 273-105 ? Probably, but maybe someone can do better than with my test setup.

At any rate, for the 105's, and for that matter most of my other type-43 ferrite material chokes I had laying around, the results were about the same. So to be conservative, for projects that use the type-43's, I'll mentally calculate that each has about 200 ohms and for good measure, add one more.

Not very scientific, and I wish I had the equipment to do a better test.
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