On Wed, 21 Jul 2004 07:17:46 -0500, Tom Hammond wrote: >Does anyone happen to know of a way to do some cursory testing on these >beads (possibly using the MFJ-259) to tell if they might offer a decent >impedance at HF? And if, how?
I don't believe in "off-list" replies except for buying/selling stuff. I have done a fair amount of work on this, and can give you some very simple methods using the MFJ-259B or the AEA CIA-HF. So here goes. The equivalent circuit of a choke formed by passing a wire through a bead is simply a series R and L (or R and X L). Both R and L vary widely with frequency. That is, there is not a single L value, nor is there a single X value that describes the choke at all frequencies. Rather, there is a set of curves that describes both R and X. In general, R and X both start off small at low frequencies, both increase with increasing frequency, and at some frequency X (or L) will drop off to a pretty small value leaving only R. To measure these curves for any ferrite, simply take a short piece of wire, pass it through the ferrite, and connect the wire to the analyzer. Then read the impedance over a range of frequencies, put the values of R and X in a spreadsheet, and compare them to the datasheet for the material. There are excellent data for Fair- Rite beads in the Fair-Rite pdf catalog on their website. I strongly recommend this catalog -- in addition to the data, there are some excellent applications notes that talk about how ferrites work and how to use them. You can also wind multi-turn coils through many ferrites (depending on their size and shape). This will increase the impedance (both R and X) at any frequency, but will also shift the peak in both R and X to a much lower frequency that is sort of proportional to the square of the number of turns. For example, the #43 mix peaks somewhere around 100 MHz for a single turn, but 8-10 turns will pull the peak down around 80 meters, and 14 turns or so will get it down in the AM broadcast band (you can easily do this with the 2.4" OD toroids commonly used for baluns. The limit of measurement for the MFJ-259B is 650 ohms, and for the CIA-HF is 1K ohms. Many multi-turn chokes will result in impedances well above this. Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
