RF Suppression for Microphone Circuits (and more)

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RF Suppression for Microphone Circuits (and more)

RF Suppression for Microphone Circuits (and more)

  System Dependencies
  Minimum PowerSDR Version:     N/A 
  Applicable Hardware:     All transceivers 

Content provided by:  Jim, W4GB

If you are having trouble with transmitter RF energy getting into your microphone circuit, and causing audio distortion, please read on.

I have found some parts for RF suppression that are effective over ALL of the high-frequency amateur radio bands. These parts are equally effective on all cables; power, Firewire, powered speaker, etc., not just microphone cables.

The cores are made by the Fair-Rite Corporation, and utilize a new "mix" of lossy, suppressor ferrite materials designated as Type 31. The material is specified to be effective from 1 MHz up to 300 MHz. The two recommended parts are identical, except the "Snap-On" part is split into 2 parts lengthwise (split core), which allows more turns to be easily applied. Of course, the Snap-On part is more expensive, but the utilization is easier. And, in some cases, for instance when there is a connector fixed to the cable, it is well worth the extra cost.

I did a 7.169 MHz lab test on a similar part, F-R # 2631102002, and it had ~6 dB loss for RF on the OUTSIDE of the shield per pass through the inside hole due to the skin-effect. This skin-effect applies to the distribution of alternating currents on the outside surface of the shield whether on coaxial cable or a shielded pair.  The parts listed below are 50% longer, so, I estimate the loss may be ~8 dB per pass. Use 4-6 passes through the core, and gently secure the turns with a TY-Wrap.  Ferrite doesn't affect the RF signal on the inside of a shielded pair, it just attenuates the RF current on the outside. However, the ferrite will equally affect the signal on a NON-shielded wire, such as a DC power cord. Do Not put a core on a NON-shielded Internet DSL line for instance, as that is a very high frequency signal, and it will be choked. The Firewire cables ARE shielded, so a core can be used there.

In comparison, the commonly used Type 43 material is not specified for use on frequencies below 10 MHz. Unfortunately, the only readily available RF suppressor, at "Radio Shack", utilizes Type 43 material. It may have some beneficial effect on 7 MHZ, but it will be even less effective on 3.5 MHZ, and less again on 1850 KHz.

The recommended parts are as follows:
Fair-Rite Part Number
 O.D (in.) I.D. (in.)
Length (in.)
Solid Cylinder
 2.0  1.0  2.0
Split w/Plastic Case
 ~2.2  1.0  ~2.3

If you want to make an outstanding "choke" Balun for use at the feedpoint of your antenna (160M-40M) that is 5 to 10 times more reactive than anything on the commercial market for Amateurs, use a stack of 5 of the following part number, with 6 passes (only 5 external loops) of RG-8, RG-213, RG-8X, 9913, etc. through the core-stack as illustrated in his paper, A Ham's Guide to RFI, Ferrites, Baluns and Audio Interfacing  by K9YC, Jim Brown.

These chokes work very, very well, and they don't ever get hot. No more TVI or RF hot items in the shack (unless your antenna is really close by). It doesn't need to go in a box, just put it in series at the feedpoint. I just wind the last 5 feet or so, just before the antenna through the cores and secure the turns with black (outdoor use) Ty-Wraps.

Use F-R # 2631803802, 2.4 inch, type 31 core; $6 to $10 each, depending on where and how many you buy.

A list of dealers can be found on the Fair-Rite Corporation Authorized Dealers homepage and there are many.

Try the dealer nearest to you. They may not want to sell you small quantities, but it never hurts to ask.
The items come prepackaged in a special cutout cardboard shipping form as a "unit" that may hold 10 or 12 parts. If you want to purchase just a couple, try Amidon in California. However, the price will be full retail. These ferrite cores are not inexpensive, but then, neither is a gallon of gasoline, and both are necessary.

I bought my cores of various types from Kreger Components in Salem, VA, 540-389-7979, ask for Sherry. Other companies may have lower prices, or may be closer to your location.

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Last Modified:Wednesday, February 10, 2010
Last Modified By: Administrator
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