How many chokes do you need?
A ferrite bead, Snap-On bead, or toroid, (all RFI chokes), can be modeled as a series resistance and a reactance (capacitive or inductive). Likewise a wire or cable also has series resistance and reactance. The impedance (resistance + reactance) are frequency dependent. At the interfering frequency, the cable has a calculated impedance and this determines the interfering current in the cable. To reduce the effect of the interference we must reduce the interfering current by adding chokes which have their own impedance at the interfering frequency. So to achieve good RFI suppression, we need the choke impedance to be much higher than the cable impedance at the interfering frequency.
Example: If you are using the Snap-On beads for RFI suppression, let’s say on an ethernet cable with a series impedance, at 28 Mhz (the interference frequency), of 300 ohms, then if we can reduce the RF current by 6 db (one half), the interference radiated by the cable will also drop by 6 db. To accomplish this current reduction we must increase the series resistance to 600 ohms (or more) to reduce the current by 50%.
Solution #1: Looking up the impedance of a Palomar FSB-1/4 on the website or the catalog, we find the impedance to be roughly 170 ohms. So to drop the interfering current by 50% we could add 2 FSB-1/4 and the “new” impedance to the interfering current would be close to 300 + 320 = 620 ohms. The FSB-1/4 will clamp on over the Ethernet cable – see picture below.
Where to place the chokes? At 28 Mhz, one wavelength is roughly 35 feet or 420 inches. If the cable is less than 1/10 wavelength (3.5 feet or 42 inches), then one choke in the middle of the cable may be enough. A cable greater than 1/10 wavelength generally needs chokes at both end of the cable.
Solution #2: Instead of using a FSB-1/4 Snap on choke, use a FSB-1/2 Snap on choke and pass the Ethernet wire through twice thereby increasing the effective impedance 4X. The impedance of a FSB-1/4 choke at 28 mHz is about 160 ohms. Passing the cable through 2 times give an effective impedance of 640 ohms with only 1 choke. Passing the cable through 3 times gives 1440 ohms (9X) the impedance for even better current suppression (see picture below). Chokes may be needed at both ends depending on the electrical length of the cable as described in Solution#1.
Solution#3: Use 2.4” ferrite toroid with mix 43 and wind the cable through 7 turns to get an impedance over 1,000 ohms (measured data). Each time the cable goes through the center of the toroid counts as one turn. Depending on the size of your cable, the larger inner diameter toroid may be the most effective. (see picture below)
It is quite possible that additional chokes may be needed to increase the series impedance enough to reduce the interfering frequency current. In addition the power supply lines that supply DC/AC power to the unit will also need to be “choked” as the negative, or ground “return’ is also part of the radiating/receiving antenna in this system. A good rule of thumb is that it usually takes 1000-5000 ohms of choke impedance to make a difference in RFI suppression. Higher values over 5,000 also help in reducing induced common mode noise reduction from external noise generators thus reducing the noise levels in your receiver. Remember choke impedance varies with frequency.
See typical choke installation example pictures below.
For your convenience, Pre-configured RFI kits are available for various electronic devices which are susceptible to RFI or generate RFI that effect other devices – see RFI Kits on the website at www.Palomar-Engineers.com