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RF Chokes


<< My present hottest one [choke] was on func gen to CB mike input coax. So I added a couple more to that cable - now have 3 on the 18" RG59. >>

Would not a choke on this line interfere with the leading edge of the square wave as the function of a choke is to impede high frequencies (which is what the leading edge of the wave actually is - no?)?

Regards -David F-


I use 90 degree connectors and there are no problems with regard to the devices effectiveness. 



>In a bare-rife generator, the square waves produce harmonics in the >range of interest (for me) of 70K to 1.2M (based on Clark's findings of >pathogen MORs.) I assume that there are no harmonics produced by the >27MHz. Are you saying that there is no appreciable loss around a 90 >deg bend of the 27MHz with the (up to 1.2M) harmonics?

The harmonics you are talking abt. are in the modulation, the RF freq is fixed and tuned by the tuner. The 90 degree bend is fine as long as you don't crimp the coax. A few inch radius is fine as long as you don't distort the coax which isn't a problem with RG-8 type. Does your tuner have a dummy load? If so you can select it and watch the power while bending the coax, you'll see no changes.


Tom McCoy KN6KK Symation Technologies


D><< My present hottest one [choke] was on func gen to CB mike input coax. So D>added a couple more to that cable - now have 3 on the 18" RG59. >>

D>Would not a choke on this line interfere with the leading edge of the square D>wave as the function of a choke is to impeed high frequencies (which is what D>the leading edge of the wave actually is - no?)?

No, a choke around the power leads or the mike cable will stop Common Mode currents, that is, any signal traveling in the same direction on BOTH wires at the same time. If the signal goes the same way on both wires, the magnetic field of each wire adds together, and the choke stops the signal. If the signal travels in OPPOSITE directions, as it will for the audio signal in the mike line, or for the power leads, the magnetic field of each wire is equal to the other, but opposite in phase, so they cancel each other out. The choke, seeing no magnetic field from the desired signal, simply ignores it and lets it go through the choke.



Remember that current moving in a wire has a magnetic field oriented around its perifery. When the current comes to a corner you can picture the magnetic fields involved as being squished on the inside corner like an accordion. The magnetic field tends to resist that turn. At high currents that effect is more pronounced. Keep in mind that a 90 degree turn is essentially one quarter of a loop. Thus it is a measurable inductor with a measurable inductance value.

When frequency is introduced into the equation, inductance plays a role in terms of resonant frequencies (the natural frequency at which a circuit will "ring" electrically). It could be that the amount of inductance you were introducing to the circuit was bringing your overall setup closer to a resonant frequency for a given area on the frequency spectrum.

Imagine, a smart sensing circuit could watch current in front of the tube and feed back to a mechanism that would vary an inductor as frequency is varied to maximize the current and thus overall resonance at any given frequency. I believe that maximum resonance means maximum efficiency, but I'm not that familiar with the complexities of all of the components of this setup.

Gary Hawkins 


In this case since square waves are desired, the less inductance (and the less resistance) the more purely the square wave can be preserved.

A choke is another term for an inductor. It's effect on a square wave is to round it out toward that of a sine wave. On the other hand resonance for a given frequency loves a choke/inductor of the right value when speaking of sine waves, where capacitance is also involved. 

A choke of the right value when it comes to resonance would not really represent a loss (except for its resistance value) but instead an alternating conversion of current to magnetic field and back to current again in the opposite direction. Inductors likely began to be called chokes in their use for limiting stray high frequencies for the relatively low frequency of 60 Hz in power supplies. Compared to the frequencies they are impeding, the 60 Hz is virtually standing still and has no problem moving through the choke.

Why that? Switching hats from presumed teacher to student I would love to understand better in slow motion exactly why high frequency is impeded by a choke. Long coils of fine wire are used between the supply to a Tesla coil and the coil itself (and capacitors) to keep the high frequency (and high voltage) from making its way back into the supply line, which would arc across gaps that were otherwise able to insulate against a mere 220 volts.

Gary Hawkins


<< No, a choke around the power leads or the mike cable will stop Common Mode currents, that is, any signal traveling in the same direction on BOTH wires at the same time. >>

In your directions for the uniden 510 you specify that ferite beads should be used on the signal carrying wire to the CB.....(therefore they would not be present not on the ground/return wire).... since the beads are not covering "both" wires, how do they impede RF without impeding the signal?

Regards -David F-


MFJ sells an RFI choke kit (MFJ-701) which is a package of 4 that's supposed to be good From .5 to 200Mhz...$14.95... I haven't tried but I'm gonna order...snap-together halves..

Request their latest catalog... http://www.mfjenterprises.com 


> a type or model or part number or a mfg. or all of the above

They only sell one snap-fit at typical Radio Shacks as far as I know. It is smallish and fits snug on a 120V power cord. It is a snap-open box which holds a 1.5" long cylinder through which the cord runs. This one is rated at (I think) 2MHz, so it would likely be better to find a source for a 25-30MHz filter, although I have used a couple of the 2MHz ones and they seem to work fine. At a large Radio Shack, I found another type which is meant to have the cord wrapped through it multiple times, and is a picture-frame-looking thing - it is rated on the back as to freq but I did not notice. I suppose this type is best for thin lines like the CB, but I just use the snap-type and wrap around it a couple of times with the thin wires.

Other chokes I have bought are larger and rated correctly, at Ack Radio Supply and Tech America. Although I have gotten them at Tech America, I don't see any in their catalog at the moment.


Yes, the toroids work better because there is no break in the toroid as in the clam-shell type. It is easier to use the clam-shell, but...

When I have had a severe case of RFI (Radio Frequency Interference), I have gone into the equipment and disconnected the AC line, wrapped each side of the AC line in a toroid, and reconnected the power lines. While I was in there, I also connected some .01uF ceramic caps from each side of the line to circuit ground. If I really felt paranoid, I connected two 175V MOVs (spike suppressors) across the caps. I did this for the TV in my ham shack...use it to show neighbors that my transmitter is NOT causing problems with their TV reception. :):)


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