Using crossover coils as RF chokes

Discussion in 'Amateur Radio' started by W5HRO, Aug 25, 2017.

  1. W5HRO

    W5HRO Administrator

    Has anyone ever tried using big crossover coils used for speaker building for RF chokes instead? It dawned on me many of them are exactly 2.5mH. I came across the ones below and they are 2.5mH using AWG18 wire and they would sure handle the current. They are air wound and I don't really see any reason why they wouldn’t work in RF circuits too. Of course, they are not pi-wound, but I don’t think it would really matter and their self-resonate point should still be well below 1.8MHz. You could make them pi-wound though by using 3 of them slipped over a wooden dowel and connecting them in series for 7.5mH total.

    These things are only $11 each and they also have a bigger one with AWG14 wire that handles 750W for $28.

    2.5mH with AWG18 wire

    I think these would work really well for the 2nd choke in series with the cold-end of plate chokes in big linears, at the output connections in antenna matchboxes, and as corona discharge drains at the base of vertical antennas to ground etc. The only issue I can see might be HV breakdown if too high of a plate voltage is used, but they should have no issues for the other 2 applications.

    2.5mH with AWG14 wire
  2. WD5JKO

    WD5JKO Member

    Those coils might be good for a PWM modulator where a filter at the PWM frequency is needed.

    The coil turn to turn capacitance will be a bunch, not good for HF RF.

    A inductor above it's self resonance frequency behaves more like a capacitor
    A capacitor above its self resonant frequency behaves more like an inductor
  3. W5HRO

    W5HRO Administrator

    Well, what I'm looking at is will they effect HF RF if placed across the input or output to ground?

    I looked around the web and couldn't find any information where people have even tried it or why they won't work if they have. If you take a normal 2.5mH RF choke and place it across an input or output it's invisible to the RF all the way down to 160-meters. Will these coils act the same way? That's basically what I was trying to find out. I did order one to play around with.

    And/or what if you were to take 3 of them connected in series on a wooded dowel to make it like a pi-wound coil instead?
  4. W5HRO

    W5HRO Administrator


    What I'm going to do is play around with the coil when I get it by installing alligator clips on each end and clip it to the output of my Alinco first and see if it attenuates the receiver. Then I will place it on the input, then the output of my little MFJ antenna tuner and see if it effects the adjust in anyway with like 50W going through. If they are capacitive then I should be able to see it. All I'm really trying to find out is can they be used for protection and corona discharge drains. They also make 0.82mH and 0.91mH ones with AWG14 where you could use 3 of them in series on a wooden dowel or ceramic rod to make them a pi-wound at 2.46mH or 2.73mH. They also make them in 1.0mH too. The tolerance is 5% so the 0.91mH ones would work best to make sure they stay above 2.5mH or equal to it.

    My reason personally for wanting to use them if possible is mainly for the output and input of my big home-brew antenna tuner and then at the base of the Zero-Five vertical ground plane. I'm not putting up a tall vertical without a hefty and constant corona discharge drain going to ground at the base. P-static and of course man-made noise can raise your receive noise floor by as much as 40db to 50db at times during extreme conditions.
  5. W5HRO

    W5HRO Administrator

  6. W5HRO

    W5HRO Administrator

    Received the 2.5mH crossover could today. When I get the chance I will try it and then publish my results. Regardless of the results I still say three of the 0.91mH coils in series on a rod would work like gangbusters.