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The Ol' String of Beads

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W2WDX, Sep 13, 2015.

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  1. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Hi all,

    So I was toying around with little 2m loop antenna I have a bunch of and seeing how well it works up on the balcony of my apartment. I have 2 of the ubiquitous 48" military fiberglass masts stuck into one those "concrete poured into a plastic bucket" arrangements. I'm using about 75' of LMR-400. This antenna is one of those .25" diameter solid rod dipoles folded into a square with a gamma match. The "ends" of the dipole are open and facing each other with about .8" gap left to tune the end capacitance.

    So I'm tuning the thing and I notice the SWR was acting a bit oddly across the bandwidth. This is an antenna I am very familar with, (Mind you I'm one of those who do not put a lot of worry over SWR as a tuning method. It just tells me what's going on over the coax and not really the antenna).

    At first I thought it was the railing on the balcony affecting the antenna, but moving around the antenna to different locations didn't make any difference. So then it dawned on me. Since there is nothing on this antenna except a gamma match, was I getting a false reading on my Bird from common mode currents on the coax shield? So I rummaged around the shack knowing I had a bunch of 2" long Fair Rite #43 ferrite beads with .400 inner diameter which could be squeezed over the LMR-400.

    After cutting a piece of coax and putting on about 8 of these and attaching the coax connectors I had a little String of Beads common mode choke with about 2400 ohms of choking impedance at 2m. That should be sufficient to not only remove any radiation pattern affects, but should also choke off any common mode current flowing on the coax. I went back in the shack and sure enough the SWR curve was now proper for this antenna.

    So that tells me one thing, SWR is generally only useful for seeing what's happening on the coax. It doesn't really have any meaning for how well the antenna is radiating. Nor does it create any significant losses, even at high ratios. Besides, RF is dumb. If you do have a high SWR on the coax, where does the reflected power go? It's gets radiated by the antenna, assuming the antenna is an efficient radiator in the first place. Or, as in this case, the coax shield.

    So I made a few and stuck them in various places on the line; after the amplifier, after the Bird meter, etc. Good thing is I stopped hearing myself coming over my crappy Chinese made computer speakers. And probably so did my neighbors. Hehe.

    73

    John, W2WDX
     
    Last edited: Sep 13, 2015
    KK4YWN, KJ4VTH and KC4JXH like this.
  2. KK4YWN

    KK4YWN Ham Member QRZ Page

    I have a "low noise" yagi for 2m. Since constructing it, I've always been a big fan of "low noise" yagis.

    But, I did not install the t80-0 toroid mentioned in the instructions. I had it, I just never installed it (I ordered 20 of them and they didn't arrive soon enough. I installed the antenna because waiting sucks).

    Last week I took the antenna down and installed the toroid. I'm so happy I did. I was happy with the antenna when I first put it up, but its a much quieter antenna now.

    Imagine what my old antenna was like.

    Yeah common mode currents just suck.
     

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