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Feedline Ferrites: what noise do they suppress and _not_ suppress?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI6EQ, Feb 1, 2017.

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  1. AI6EQ

    AI6EQ Ham Member QRZ Page

    This is basically a question about "signal processing". But since a potential solution comes to mind involving ferrites on a radio's transmission line, I am posting the question here in this "Antennas, Feedlines, Towers & Rotors" forum.

    From comments I've read about digital signal processing (DSP) and software-defined radio, I imagine an excellent, easily adaptable, general solution might be DSP in a digital processor interposed between the feedline and the radio (especially the radio as receiver and especially for doing DX communications). But that approach seems likely to be fairly costly and may perhaps involve more rigmarole than necessary.

    So the legacy, analog approach to signal processing of adding ferrites (ferrite beads and/or ferrite toroids) to the feedline near the radio comes to mind as potential solution for all noise problems arriving over the feedline.

    This question applies to both HF and VHF reception. I am planning installation of an HF antenna system ("fan" inverted V for 40m/20m), and so this question is applicable to that system as well as my existing VHF/UHF system.

    Basically, the question comes down to this: Can ferrites on the receiving feedline suppress "scratchy" noise that is transmitted by a remote station as part of the transmitted signal?

    The main source of my question comes from the fact that I received a fairly noisy ("scratchy") yet still readable signal via listening to a cross-band repeater on a 70cm frequency but when I switched to listen on the 2m frequency that the originating station was transmitting on, his signal I heard was very clear and noise-free, and moreover, I received other stations very clearly via listening to the cross-band repeater's 70cm output.

    So it seems the noise was not originating in the cross-band repeater's transmission or from extraneous sources after that transmission but was originating from the cross-band repeater's signal as received, perhaps with noise mixed in from the repeater's receiver itself. What was the exact originating source(s) of that noise is not clear, but it seems that the noise I heard was being transmitted as part of the repeater's outgoing signal. And I'm guessing that the noise could be characterized as "noise-modulated RF", but I'm not sure what that really means in the case of this kind of noise. ( In other words, I'm wondering if "noise-modulated RF" is really a well-defined concept when the noise may have a very broad spectrum.)

    The character of the "scratchy" noise did not sound like the "soft", pure white-noise "hiss" like one might hear in an audio system amplifier with the volume turned up. But rather the noise sounded more harshly like hot, frying bacon with perhaps some moisture in the mix -- i.e., so that the noise tends to have a "crackling" or "popping" component. So I image the noise spectrum will look very different in these two cases (pure hiss or frying bacon).

    Now this question is also relevant to the HF antenna system I am planning. I have a vintage Yaesu FT-767GX transceiver which I have used to receive SSB on 40m via my Comet CX-333 tri-band VHF/UHF antenna. I can hear SSB stations, but I generally have to turn down the noise squelch all the way in order to hear those stations. So I get an awful lot of received noise as a result. If I recall correctly, it also sounded a bit like frying bacon.

    I am hoping the noise will be greatly reduced when I get my HF antenna system set up.

    So I am wondering just how much noise can be suppressed with ferrites in the new HF set-up. Will there still be noise that cannot be eliminated using ferrites?

    Would using a microcontroller package (such as, say, Raspberry Pi with appropriate DSP open-source software) as a front-end to the receiver be needed to substantially remove a lot of the noise?
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    If the signal is transmitted by the distant station it is part of the station's modulated signal. IOW, nothing on the receive end can strip that off the raw RF signal.

    Sure things can be done in IF and baseband to limit bandwidth and even to do some pretty creative DSP stuff like removing noise based on degree of correlation. But no antenna or feed line choking or other similar system can remove information that is actually modulated onto the carrier at the transmitting side of the system. Once modulated on to the transmission signal it IS part of the information being transmitted even if it is not information we would care to receive.
    KD6RF likes this.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Ferrite chokes cannot preferentially block "noise" over desired signals. They block all signals within the band which they are effective, controlled by their 'mix', number of turns of the coax thru the choke.

    Coax is designed to have our RF signal carried on the center conductor and the INSIDE of the coax shield. Depending on various factors signals may be carried on the OUTSIDE of the shield (particularly if the antenna is unbalanced). Signals carried on the outside of the shield are said to be "Common Mode" transmissions because the shield represents our 'common conductor" (or ground) our radio circuits.

    The common mode signals on the outside of the shield can mix with the desired signals and increase our total noise level. It can also pickup local noise like electrical and electronic hash.

    We can use external chokes or 1:1 isolators or BalUn (Balanced to Unbalanced) transformers to reduce common mode signals. None of this will change or reduce undesired signals (noise) in the RF spectrum coming from our antenna.
    KD6RF likes this.
  4. KD6RF

    KD6RF Premium Subscriber QRZ Page

    In general, coax-fed doublet or end-fed antennas w/o baluns or chokes at the feedpoint will have current flow common-mode on the shield of the coax feedline - the extent is determined by the details. (Even balanced antennas with balanced line can have significant common-mode current flow if there is less than perfect symmetry).

    It's no more complicated in concept than noting that common-mode current flow on the coax shield means that the coax shield is not just a feedline any more, it's now a significant portion of the antenna.

    So, for TX we worry about RFI and bitey-mic RF in the shack and so on.

    For RX, every thing that can be picked up by an antenna IS picked up by the coax shield - because the coax (with common-mode current flowing) now IS an antenna. Everything from desired signals to lightning to near-field lighting to wall-wart switching supply noise can be picked up.

    Best practice is to effectively choke at the feed-point first, and worry later about a choke near the radio if need be. A choke at the radio (in the absence of a feedpoint choke) may help in cases of RFI and bitey-mic, but doesn't prevent the rest of the coax shield from picking up noise from every device it comes near - lighting, plasma TV, switching supplies etc...

    We really can't predict how many of these things are a problem at your installation as the extent is implementation dependent.

    The effectiveness of chokes is always of concern - Steve G3TXQ has a good article on the subject ===>

    K2XT and AK5B like this.
  5. AI6KX

    AI6KX Ham Member QRZ Page

    If I had a buck for every suggestion that the problem is common-mode feedline pickup, I would buy myself a decent noise-cancelling gadget. If you have a battery-powered shortwave receiver, take it outside and see whether the noise is just something very local or generally around in the neighborhood. If it's the latter, it's not a common-mode issue and no chokes are going to help, because the noise will be coming down the feedline just like the other signals. I have a S9 noise level on 40 meters 24/7 and have installed properly designed chokes at both end of the feedline - no help at all. (There was no noticeable change either when I switched from a horizontal dipole to a vertical at the same height). All I can do is narrow the receive bandwidth as much as I can.

    Good luck - I hope you find your noise source.

    KD6RF likes this.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Agreed !

    I have used dipoles on all HF bands for over 30 years, and never used any kind of Baluns, just coax straight to the antenna wires with no problems.

    Last year I installed Ferrite Baluns at the feedpoint of my 160m dipole and my Multiband trap dipole . . . made no difference whatsover !!

    Roger G3YRO
  7. AI6KX

    AI6KX Ham Member QRZ Page

    And now on a piece of paper that my trusted assistant cannot see, I have written a list of five responses to your post, Roger. At the end of the act, she will reveal them to the audience and we will see if I got them right.
    K2XT likes this.
  8. KF6A

    KF6A Ham Member QRZ Page

    That settles it then. Ferrite does nothing at all.
    KA0HCP and K2XT like this.
  9. KD6RF

    KD6RF Premium Subscriber QRZ Page

    Well, I'm going t go out on a limb and suggest that you probably have no way of knowing whether or not that's true :)

    One of the take-aways from all the stuff written above is that noise ingress is kinda sneaky, and there's no way of knowing whether or not you've had "no problems" if you haven't tried the ferrite test or the walk-around test or the let's-turn-off-all-the-breakers test :eek:

    Unlike RF-in-the-shack and Bitey-Mic problems, noise ingress (and efficiency loss) is not apparent unless you have something to compare to. There's no way of knowing whether your S5 noise level really should be S2 if only you had proper balance and isolation/choking!

    (My profuse applogy if in fact you did do those tests and in fact really did have no problem :) :) :) )
  10. W1VT

    W1VT Ham Member QRZ Page

    I've used A/B testing with dipoles a couple feet apart from each other to see the need for a balun. The dipole without a balun would pick up an intermittent noise, while the one with a balun didn't notice the noise. When I installed a balun with a foot of vertically oriented coax between the balun and feedpoint, the intermittent noise was significantly reduced, but still easily detectable. Needless to say I made another balun with a much shorter section of coax to the feedpoint, eliminating the noise issue. Reducing the noise an S-unit or two makes a huge difference when trying to work weak stations. And, if you can hear well, there are a lot more weak stations than strong ones that can be worked.
    Last edited: Feb 1, 2017

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