Balun and hairpin match

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WB5NEN, Jul 1, 2020.

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  1. WB5NEN

    WB5NEN Ham Member QRZ Page

    I’m getting ready to adjust my trusty 6-meter “squalo” omnidirectional antenna. I know the hairpin is a balanced feedpoint, but I’ve never used a Balun because, well, the antenna is omnidirectional. Any thoughts? Should I build up a Balun, or leave it without?
  2. KD0CAC

    KD0CAC Ham Member QRZ Page

    Never played with one of the Squalo antennas , is your factory build ?
    Never a.s.s.u.m. , but for the sake of discussion , I would think that most factory built antennas are complete - relative to having add devices to it ?
    My understanding is the hairpin is to adjust for narrow bandwith , high end for phone or low end for CW , generally speaking .

    As for the balun , what is your thought for the use ?

    Again my current view of baluns , for discussion we use the word balun , but there is much more to it , bal-un = balanced to unbalanced , un-un = unbalanced to unbalanced .
    Then there are current & voltage baluns .
    The ratio baluns 1:1 , 4:1 , 6:1 , 9:1 the most common = these are RATIO baluns to help move impedance of antenna that for some design reason are out of impedance range for transmitter / the 1:1 are choke baluns either voltage or current .

    Now for the guys that have more info ;)
  3. WB5NEN

    WB5NEN Ham Member QRZ Page

    Thanks. Think of a hairpin as a coil L to offset C in the antenna and provide a match. It’s kinda like an L network, as I understand. To clarify, I would use a 1:1 current Balun, or a designed coil of coax. But my thinking has antenna is omnidirectional, so I don’t need to worry about pattern distortion from using unbalanced coax to feed a balanced antenna. Or do I need to worry? What am I missing? That’s my question.
    Last edited: Jul 1, 2020
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    The highest and best use of a Common-Mode Choke (forget abt balun for the moment) is to block noise originating in your house/shack (computers, monitors,switching power supplies) from coupling to the coax shield in the shack and then being conducted as a common-mode current on the outside of the coax to excite the antenna, thereby creating a differential mode signal that comes back down the inside of the coax to get to your receiver. This is much more of a problem on 160 to 40m; less so at 6m, but it can help lower received noise level up to 2m... To accomplish this, the optimum CM choke placement could be at the feedpoint or sometimes closer to the shack...

    A CM choke and a 1:1 current-mode balun are one and the same..., so frequently one device can simultaneously improve SNR during receive and keep RF out of the shack/clean up the antenna pattern during transmit.\

    I have a squalo that employs a Gamma Match, so no balun is required to feed it. Post a picture of your squalo's feed system, and we can see if it is intrinsically balanced or unbalanced.
    Last edited: Jul 1, 2020
    K3RW likes this.
  5. WB5NEN

    WB5NEN Ham Member QRZ Page

    Yessir, it’s balanced. But it’s also up in the air right now...picture a challenge. My apologies. But it’s a standard, balanced hairpin match...elements are ungrounded ... “hairpin“ goes across the two elements to provide the adjustable shunt inductance. But, back to my original question. Do I need to use a Balun to feed it, remembering it’s an omni-directional antenna? I acknowledge the hairpin is a balanced feed. And my coax is unbalanced.
  6. WB5NEN

    WB5NEN Ham Member QRZ Page

    I think I have the answer. My situation is much like using a simple dipole, with or without a Balun. As WA7ARK pointed out, feedline radiation and/received noise could be a problem without a Balun. I’ve not worried about feedline radiation until now, but because my home brew 6-meter amp is about finished, I might experience problems I’ve not seen at 100 watts. I think I’ll build a Balun. Thanks to all.
  7. KD0CAC

    KD0CAC Ham Member QRZ Page

    I would ideally aways try to match bal - un .

    As for choke and common mode , now the above just starts another question in my mind to fill in , and or correct understanding .
    Always [ willing to add or change with good evidence ;) my thinking is common mode is created by some mismatch of antenna , feed-line etc. - putting current [ common mode ] on the outside of the coax shield [ skin affect ] and this attracts other RFI / EFI , and now becomes part of , mostly received signal ? and to a lesser degree the transmitted signal ?

    Coax chokes tend to be narrow banded - not an issue with 6m , but if your trying to have one antenna & use 6 - 9 + bands , a coax choke is problematic .
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Common mode RFI noise conducted up the coax shield during is independent from RF conducted down the coax shield during transmit. A good choke blocks both... During transmit, the source of CM current on the coax is unbalance at the antenna's feedpoint (e.g. dipole with no balun) or mutual induction (coax not routed symmetrically (orthogonal to) the antenna axis. During receive, the coupling from a RFI generator to the coax is just proximity (capacitance) .

    A ferrite choke as taught by G3TXQ (sk) is quite good for three octaves and still usable up to a decade of frequency. Good news is that you can easily series two; one centered say at 60m, and a second centered say at 15m to get good choking from 1.8MHz to 30MHz...
    K3RW likes this.
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    I left out a thought , that was , I was not of the mind that RFI can not get into the system - that is what good grounding is part of - more so than a choke ?
    Not that a choke wouldn't help , but each has a primary use / function choke - to stop CMD , ground for RFI .

    So my thought , not previously , that the common-mode acts like a magnet to RFI / EFI ?

    Kinda thinking like free-space antenna design - once an antenna designed [ in free-space ] then we have to deal with that antenna needing help in the environment " we " put it in ;(
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Blocking CM on coax in either direction is enhanced by grounding the coax between the station and the antenna. I look at it this way: Placing a CM choke in series with the coax puts a high series impedance in-line on the coax. Grounding the coax shield (as you would for static drain/lightning protection before the coax enters the house wall) shunts CM into the dirt. If you combine a high series impedance (>~1000 Ohms) with a low shunt impedance (<~50 Ohms), you have created a pretty good voltage divider (attenuator).

    No, I would say that RFI couples to the conductor that is the outer jacket of the coax. That induces a current onto the coax, which creates a current standing wave along the coax, just like the current standing wave along a dipole wire. When such a current flows along the outer part of coax it is called "common mode" to differentiate it from the current that flows inside the coax, which is called "differential mode".

    The advent of switch-mode power supplies has greatly changed the radio environment.
    KD0CAC likes this.

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