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Base Using choke on A99 and clones? Effects?

Discussion in 'CB Antennas' started by darklife, Jun 19, 2020.

  1. darklife

    darklife Modulating Madness

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    I got to wondering if anyone has tried the A99 or IMAX2000 or really any variation of these end-fed dipole antennas and if a choke at the end helped or not?

    Also for those who tried using them with or without did it affect your tuning and where SWR is lowest?

    I got to thinking how a choke might affect the resonant frequency of end-feds since part of the coax acts as the return current because of Kirchhoff's law. Putting the choke 0.5 (~ 2' from end of antenna for 27MHz) wavelength is recommended by quite a few websites, but many people seem to be using these antennas without any choke at all with good results.



    Wondering if not using a choke could make the antenna element look longer to the transmitter because part of the radiation becomes the coax which adds to the overall length? Unless I am misunderstanding this would make the antenna look more like an off center fed dipole rather than end fed which surely would screw up the performance. I might be overthinking this though as I am using mine without a choke and it's satisfactory.
     

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Can Kirchhoff's law be applied to RF? ( I really don't know, not being sarcastic) I have heard of people winding a coax choke nine feet below the feed point. I have also heard of people isolating the base of the antenna from the mast with a piece of PVC. I don't know if people do these things to improve performance or to try and eliminate a problem. Tuned in to hear what people have to say about this.
     
  3. groundwire

    groundwire Well-Known Member

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    A choke at the feed point on a vertical will help eliminate common mode current and nothing more. Not a bad thing
    .
     
    SIX-SHOOTER likes this.
  4. M0GVZ

    M0GVZ Sr. Member

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    Yes Kirchoffs Law applies. RF is nothing more than alternating current at a specific frequency which is fed into an antenna that radiates a charge.

    Going back to the first post...

    First of all...

    The Imax 2000 and A99 are not end fed dipole antennas, they're an end-fed antenna. The only end-fed dipole I can think of in CB land are the Gainmasters. ALL end fed antennas require a RF ground. They all require a RF choke to ensure that the feedline doesn't form part of the antenna.

    The choke only would affect the resonant frequency of the antenna if the coax feeding it had been forming part of the RF ground. So if you use a choke or coil up your coax or alter the length or even say put in a linear and use a patch lead and that alters the SWR then that's your antenna system telling you you've an insufficient RF ground and your coax is being used to compensate which is a bad thing.

    People use them without a choke and say they get good performance because they've either been lucky with the combination of mast and coax length they've used or they have very low expectations and don't realise that things like the noise level they have and any interference they cause to their TV or things like neighbour's burglar alarms are because of common mode RFI caused by the stuff mentioned above.

    A bit of light reading....

    https://www.w8ji.com/end-fed_vertical.htm#IMAX
     
  5. darklife

    darklife Modulating Madness

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    From what I understand it's still a dipole antenna, just fed at the voltage node where impedance is in the kiloohms instead of the centered current node where its around 70 ohms, which is why it requires the transformation. If it weren't a dipole then hams wouldn't be calling the extremes like off centered dipoles a dipole, even though the impedance rises greatly toward one leg of feeding. Maybe this is a matter of semantics though. Either way it's an end fed half wave antenna, and traditionally a dipole is called a half wave antenna. The EFHW is just the extreme example of an off center feed where impedance rises to 1500-4500k ohms.
     
    Shadetree Mechanic likes this.
  6. M0GVZ

    M0GVZ Sr. Member

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    It isn't. It's a ground plane end-fed antenna. Stop thinking of it that way as you're just confusing yourself.

    No it isn't, it's called a dipole. "Dipole" refers to an antenna fed in the centre with two identical conductive elements (hence the reason that an A99 on a mast isn't a dipole because it doesn't have two identical conductive elements), nothing about the length. Now you can have a half wave dipole which signifies that it's total length is one half wavelength long but equally and without any matching circuitry needed you could also have a one and a half wavelength dipole where each leg is a 3/4 wavelength.

    Whilst the semantics may seem pointless nit-picking to you, they're actually what prevents you from getting confusion over how something functions because it stops you making false assumptions such as thinking that an Antron A99 on an 18ft mast works like a dipole does.
     
    #6 M0GVZ, Jun 21, 2020
    Last edited: Jun 21, 2020
  7. secret squirrel

    secret squirrel Lustrous Potentate

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    Ok, no science at all here. I am just speaking from experience. My first base antenna was a old Radio Shack 1/4 wave GP. “The big metal jack” in the yard as my ex-wife called it. It was up at 15 feet. I did get a lot of noise. Antenna number 2 was a A99 with the Firestick “Fire Up” top section. Sparky sold it to me. It’s 30 feet up in a pine tree. I had the extra 18 foot section of RG 8x on hand so I made a open air choke and attached it before we put everything on the tree. My logic was the open air choke was cheap to built. I had a tree climber mount everything. If it helped. Then good. No harm done. The A99 was not noisy at all. SWR pretty flat 1.4 to 1.3 across all 40.
    Like I said no basis for what I did. Just tried it figured if it help then I did everything right first time.
     
  8. Highway Man

    Highway Man Well-Known Member

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    So would a A99 be considered a colinear antenna ?
     

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