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Thread: velocity factor calculations

  1. #21
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    This is fun for me butcher , it sorts the bs from the "real people" . Your info is not lost on me and I have used it before ! The most interesting point is so many dont know and try to bullshit their knowledge hehehe , Yes i will proberly replace the elements with some 1" heliax etc I have stripped down the whip and thought it may be possible to change the v/f of the sections , The maths say it is possible by changing the vf but very awkward to achieve what i want and then more work than worth it , I may try later just for kicks to see where it works out , nothing to lose ? thanks for the reply Don
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    Don you might need to buy an "ACME pole stretcher". There is plenty on ebay.

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    playing with one section of the antenna going to fill it with foam and see what happens my cals/ guess is about 2.8mhz we will see , what do you reckon ??? don
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    Velocity factor is the speed of a wave traveling through a medium, as a percentage of the speed of a wave in a vacuum (not air). Just saying.

    Also, coax doesn’t change the speed of light, it changes the speed of the wave traveling through it.




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    yep thats what we said thats why air is 0.96?vf
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    Quote Originally Posted by The Butcher View Post
    And as stated by another op, coax (any feed-line) slows down C (speed of light which travels at roughly the same speed as radio waves), hence the conversion factors. This is directly related to Da due to atoms bucket-brigading electrons (simply put).

    0.96(X) is 4% less and 0.85(X) is 15% less. And yes, air is 1(X), or just X, to simplify. This (air) is what is used as reference. To make issues harder, air changes with pressure (Pa) and temp (etc).
    The Butcher seems to be a little confused on this issue.


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    nah southern hemisphere we think upside down he said 0.96 x = x is air not vacuum cos whos gunna build a coax in a vacuum ? lol
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    Default velocity factor calculations

    Sorry, nope. Air is not a vacuum. The speed of a wave in a vacuum will always be the same, and a constant is what you need to use as a reference. Just “air” means a lot of different things.




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    yep the 0.96 is a pretty nominal figure as some time they have a spiral insulator , sometimes a plastic disk all sorts so we regard it as ""about right "" on large transmission coax there is only insulators on the corners and maybe one or two an the length but them we are talking 3"-4" plumbing in these cases the v/f can be as high as 0.99 but thats unusual they can also be pressurised with nitrogen or argon which upsets the v/f as well there you go some useless info lol
    Last edited by hinekadon; 16-05-19 at 03:21 PM. Reason: spelling
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    have a play around on ytube they had a bunch of hams that went thru the old am radio station 5000kw i think cant remember but they had some big coax there ???
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    "The Butcher seems to be a little confused on this issue."

    Nope, just lazy. I did not want to quote formulae nor working examples as it is all published.


    To quote myself...

    "pps. Get a hold of some ARRL books and stick ya head in it real deep with calculator in hand!!!." -Very useful.

    "To make issues harder, air changes with pressure (Pa) and temp (etc)."

    That is why the formulae needs to use vacuum. To remove said factors.

    But yea, in a vacuum is what it refers to. Thanks Mr. Wheel43. I seem to recall having a discussion with you a few months back...


    "Also, coax doesn’t change the speed of light, it changes the speed of the wave traveling through it."

    Correct. C is a constant...






    Filling with foam IS an interesting concept. Give it a go. Foam may contain a fair bit of air (vacuum or whatever). One can use a GDO. By tuning in an Rx to the oscillator in the GDO and find the carrier, one can work out the exact resonant f. Or may be an ant analyser could be used? I have seen them in use.

    One could use deionised water. That would be trick.

    And collinear aerials are very broad band anyhow (-3dB points) and SWR below 2:1 is acceptable (bugger-all loss).


    Consider building a coaxial collinear from scratch. They work great. Slip into an old fishing rod or attach to a fibreglass rod or similar. Doing so lowers the resonant f by a small percentage. May be this could be another idea or two to try?

    The ones I have built work great! 1/4L above a GP feeding 4 1/2L (2, 4 or 8. More = lower angle of radiation). then a 1/4L on top. I can provide links if required. But google is your friend!

    Feedline impedance is to do with the ratio of spacing between both conductors. Closer they are together the lower the Z.


    And C can vary too (so they say). But that is not a topic for this thread!

    Last edited by The Butcher; 16-05-19 at 07:54 PM.

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    Rite on the butcher > I have a rigexpert aa1000 that i use , the interesting point is that the stationmaster I have just dis-assembled has a 5/8 on top not a 1/4w so some investigation into this as well but its raining here so may be a while before i try the foam trick , I live alone with a house keeper once a week she would most likely skin me if i did it in the lounge Hahaha
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    1/2L + 1/4L = 3/4L

    5/8L + 1/8L = 3/4L <=== This is because 5/8L + 1/8L = 6/8L, which is 3/4L


    Point is, a 1/2 wave end fed and a 5/8L end fed are actually both 3/4L. 1/2L uses a 1/4L loading coil and a 5/8L uses a 1/8L loading coil. Both are 50Z at the feed point then.

    It is interesting seeing the extra bit on top of the 1/2L section though. Some plans don't use it.

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    I hadnt thought of it in those terms yes that explains the 5/8 on top , this will give a slight down tilt to the radiated wave . To bloody cold { frost} outside at the moment to do any thing but look at it ?
    thanks for the thought
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    "this will give a slight down tilt to the radiated wave"

    Interesting... Good we trick that... Most of the radiation occurs at the lowest elements though. Bit like a tube with a bunch of holes in it with water forced up it. Most of the water escapes from the base. Bit of a bad analogy, but it is late. I guess being a resonant length away from the lower elements, it would offer lower absolute angle of radiation?

    Yup we had a frost here in NZ yesterday. Will kill of bugs, I guess...
    Last edited by The Butcher; 18-05-19 at 10:26 PM.

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    ok managed to lay my hands on some likely looking coax but no labels on it so who can identify it
    overall outside =0.8525 or 21.64 mm
    inside outer sheath =0.61" or 15.60 mm
    inner diameter =0.1730 or 4.44 mm
    its spiral poly insulated
    aliminium solid sheath alkathene covered
    solid inner of copper
    Got enough to make new guts for the whip but dont know the v/f of it
    ,came of a large transmitter ( may even be 75 ohms ) cant find it on google
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    Sounds like sat-tv coax. This is good. 75 ohm? Low-loss, I bet.

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    yep cant find rg# for it or anything to sort it the cals say its 75 ohms
    just a bit worried about the copper to ali joints tho whats your thoughts ?
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    Copper ==> Tin / Silver ===> Alluminium?

    Use a solder tag.

    Galvanic reaction etc. There are charts for this on the net / in books.


    Impedance can be calculated by the distance from the inner to the braid. You are on your own with this! But 75Z uses a greater dielectric distance compared with 50Z.

    If she is solid copper inner, you might be on to a winner? (50Z?) Cable / sat stuff I have seen uses copper coated steel or aluminium for the inner.


    Strange how there is NO numbers on it?


    "From a large tx". It must be 50Z then...


    Google how to determine v-factor of coax google. Lots of stuff there.




    I read somewhere that shitty old RG58 can be used to no detriment. No need for expensive coax. Use it for your feedline on VHF/UHF, if you can get the plugs...


    For reference...

    Last edited by The Butcher; 24-05-19 at 02:15 PM.

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