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Thread: COAX LENGTH...Yes this topic again

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    COAX LENGTH...Yes this topic again

    So…I have a question for the gurus here…and yes, it’s the always asked question…What length should I cut my coax? And yes again…I’ve been reading and researching. But I still have questions…remember, I’m just a tobacco spitting M.S. Degree in Biology holding redneck…so don’t criticize me for not reading for my answer somewhere else here on the forum! Lol

    I just ran 200ft of RG213 (VF = 0.66) from my radio shack in the basement out to my dipole in the oak tree. 200ft was at least 30-50ft more than what I needed. So…the extra RG213 that’s lying on the ground all laid out before heading up into the tree, I’m planning to whack it and create myself some jumpers for in the shack.

    In thinking about “tuning” the coax and cutting it to the correct lengths…I’m looking at the ½ wavelength formula 492 x (Velocity Factor) / Frequency (MHz)

    My question is what frequency (MHz) to plug into the formula?

    I operate on 26.735 AM in talking local with my dad and his ol geezer buddies…then I operate on 27.355 – 27.555 LSB. So that’s quite the spread on frequencies. Do I just go with say 27.145 MHz as the freq to use in the formula since by my math is the “halfway” point between 26.735 and 27.145?

    My Math:

    27.555 – 26.735 = .820 / 2 = .410

    26.735 + .410 = 27.145 MHz

    27.555 - .410 = 27.145 MHz

    ½ wave length = 492 x .66 / 27.145 MHz = 11.96 so Round up to 12ft to make it easier

    Then I’ll use 12ft jumpers from radio, amp and meter and the feed coax cut it to whatever interval of 12ft works out for the length that I need from shack to the antenna.

    Does this all sound okey dokey or do the gurus have suggestions / comments from the peanut gallery to share?

    Sent from my motorola one 5G UW ace using Tapatalk

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    whiteastro (06-19-2022)

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    My experience has been that any length of coax that is at least two wavelengths long, it can be terminated at any length. When working with a living antenna support, leave some extra length to allow for growth. My experience here is that a tall tree, 50 feet or more will grow an average of 6 inches a year. My last install in a tree had 20 foot extra but after 20 years you can see the slack is taken up.

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    whiteastro (06-19-2022)

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    cheap vswr meters can be skewed by coax length . good meters aren't .
    use an electrical half wavelength if you're concerned about using a cheap meter .
    an antenna analyzer is the most accurate route .

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    1/4 wavelength of coax and multiples there of will act as a transformer, ie: when terminated with a known impedance, It presents at its input approximately double the impedance with which it is terminated. 1/2 wavelength of coax and multiples there of will show the terminated impedance on the input equally. ie: it will show the actual feed point impedance of the antenna (approximately) and not transform it

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    Quote Originally Posted by BOOTY MONSTER View Post
    cheap vswr meters can be skewed by coax length . good meters aren't .
    use an electrical half wavelength if you're concerned about using a cheap meter .
    an antenna analyzer is the most accurate route .
    that makes no sense at all, and is inaccurate.

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    Yesterday evening I installed a radiowavz double bazooka dipole...tuned the antenna length to the following SWR readings on my AA55...


    Sent from my motorola one 5G UW ace using Tapatalk

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    I take it your just using what is needed to run from the antenna back to the radio since you said you were going to use the left-overs for jumpers...so if that length didn't mater to you...why should jumper lengths mater?...

    You were on the right track at first...use whatever it takes to get from A to B...or you can listen to the so-called experts and waist allot of time and brain cells trying to come up with some mathematical theory on how long the jumpers should be...

    In the end...which ever way you go, you won't notice any difference and nether will anyone else on the receiving end...

    have fun.

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    whiteastro (06-19-2022)

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