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Author Topic: 468/f-mhz or 472/f-mhz? Which is better  (Read 3408 times)
KT0DD
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Posts: 278




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« on: June 04, 2013, 03:54:05 PM »

I have heard people use several different variances of the 468/f formula and I'm wondering which best represents physical length for antenna building. Someone told me that 472 works better?

I'm building a multiband ladder line fed doublet for field day and I'd like to cut my wires as close as possible the first time to minimize trips up and down for tuning the antenna. I plan to be 40' in the air with 20 ft extra to reach the tent. I will be using 440u 14 guage stranded ladder line and 12 guage stranded and coated antenna wire, both from the Wireman.

Any info is appreciated.

73.  Todd - KT0DD
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K1WJ
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Posts: 458




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« Reply #1 on: June 04, 2013, 04:06:00 PM »

I always use 468/F, example if voice on 20m = 14.150 to 14.350. So I would calculate the formula using 14.150 so I would tend to be on the long side, & make a few cuts to get to lets say 14.250. If inverted vs horizonal may be 3%+- shorter. I have never cut a dipole & been way off swr wise. If using ladder line a tuner is involved - no biggie. 73 K1WJ David
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WB6BYU
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Posts: 13341




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« Reply #2 on: June 04, 2013, 04:36:59 PM »

Quote from: KT0DD

I'm building a multiband ladder line fed doublet for field day and I'd like to cut my wires as close as possible the first time to minimize trips up and down for tuning the antenna...



[My bold]

I think you are missing the point here.

If you are using ladder line feed with a tuner in the shack, the exact length of the antenna
doesn't matter.  The antenna will work about the same on the fundamental with +/- 20% or
more variation.  The only reason you have to tune an antenna is to minimize the SWR on
your feedline when you aren't using a tuner, and with ladder line you'll have a high SWR
and need a tuner anyway.

I did just what you are proposing last Field Day:  after considering the weather (rain) and
resources available (I was on crutches) I threw together a 40m doublet with ladder line.
I just estimated the length as somewhere between 30' and 40' each side based on the length
of my arms (but making sure both sides were the same length), tossed it up in a tree, tied
off the ends, threw it on the tuner and started working stations.

Don't make it more difficult than you need to!
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NO2A
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Posts: 801




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« Reply #3 on: June 04, 2013, 06:17:22 PM »

The 468 formula is for hf. The 472 formula is for vhf/uhf.
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W4OP
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Posts: 436


WWW

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« Reply #4 on: June 04, 2013, 06:26:38 PM »

The 468 formula is for hf. The 472 formula is for vhf/uhf.

I cannot follow the reasoning as to why  472 would be used for VHF/UHF. The radiator, whether wire or tube/rod is a larger percentage of a wavelength at V/U (i.e. fatter) which would then require a shorter length than a thin radiator. 472 would result in a longer radiator.

Dale W4OP
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WB6DGN
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Posts: 619




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« Reply #5 on: June 04, 2013, 08:20:30 PM »

You'd have a hard time accurately measuring the difference between the two formulas.  Quite likely the difference came about because of "rounding error" when deriving the formulas involved.  If you can accurately measure a 30+ foot piece of wire lying on the ground to +- 3/8" then split the difference and let it go at that!  And, by the way, I've got some matching sections I'd like you to build for me if you're consistently that accurate.
Tom
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K5TR
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« Reply #6 on: June 04, 2013, 08:25:22 PM »

I have heard people use several different variances of the 468/f formula and I'm wondering which best represents physical length for antenna building. Someone told me that 472 works better?

Ward Silver - N0AX wrote a great article for eHam.net a few years ago abou 468.

http://www.eham.net/articles/23802

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George
K5TR
WB6BYU
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Posts: 13341




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« Reply #7 on: June 04, 2013, 11:41:35 PM »

The resonant length for a half wave antenna varies depending on the diameter of
the conductor, the height above ground, and whether the wire is insulated or not.  
In practice it also depends on how you make the connections to the center insulator
and tie the wires off at the ends.

Unless your formula takes those factors into account, then it is just a rough estimate
anyway, and it really doesn't matter which one you use.
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EI2HEB
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Posts: 58




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« Reply #8 on: June 05, 2013, 09:30:12 AM »

For me 300/f works the best for initial measurements... really!

 Wink Wink Wink Cheesy Cheesy

Map of countries which have not implemented the metric system: http://able2know.org/topic/121197-1

... just saying ...

« Last Edit: June 05, 2013, 09:36:51 AM by EI2HEB » Logged
AA5TB
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Posts: 81


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« Reply #9 on: June 05, 2013, 02:17:08 PM »

To the metric dudes, 300/f(MHz) = wavelength, not dipole length. Should be 150/f(MHz) = 1/2 wavelength in meters.
 Wink

73,
Steve - AA5TB
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EI2HEB
Member

Posts: 58




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« Reply #10 on: June 06, 2013, 01:35:45 AM »

To the metric dudes, 300/f(MHz) = wavelength, not dipole length. Should be 150/f(MHz) = 1/2 wavelength in meters.

Fair enough Steve! I guess I posted to quickly; you  are spot on, 300/f will give you  wavelength.

and if you cut each side of the dipole seperate then it is 75/f for a 1/4 wave-L

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W6RMK
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Posts: 656




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« Reply #11 on: June 08, 2013, 09:54:12 PM »

sloppy, sloppy, what is this 300 stuff?  299.8 is the number, unless you want to compensate for the dielectric constant of the air, factoring in the humidity (you DO re-prune your antenna when the humidity and temperature changes don't you?)

As others have pointed out, 468 is historical, and anything close will work just fine. I doubt that most dipoles are perfectly horizontal and suspended in free space many wavelengths from anything else.
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EI2HEB
Member

Posts: 58




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« Reply #12 on: June 09, 2013, 03:03:24 AM »

you DO re-prune your antenna when the humidity and temperature changes don't you

Here in Ireland we have Leprechaun's (http://en.wikipedia.org/wiki/Leprechaun) to do that for us!
 Wink

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