[amsat-bb] Re: Polarity questions

Edward Cole kl7uw at acsalaska.net
Sat Sep 20 11:07:03 PDT 2008


I am surprized no one caught the error!  I even gave you a clue in 
the sentence after the computations.

the formula for 1/4 WL = Vf * 248/Fmhz, not Vf * 492/Fmhz
So if you make your 144-MHz phasing line 27 inches of RG-11, it WILL 
be too long!

It is true that manufacturing variations in Vf occur from roll to 
roll of coax.  If you can measure it, this is prefferable to assuming 
the mfr's data.  But, using the published numbers is fine if you do 
not have special equipment.

73, Ed - KL7UW

At 09:59 PM 9/19/2008, Edward Cole wrote:


>You are making too much of a big deal out of this.  Velocity factor
>is published by coax manufacturers so you do not need instruments to
>get close.
>1/4 WL = Vf * 492/Fmhz in feet
>1/4 WL (RG-213) = 0.66 * 492/144 = 2.25 feet or 27-inches
>
>If you are off 5% it will not destroy the circularity.  Most hams get
>it "close enough".  Of course if you are off by a quarter wavelength
>or more it will matter (20-inches at 144-MHz).
>GL
>
>*****************************************************
>73, Ed - KL7UW              BP40iq, 6m - 3cm
>144-EME: FT-847, mgf-1801, 4x-xp20, 185w
>http://www.kl7uw.com     AK VHF-Up Group
>NA Rep. for DUBUS: dubususa at hotmail.com
>*****************************************************
>
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