Try this... First get the 1/4 wave length in free space for your frequency from this link:
http://www.csgnetwork.com/freqwavelengthcalc.html Then multiply by the velocity factor of the cable you are using which should get you really close to what you want. For example 146.000 MHz = 20.21917808219178 inches X .695 for RG-400 = 14.0523287671232871 inches for 1/4 wave. 14.052 Inches of RG-400 should be very close to a 1/4 wave @ 146.000 MHz. I used this method to make some 1/4 wave jumpers using Andrew FSJ2-50B and when we checked the cables with a very expensive piece of test gear I was only 2 degrees off from a perfect 1/4 wave on my frequency. CABLE VF RG-8 .66 LMR-400 .85 RG-8X .84 RG-11 .75 RG-58 .66 LMR-195 .83 RG-59 .82 RG-62 .84 RG-174 .66 RG-213 .66 RG-214 .66 RG-217 .66 RG-218 .66 RG-316 .79 RG-400 .695 LMR-500 .85 LMR-600 .86 1/2 HARD .81 7/8 HARD .81 LDF all ver .88 ----- Original Message ----- From: DCFluX To: [email protected] Sent: Friday, February 05, 2010 9:23 PM Subject: Re: [Repeater-Builder] Test rig for cable length at velocity Try measuring a shorted coax. On Fri, Feb 5, 2010 at 7:20 PM, Kris Kirby <[email protected]> wrote: > > Hi all, > > I've been trying to figure out how, using relatively simple equipment, > to determine proper cable length for one-quarter wavelength cables of a > given type of cable (i.e.: RG-400). One of my thoughts was to measure > the open circuit resistance using an older MFJ-259, however this results > in strangely low impedances that I believe hamper proper readings. > > Basically, I'm trying to figure out if I need to cut the cable shorter > or if it's too short, and right now nothing I have gives me any clue as > to exactly where it is, short of transmitting a few watts and watching > the SWR as I change frequency. > > > > -- > Kris Kirby, KE4AHR > Disinformation Analyst > > > ------------------------------------ > > > > Yahoo! Groups Links > > > >
