Use the following formula:
V = (length in feet of the cable x frequency)/246
V= velocity factor
Another method using the GDO:
From "CQ Magazine" May 1974 here is another method:
Measure the length of the coax carefully. Place a "hairpin" short at one end of the cable and a short loop between the shield and center conductor at the other end...The loop should be large enough to insert the GDO. If you can couple into a very small loop, the readings will be much better...Use of a large loop with show a slightly lower frequency on the GDO.
Suppose that you are measuring 1ØØ feet of cable. Calculate what you believe the resonant frequency using a table of known values.
Dip the cable.
For our example the cable dips at 3.9 MHz
The calculated resonant frequency is 4.Ø83 MHz (based on 1ØØft and a velocity factor of Ø.83)
frequency = (492 x velocity factor)/ feet
frequency = (492 x .83)/1ØØ = 4.Ø83 MHz
Now using the described method, find the resonant frequency of the unknown cable an perform the following calculations:
V = (feet x frequency of the dip)/492 = (1ØØ x 3.9)/492 = Ø.792
Again look for a loose coupling (shallow dip)
I can see the MFJ method as a possibility. The 5Ø Ohm load provide purely resistive load at a fixed value. The meter relies on a 5Ø Ohm load.
The method described in the magazine article, utilizing a GDO is not impedance dependent.
BTW, the GDO should (after measurement are taken), be placed near a good receiver to obtain the exact frequency of the Dip.
73,
Jay/AF2C
At 12:14 PM 12/11/05 -0600, you wrote:
"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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That doesn't sound quite right, but the MFJ instructions seem a little
wierd.
On Sun, 11 Dec 2005 09:31:29 -0700, Peter Hoon wrote:
>Connect one end of your coax sample to the MFJ 249 or similar unit. Connect
>the other end with let us say 27 feet of coax to a fifty ohm resistor,
>between ctr and shield. You can coil up the coax on the floor.
You won't get ANY dip with the 50 ohm resistor at the end of the line.
That would be a flat line. MFJ wants you to connect an open line to the
analyser with a 50 ohm (Non-inductive) resistor in series with the
center conductor at the antenna connector. Not sure why, I cut a 144.39
stub filter for my television line by just connecting an open line
directly to the analyser; I didn't see any benefit to using the
resistor.
>Measure precisely the length of your contaminated coax sample.
>
>Find the dip frequency using the 249 analyzer. For example, a 27 foot
>length has a dip frequency of about 7.3 Mhz.
Start at the LOW end of the range and find the first dip. The line will
dip at 1/4 wavelength and multiples thereof.
--
Jim Shorney -->.<--Put complaints in this box
jshorney (at) inebraska.com
Ham Radio NU0C
Lincoln, NE, USA
EN10ps
http://incolor.inetnebr.com/jshorney/
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