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VF of CATV Rg-6




 
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WA2SQQ
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« on: July 26, 2021, 07:23:34 PM »

Playing around with a delta loop for 17m. I have reel of CATV RG6 cable which I figured I could use to make a 1/4 matching stub. Every chart I can find shows the VF for RG6 as .66.  My antenna analyzer shows 0.8 ohms, but the measured length is about 16 inches longer than the calculated length.

Does anyone know if the CATV cable differs that much from standard RG-6.
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W1ITT
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« Reply #1 on: July 26, 2021, 08:52:38 PM »

If you strip a bit of the jacket and braid back and look at the dielectric, what material is it?   If it's  a semi hard milky polyethylene like the RG8 and RG58 cables we've been messing with for years, that would indicate that a 0.66 velocity factor is in order.  If it appears to be foam or a bit squishy with little or no transparency, that would be indicative of a velocity factor closer to the 0.8 value that you measured.  But plastics are fantastic and many things are possible.
Assuming decent technique, I've always maintained that a measurement beats an opinion every day.  In any case, it's good to investigate as RG6 is commonly supplied by some vendors as whatever they want to make that week.
73 de Norm W1ITT
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KD6VXI
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« Reply #2 on: July 26, 2021, 11:41:46 PM »

Coaxial cable velocity factor is a varying number depending on the frequency you measure it at.

This is why for things that matter I trim my stubs using either the MFJ or one of my VNA's.

Most (all) manufacturers don't indicate this, but it is a simple fact of life.

A mfj259 can help here, a TDA, VNA or SNA will be your friend.

--Shane
KD6VXI
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WA2SQQ
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« Reply #3 on: July 26, 2021, 11:44:50 PM »

Im going with the actual measurement. My VNA and my AA-230 analyzer agree so much for published numbers. Thanks .
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KA2DZT
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« Reply #4 on: July 27, 2021, 03:02:36 AM »

If the cable has a solid dielectric the VOP would be .66.  A foam dielectric cable the VOP is usually around .83-.85

I use all full wave loop antennas at my QTH and I've made many 1/4wave matching stubs

Fred
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