Question about Velocity Factor of coax

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Re: Question about Velocity Factor of coax

Post by Ivan on Sat Dec 07, 2013 1:00 pm

Hi Bernie,
RG-59 has 75 ohm, after 1:4 transformation it will be matched to 300 ohm instead of 200 ohm. Nearly the same coax having 50 ohm is RG-58.

BR from Ivan

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Re: Question about Velocity Factor of coax

Post by Bernie on Thu Dec 05, 2013 4:19 pm

Ivan, thanks

The cable is RG59-foam, and I was measuring with the "far" end open (mainly because the dips seemed to be more apparent). I did two things:

Firstly, I confirmed the 15.2 Mhz resonance on an hf receiver. This tells me that for open-end the calculated VF would be the previous 0.865.

Secondly, I shorted the stub-end, and searched for the resonance points. This time I was looking for a dip near 2.4 Mhz and harmonics. The dips were not visible except 20.5 Mhz and 35.5 Mhz (as you said there may be something wrong with the GDO). The 20.5 Mhz I also confirmed on the receiver. I guessed they were the 9th and 15th harmonics. Then doing the math I found these would correspond to calculated VF of 0.75 (this based on freq measured with receiver) and 0.78 (this with GDO only).

I think I will stick to 0.75 for cutting the cable (shorted better than open, and receiver better than GDO scale).

Thanks again for your help.
Regards

PS: I plan to use multiples of 1/2 wave for the feeder, and connect feeder + 4:1 balun to a point on the slim-jim having near 200 ohm impedance. Still to see. The SWR and feeder loss will be a project for the future, as I finish the SWR meter ...

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Re: Question about Velocity Factor of coax

Post by Ivan on Wed Dec 04, 2013 7:51 am

Hi Bernie,
the VF given by manufactures varies from 0,66 (solid PE dielectric) to 0,82 (PE foam). It is well possible your cable has VF= 0,865 if it contains some more air.
Did you measure an open or shorted stub ? The open end tends to radiate and therefore the stub seems to be electrically longer than it really is. The shorted stub usually gives better results.
Your measurement seems to be OK. I have no clue why your GDO did not catch the resonance at the 1st harmonic. Maybe it has to do with its construction ? Or its coupling with the stub was too weak at 5 MHz ?
Be aware that RG59 has characteristic impedance 75 ohm.

BR from Ivan

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Question about Velocity Factor of coax

Post by Bernie on Tue Dec 03, 2013 9:00 pm

Good evening. This is my first post.

I have a spool of cheap RG59 I wanted to use as a feeder and 1/2wave balun for a 2m slimjim antenna.
The velocity factor should be typically 0.8, but I wanted to confirm it for the particular cable prior to cutting it.
Also I heard that low VF can be associated with high loss, and I am suspicious about the cable.

The measuring instrument was my homebrew GDO (good upto 50 Mhz and reasonably calibrated), probably the single instrument I could use for this. I have also a homebrew frequency meter, but it is not able to read the frequency while the GDO is "dipping".

So I took a lenght of the same RG59 coax that was laying around, 12.80 mts long. I calculated the 1/4 wave resonance frequency to be 4.68 Mhz, and thought that measuring the "actual" resonance frequency would be easy on the GDO, and enough to check/calculate the VF.

Then something happened that I was not expecting. I found no resonance dip on the GDO near the calculated frequency. Nothing at least from 3 to 8 Mhz. However I did find several other very noticeable dips at 15.2 Mhz - around 36 Mhz - and around 45 Mhz.

I guess that 15.2 Mhz was the third harmonic resonance, then the frequency at which the test-lenght was a 1/4wave was 5.07 Mhz. This could mean the other dips were the 7th (35.5 Mhz) and 9th harmonic (45.6 Mhz).

This would relate to a "measured" VF of 0.865. Probably too high for a cheap coax, maybe because of the rough mesurement I made with the GDO scale. I should repeat this test but find the resonance on an hf receiver.

But still confuses me that I found no resonance at or near 4.68 Mhz.
And in turn found very noticeable dips at the harmonic frequencies 3rd, 7th, 9th (fifht is not covered by my GDO).

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