RG8X velocity factor

[K6JEK]

The books and catalogs say RG8X has a velocity factor of .78.   My MFJ 259B says .70.   What should I believe?

Jon

[KF1Z]

If you know the manufacturer of the coax....
go with the given value.....

Remember... the 259b can give "false" readings if it's out of calibration
(VERY common!)  or the battery is low.....

Also, coax specs are implying the cable is NEW.... Straight run.... and never been kinked.... and  not out in the weather.....


As always... start a little long.... and trim to suit......

[WD8BIL]

Yup...... go with the manufatures spec.

[Tom WA3KLR]

Jon,

The RG8-X that I use here I measured the velocity factor as 0.79 a couple years ago.  The spec is 0.78.

I used my MFJ-259B to get the answer, but that unit does not read out the velocity factor directly, the unit would need to know the exact physical length of the coax under test.  The only transmission line parameter my MFJ-259B reads out is the coax loss in dB.

I started at a low frequency and found the half-wave resonant frequency for the cable.  Then I measured the physical length and did the cipherin' from there.

[W4EWH]

The books and catalogs say RG8X has a velocity factor of .78.   My MFJ 259B says .70.   What should I believe?

Jon

Jon,

If you doubt the MFJ analizer, I'd say you should get better test equipment: a measured value under actual conditions is always better than a nominal value from a spec.

I suggest you buy or borrow an antenna noise bridge, and use it to confirm or deny the accuracy of the MFJ reading.

HTH.

73, Bill W1AC

[Tom WA3KLR]

Borrowing a noise bridge to confirm a reading on an MFJ-259B is like borrowing a Weston 1910 meter to confirm your DVM's accuracy.  The bridge accuracy in the -259 isn't much of an issue for spotting the half-wave resonance, the result is a frequency number which accuracy depends on the time base - no sweat for the velocity factor determination.

I have made many checks on the accuracy of my -259B and it is a very good piece of gear.  There always is a possibility of a problem though.  Tom W8JI has a re-alignment procedure on the web.  I touched up the 2nd harmonic suppression on mine to ensure that the R + X result was not being thrown off by that harmonic.

[K6JEK]

The MFJ has a mode "distance to fault" under "Advanced" and a procedure to use this mode to determine Vf.   I got a different chunk of RG-8X, new stuff from the Wireman that they are proud of, 95% braid + foil.   They advertise .78 Vf.   I measured .78 Vf.   I've decided to believe the MFJ and conclude the other RG-8X is just slow.

Jon

[KB2WIG]

"and conclude the other RG-8X is just slow."

Its not slow, its just  "special".              klc

[WA3VJB]

Not to put a kink in your coax, Jon, but I've heard horror stories that the dielectric breakdown voltage is pretty low for that stuff.

[K6JEK]

Not to put a kink in your coax, Jon, but I've heard horror stories that the dielectric breakdown voltage is pretty low for that stuff.

Indeed.   I'm not running real power through it.   I'm using RG-8X (because it's ubiquitous) for a delay line on the input side of one of two amplifiers feeding separate phased dipoles.   That is I'm doing the phasing on low power side of things.  The high power side goes to equal length ladder lines.

Jon

[W1ATR]

What the hell is rg8x anyway? never heard of it. Must be something new. That's why I like this forum because I learn something new every day. I run the old RG-218 3/4" stuff, but it's awfully stiff to work with. If you hook it up to anything that doesn't have a little weight behind it, that big coass will yank it off the table and drag it out the window.

[K6JEK]

What the hell is rg8x anyway? never heard of it. Must be something new. That's why I like this forum because I learn something new every day. I run the old RG-218 3/4" stuff, but it's awfully stiff to work with. If you hook it up to anything that doesn't have a little weight behind it, that big coass will yank it off the table and drag it out the window.

Is that a BNC connector I see on the end of that?

[KF1Z]

RG-8X is very small, very lossy (even at HF)  really cheap coax.....

Sorta like the mfj  of  feed line.....   :-)

RG-58 is more reliable for tiny coax...





That connector on ATR's line looks more like a "C" or something.

[The Slab Bacon]

RG8X is also refered to by some manufacturers as "mini 8" it is smaller diameter than 8 but considerably larger than 58. IIRC it is 1/4" diameter with a #16 (i think) stranded center connector. It is great for short jumpers, but I dont use it for much else. It will handle much more current than 58, but the capacitive loss is MUCH higher than regular rg8. Use it for short patch cables, interconnecting cables and test cables, but DONT use it for long run feedlines, especially if you are running any power.
                                               The Slab Bacon

[WD8BIL]

If you use the manufacture's spec for the cable it'll probably end up being one of the variables that least affect an amateur antenna system. Those you can't control will have affects that far outway any VF differences.


I wouldn't worry about it.

[Bill, KD0HG]

I'm using a run of 15 year old RG8X to feed my 160 sloper because of its light weight, RG-213 pulls the dipole center down too much. It's also easier to hang ferrites on the smaller coax for choke purposes.

The line doesn't have any problem handling the full output of my 304-TL rig, and has lasted several years. It *does* get somewhat warm, but it hasn't failed.

In any case, you'd be surprised how much power it can take on HF.

..

[Steve - WB3HUZ]

RG-8X is not much more lossy than RG-8 or 213. For runs of less than 100 feet it's within 0.5 dB of the others. I've used it on my 160 meter dipole for five years with FB results.


Length: 100 ft
Freq: 10 MHz

Cable Type                             Loss (dB)
RG-58 (Wireman CQ125)                    1.333
RG-8x (Belden 9258)                      0.908
RG-213 (Belden 8267)                     0.622
RG-8 (Belden 8237)                       0.556
 

[The Slab Bacon]

RG-8X is not much more lossy than RG-8 or 213. For runs of less than 100 feet it's within 0.5 dB of the others. I've used it on my 160 meter dipole for five years with FB results.


Length: 100 ft
Freq: 10 MHz

Cable Type                             Loss (dB)
RG-58 (Wireman CQ125)                    1.333
RG-8x (Belden 9258)                      0.908
RG-213 (Belden 8267)                     0.622
RG-8 (Belden 8237)                       0.556
 

It would be interesting to see  loss v frequency and/or loss v mismatch.
I used a small run of it to my 160 balun and melted the foam in it. It couldnt handle the heavy current load. Regular 8 handles it fine.

One nice thing about it is that it is much lighter than standard rg8 for the coax fed dipole types.

[Steve - WB3HUZ]

Not all RG-8x is the same. Some has foam dielectric, some has solid dielectric. I'm sure the melting points are different for the two materials. That said, RG-8 is larger and will handle more power. Size does matter. 

Mismatch loss is related to matched loss by the following.

Mismatch Loss (dB) = 10 log [1- abs(reflection coefficient)^2]

reflection coefficient (Rf) = (Z_o - Z_l)/(Z_o + Z_l)

and is related to SWR by

SWR = (1 + Rf)/(1-Rf)

You can run some numbers here.

http://fermi.la.asu.edu/w9cf/tran/index.html


RG-8x is a nice cable to use, especially at lower HF frequencies. It's much lighter than RG-8 and about the same diameter as RG-59. As Bill noted, it puts a much smaller weight load on the center of dipoles and such. That's why I use it on my 160 meter dipole

[WA1GFZ]

Small cable will heat up faster and hotter when there is a mismatch since there is less surface area to dissipate the heat. 1/2 the diameter is 1/4 the surface area so would assume 4 times the temperature rise during a mismatch.
PWers need not worry.

[k7yoo]

Anyone have the QSL info for RG8X and RG8U? I worked both of those guys and they never sent a card.

seriously tho.. I have had some real problems with this stuff-especially in warm weather. I had the center conductor migrate through the foam and....well  you know the rest.

If they make mini 8 with solid dielectric, let me know, because as far as I am concerned foam coax is bad news for any outside or unprotected runs. I use it for short run interconnect & patch cables only.
Skip

[The Slab Bacon]

Hey Skipper,
                 Was it the ambient heat that melted the foam, or somthin else

[W1ATR]

What the hell is rg8x anyway? never heard of it. Must be something new. That's why I like this forum because I learn something new every day. I run the old RG-218 3/4" stuff, but it's awfully stiff to work with. If you hook it up to anything that doesn't have a little weight behind it, that big coass will yank it off the table and drag it out the window.

Is that a BNC connector I see on the end of that?

Those are UG-154B/U's for that size feedline. They're like 50 or 60 bucks new, but you can find them used for almost free. I don't think they make rg-218 anymore, but the connector type is usable on a couple of different types like rg-17/u which is still avail and the same size(.870) etc.... That pic was just for sarcasm, I haven't run that bigass cable since the chicken band days a long time ago. I've got maybe 200 feet of that crap in the basement with a dozen connectors that I'm keeping just incase I want to run 15kW or whatever the east coast legal limit is on 75 meters.

[Steve - WB3HUZ]

Some low loss types use foam dielectic. You shouldn't have a migration problem unless you have bends or bends that are too sharp. Then you can get migration over time, even without heating.

Radio Works sells solid dielectric RG-8x. They also sell double shielded RG-8x. And they sell something call Super-240. It's the same size as RG-8x with a gas injected foam dilelectric, double shielded (alumimum tape and braid) for 100% shield. It has a higher breakdown voltage and power handling capability than "regular" RG-8x.

[WA3VJB]

Here's a specification sheet on a version of the 240.
I don't know what they mean by "sparker test" but if that's dielectric rating then it's 2500V, not bad, sonny.