Velocity factor of co-ax

Was this a semi air-spaced coax? John

Some coax relies on a foamed dielectric, which would have a lower dielectric constant than solid plastic.

The dielectric looks like solid polythene - almost transparent and definitely hard and solid.

What are they?

ITYF capacitance/meter dominates.

The signal propagation velocity depends entirely on the material and consistency of the dielectric. A velocity factor of 78% does not shock me. I've used cables with a foamed polyethylene dielectric with a velocity factor of 0.84. Some cables use plastic spiral wire or discrete spacers and can reach velocity factors pretty close to unity.

Jeroen Belleman

In that case it seems quite likely my cable is rather better than I anticipated. I shall start chopping it up into lengths to make a 145 Mc/s vertical co-linear array with reversals every 88.85 cm.

There are two leading-order effects that change the phase velocity in a transmission line.

One is the shape of the lowest-order normal mode. Without going into higher (but very pretty) math, there’s only so much curvature that the field plot can have. (It doesn’t mean that the field direction necessarily changes—it’s the second derivatives that are in view.)

If the boundary conditions force the transverse variation to have curvature, as in a metal waveguide, there’s less curvature available for the longitudinal variation—k_z is lower than the free space value. Since v_p = omega/k_z, the phase velocity exceeds c in the material, and can exceed c in vacuum.

As the waveguide cross-section decreases, there’s less and less for k_z, until cutoff, where k_z is 0, the phase velocity diverges, and the wave can’t propagate.

In a coaxial geometry, there’s no transverse curvature, so k_Z gets it all, and the phase velocity is c.

Which brings us to the second first-order effect, namely the dielectric. In free space, c is reduced by a factor of 1/sqrt(mu epsilon), which is about 1.5 for solid polyethylene, leading to a velocity factor of 0.65-0.7.

Foam and spiral dielectric spacers have lower effective epsilon, and thus higher velocity factors.

Cheers

Phil Hobbs

I think 93 ohm RG62 has a vf of about 0.85 - did your measurements confirm the Zo of your cable was actually close to 50 ohms? Sorry RF new to me.

The greater the dielectric air, the higher the velocity factor. The higher the velocity factor, the closer the coax's electrical length is to its physical length.

The velocity factor isn't really a determinant of the cable quality. There are also such things as loss, shielding effectiveness, accuracy of Z0 and lots of other lesser considerations. Your description of the cable construction does not suggest a high-quality cable.

Jeroen Belleman

Read the link, Jeroen listed most of them. It's all about getting more air into the space tween the inner and outer conductors.

It's ironic to find Cursitor Doom saying "I think you will find" when he clearly isn't fond of thinking.

The capacitance between the inner and out conductors of a specific length of coaxial cable probably is diagnostic, but a lower dielectric constant dielectric spacer has to be compensated by a thinner inner conductor for a given characteristic impedance, so it isn't going to be all that simple.

I suspect he's refering to pure air/gas dielectric, rather than air-filled foam.

More than just tricky.

RL

V = c/(sqrt(Er))

Solid polyethylene has Er around 2.3.

Foamed stuff is lower.

Polyethylene is awful. It melts when you solder it. Foamed is worse.

Your VNA measurement may be suspect.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I think it would be difficult to get it wrong when it is so easy to check the calibration with a simple 50 ohm terminator.

You really should use a proper calibration set: open, short and load and maybe a through too if circumstances require it. The quality of that cal kit will be crucial to the accurate of subsequent measurements. If you're doing a lot of VNA stuff, you can expect to spend a significant portion of the day calibrating and recalibrating. It's a real time thief.

Yes, but if all you are doing is looking for 50 ohm resistive then verifying with a 50 ohm terminator is all you need to do to verify your result.

Did you read it before suggesting it?

The link you gave me contained 29 screenfuls of information. None of them said anything helpful about the velocity factor; it was mentioned once, with no explanation of what it was or what caused it. There was just a formula with two variables, neither of which was defined.

Jerogen's information was helpful because it confirmed that the results I obtained were not necessarily an indication of a faulty measurement technique.