Velocity factor again

The part of the field inside the dielectric would be slower than c, of course, but the part of the field in the air would still propagate at c. You'd get some sort of superposition of fields. This is where a 3D EM field solver would be instructive.

A coaxial cable is easier to work out, because all fields are contained inside a homogeneous dielectric.

With a wire inside a dielectric pipe, you get refraction and partial reflections. The plastic is thin though. Intuitively, with the geometry you describe, I'd expect to find the VF to be essentially unity. Most of the field is still in air.

Jeroen Belleman

Right.

You could measure capacitance too.

They are old so I expect there are newer, nicer programs around.

We used atlc2 to design the multilayer pcb stackup for some cheap edge-launch SMAs, the Amazon types with with fat round center pins.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

NEC model for 4NEC program:

Yes. I've had the displeasure of building antennas that work perfectly in free space, change their characteristics once installed in a "radome", which describes the plastic plumbing pipe. With PVC, the resonant frequency of the antenna moves down in frequency. How much depends on the dielectric constant of the plastic.

Yes. You should notice an effect.

In the distant past, I used a Reflection Coefficient Bridge, typically made by Texscan:

These days, the cheap and common VNA testers are easier to use:

Notice that one does not find any commercial antennas that use PVC pipe. While it might be possible to design (or cut-and-try) an antenna using a PVC pipe radome, chances are poor that one can build two or more similar antennas because of variations in VF between PVC manufacturers and production lots. More commonly, VHF/UFH antenna radomes are made from fiberglass, which has less an effect.

You should have seen some slight heating. However, testing at 2.4GHz in a microwave oven is quite different from 145MHz. Also, were you testing gray PVC pipe or white ABS pipe?

Good observation. It's really tempting to use PVC pipe for antenna radomes because PVC is cheap, available and easy to use with all the available plumbing fittings. Watching the antenna characteristics change when a PVC radome covered my antenna was sufficient to convince me to try something better.

I think that is more suited to freqenecies well abovw 145 Mc/s

It is white pipe but it was sold to me as PVC.

Ar 145 Mc/s would it make much of a difference?

Best to test it. Cut off a small piece of the plastic, and burn it with an alcohol lamp flame. Does the flame around the burning plastic have a green halo? If yes, it contains a halogen like chlorine and so is most likely PVC. If no, ABS is most likely.

Joe

I heated up a flake on the cooker ring and the fumes that came off contained hydrochloric acid, with no trace of any styrene smell. Definitely a PVC-like material.

That's a valid test as well. So PVC it is.

Joe

Wire in a plastic pipe is a near-field effect. The distances are a fraction of a wavelength.

A radome is typically big, so any effect on the radiator is more from em wave reflections, less from capacitance effects.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Just checking. PVC pipe is rated for delivering water or air under pressure. ABS is not pressure rated and intended for non-pressurized applications, such as electrical conduits. ABS is stiffer than PVC and is best used for furniture.

Note that ABS pipe is banned in the Peoples Republic of California:

Yes. Everything in RF scales with frequency. If you find a radome pipe diameter that works well at 145MHz, both the antenna and the radome will proportionately shrink in Wi-Fi land (2400MHz). In general, an antenna design looks similar at different frequencies through scaling. If you look at the NEC4 calculations for a coaxial cable collinear vertical antenna, the 145 MHz can easily be shrunk into a 2400 MHz antenna by simply changing the operating frequency. Everything scales and you end up with a similar looking antenna, but smaller. Same with the radome which will roughly: sqrt(2400 / 145) = 4 times larger for 145 MHz. Similarly, a radome designed for a 2400 MHz antenna will need to be 4 times larger to operate at 145 MHz.

Trivia: I forgot to mention that a Reflection Coefficient Bridge will also require an RF detector. I forgot because it is built into most RF sweep generators (including all of mine). Also, the VF of air is about 0.9997. VF of a vacuum is defined as 1.0.

The problem with collinear coax cable antennas is that for a given gain, the coax collinear is twice as long as a Franklin or AMOS antenna:

I'd guess that a thin dielectric, a fraction of a wavelength, would be transparent to em fields.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Some back of the envelope guesswork. Schedule 40 and 80 PVC pipe varies in thickness depending upon the OD (outside diameter): "PVC Piping Dimension Chart for Sch 40 & Sch 80"

For a VHF coaxial vertical collinear antenna, made from RG-8/u, I would guess(tm) that the ID (inner diameter) of the PVC pipe would be about 3 times the OD of the RG-8/u cable: 3 * 0.4 = 1.2 inches

The minimum size schedule 40 PVC pipe (from the above chart) would be

I don't know how much de-tuning to expect, but I would guess(tm) that it's more than trivial. Rather than calculate, it's easy to test with a 1/4 wave (19 inch) monopole antenna (coax cable with about 19 inches of the center conductor exposed. Plug in a NanoVNA (or better VNA), set it for VSWR sweep, and you should see something like this:

Some PVC is stabilized with zinc, tin or lead and has high tan d losses so yes, I guess more than just k can be involved in detuning?

In 1986, congress removed lead from plumbing with the Safe Drinking Water Act:

"Lead-free PVC"

"Difference Between PVC and Lead Free PVC"

Calcium-zinc is commonly used as a PVC stabilizer. Calcium and zinc are not toxic, are not heavy metals, or are considered dangerous. There is some zinc in domestic water because it often passes through galvanized pipes.

I think this discussion is heading for the weeds. The original question was over the suitability of using PVC pipes for a 145 MHz antenna. The radome can change the antenna tuning by heating or it can introduce losses by absorbing some of the RF. Both will cause problems.

I forgot to mention something about "hard water" being mostly calcium (and some magnesium). If you're drinking hard water, you're drinking calcium.

It didn't heat up in the microve oven, so I don't think it will be lossy at 145 Mc/s either.

This is overflow pipe and I am also using a larger size which is intended for waste water, so any drinking water regulations wouldn't apply.

I've tried it with a wire dipole and a NanoVNA; one end of the dipole held up by a polystyrene ball-point pen and the other end hanging free. Slipping the free-hanging end into a piece of the pipe does not appear to cause any de-tuning effects that are above the general 'noise' level caused by doing the test indoors with relatively uncontrolled conditions.

The Smith Chart display over a range from 100 Mc/s to 200 Mc/s was more informative than the SWR graph.

Even if this did result in significant detuning, how would I separate the VF effect from capacitive and loss effects?

The reason I asked about VF was that I was going to try to build a Franklin version from wire inside a pipe; but I needed to know the correct lengths of the various components.

The design I based it on had the half-wave (in total) stubs wound up as non-inductive and non-radiating coils between the ends of the elements. To keep the size sensible, I decided to make it as three half-wave elements with the middle one split in half to give a low impedance feed point. To my mind, the feed point impedance should be 3 x 50 ohms but it measures about 65 ohms at the non-reactive frequency - which isn't at

I would still prefer to use polarity reversals between the elements, rather than phase change, as there are fewer frequency dependent elements in the design and less to get wrong. However, my experiments so far with co-ax polarity reversals have never resulted in anything useable. The latest version (which I call the "NBG colinear") just gave nonsense results on a Smith Chart and was more than 20dB down on the same signal received with a sleeve dipole.

I feel I'm really failing to get to grips with any kind of colinear aerial design - especially the co-ax types.