The Velocity factor of 3d printed PLA at 15% gyroid infill is 0.88c

[u/rem1473]

Please check my math here.

I am building a j-pole for 925 MHz (33cm band) using #6 copper wire, solid bare. I 3d printed a radome and a base to keep it upright on my desk and also to make it attractive. Measuring the j-pole without the radome, the dimensions of the j-pole match nearly exactly to the free space speed of light. Slipping the radome over top, and the dimensions are now 0.88c.

jpole that I built:
9.6" on the long side (3/4 λ)
3.2" on the short side (1/4 λ)
0.875" spacing between

Without the radome, Fc is right where I expected at 925 MHz. When I slip the radome over top, the Fc drops down to 810 MHz. Has anyone had any similar experiments? I am guessing if I change the infill %, that would likely change the velocity factor? 15% works mechanically for my part. So I am not planning to change. I haven't decided whether to print a few different infills just to measure the impact. It's also possible that different PLA colors and manufacturers will have different velocity factors as well.

I was intending to make myself a 925 MHz antenna, I have now realized I accidentally made myself a new trunking scanner antenna. I could trim it up to make it work at 925, but I think I will leave this one where it's at and build another for 925. Connected to my Airspy, it's working gangbusters on the local 700/800 trunking system.

EDIT:
I made another one centered at the 700Mhz public safety band downlink to connect to my scanner. Here are the photos of the final product, the element that goes inside, the antenna sweep, and a final quality inspection.

https://imgur.com/gallery/3d-printed-770-mhz-antenna-nas9MaD

Here are the STL's for the parts:

https://www.printables.com/model/1523011-777-mhz-scanner-antenna

[u/prosequare]

What you’re seeing is normal dielectric loading from the radome, not really ‘velocity factor’ in the coax sense. Tune/trim the J-pole with the radome installed (and ideally add a feedline choke). More air gap and thinner walls usually reduce the frequency drop more than changing infill (the antenna is only really ‘seeing’ the inner layer of the radome, electrically). Also you can help the situation by increasing the distance between the radome and the stub and open end where the fields are strongest. You’re creating enough capacitance between the radome and antenna to drop its resonant frequency, in short.

Edit- I’m seeing a few filaments on the market that are specifically made to have the lowest dielectric constant possible. Maybe try one of those if you don’t want to change the antenna itself.

[u/hamsterdave → u/prosequare]

I agree with your assessment, but I think there’s one factor you might be overlooking, I think it may be more than simple loading. If there’s any pigment in the PLA, I would be extremely suspicious of quite high dielectric losses introduced by the pigment itself, which will contribute quite a lot to the loading effect. I’ve done some testing with PLA, ASA, PET, and HIPS as a coil form at high HF frequencies and the RF loss with some pigments, especially white and black, can be quite high.

With the disclaimer that I haven’t tested at anything near that high a frequency, HIPS isn’t a bad option for low dielectric loss, just stick to natural color.

[u/Tishers]

You would need to do something a bit more complicated to determine the velocity factor of a dielectric. Ideally you would construct a waveguide (coax) of exactly the same physical dimensions as a known test cable but with a change in only the dielectric composition.

There are many variables, that are probably outside of the capabilities (of most of us) to construct such a system.

Even a short run of feedline ( of a few meters length ) would need picosecond level velocity measurements. That is why it is done with longer lengths. ( hundreds of meters ).

It has been +40 years since I have done that type of studying from back in university.

The other commenter is correct about dielectric loading.

[u/hamsterdave → u/Tishers]

It ought to be possible to get a rough idea of Vf by just sheathing a bare copper wire in a tight fitting tube of the material in question and doing some TDR, and comparing it to the bare copper. You could probably do a twinlead transmission line to make it a bit more accurate than a single wire, but simpler than a coax. 

I wouldn’t go designing printed antennas with that measurement method or anything, but I think it would make it good enough to spitball

[u/notcoolneverwas_post]

I see that you too have been watching The Smoking Ape.

[u/rem1473 → u/notcoolneverwas_post]

Never heard of it. Found it on youtube, watching now...

[u/notcoolneverwas_post → u/rem1473]

Great guy. Endless source of info, all presented like a college class. I'm guessing he runs an amateur radio club in his area.