Velocity factor again

It might be useful for testing out designs before they are built but I want to learn how they work and, although I thought I had a 'feel' for the basics, the results I am getting bear no relationship to the theory as I currently understand it. My understanding is obviously faulty but I can't see where.

A single half wave long dipole has an impedance of about 70 ohms. A single half wave long Franklin/AMOS antenna is a folded dipole with an impedance of: 4 * 70 ohms = 300 ohms Combine 4 such folded dipoles using multiples of 1/2 wave length phasing lines between each folded dipole and the feed impedance is about 75 ohms. 300 / 4 = 75 ohms If you must have 50 ohms, some kind of 75 to 50 ohm coaxial matching line is the easiest work:

The usual result is a tangled mess of coax cables. Something like this:

Or, parallel six 300 ohm folded dipoles to make a 50 ohm feed point impedance: "Six Element Collinear Antenna"

Dinner beckons. After that, I need to get back to work. I'll try to answer questions and offer suggestions, but I don't have the time to write a tutorial. Good luck (again).

I haven't had time to do a proper PVC in the microwave oven test. However, this might offer some clues:

I did much the same test with a frequency sweep on a NanoVNA. I took two photos. However, they're terrible and will need to be reworked:

This was also fun:

Thanks for those references, the first one was very useful but the other two explain in trival terms something that I am not trying to do.

My problem with the co-ax colinear is not straightforward: If I connected a 'floating' signal source to a resistive termination by a few wavelengths of co-ax, I would not expect that co-ax to behave as a radiating aerial and the whole power (apart from losses) would be delivered to the load. It would not matter whether I earthed the inner or the outer, there would still be no significant 'radio' radiation seen from a distance.

If I mis-terminated the line, there would be a standing wave pattern set up - so now the line would radiate alternately 'positive' and 'negative' radio signals. At a distance these would cancel, so there would still be no net propagation.

If I now cut the line to a length where the standing wave pattern was a whole number of half-wavelengths long and (by magic, not as yet invented) suppressed the radiation from alternate half wavelengths, there would be overall radiation seen from a distance. This is similar to the way a Franklin aerial works because alternate dipoles, which would detract from the overall radiation pattern, are missing from the array.

It would also be possible to insert opposite-polarity dipoles in the spaces between the others and thereby increase the radiation. This is the concept behinnd the alternate-polarity co-ax colinear array.

HOWEVER: The dipoles in a co-ax colinear array cannot be half a wavelength (in air) apart because of the velocity factor of the co-ax

AND: The co-ax colinear arrays all seem to terminate the end with a quarter-wave whip, which is matched to the characteristic impedance of the co-ax and thereby prevents any standing-wave pattern ...which in turn prevents any radiation from the co-ax sections

FURTHERMORE: The high impedance ends of the dipoles are almost touching and of opposite polarity, so there are bound to be interactions between them. No account seems to have been taken of this in any of the desighs I have seen.

Does this explain why all my efforts to build a co-ax coliner have failed or is there something obvious I have missed?

The more fundamental question is why does wavelength contract in the presence of dielecrtic loss?

A lossless dielectric reduces velocity too.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics