Custom UHF Yagi antenna build

[Baluncore]

You have better impedance matching, but the clustering of the elements into closely coupled groups, suggests a deviation from reality. The clustering makes it more like a 4 element response. You might change the Tradeoffs, and turn on the optional resistive losses.

3.15mm is the diameter of my welding rods, which I have plenty of.

Are those bronze rods?

 

[Guineafowl]

You have better impedance matching, but the clustering of the elements into closely coupled groups, suggests a deviation from reality. The clustering makes it more like a 4 element response. You might change the Tradeoffs, and turn on the optional resistive losses.

Are those bronze rods?

Mild steel. Using thicker rods (I have lots of 12mm steel) and a folded strip dipole seems to improve things:

I can’t see how to turn on the resistive losses, but I may have a different version. Yours gives dBd where mine gives dBi, for example.

 

[Baluncore]

Mild steel. Using thicker rods (I have lots of 12mm steel) and a folded strip dipole seems to improve things:

Bare iron or steel will not work for resonant antenna elements at UHF. It has too much surface resistive loss. Galvanised wire, like a wire coathanger, might work for a prototype, but it will not last. You really need aluminium, copper, bronze or brass. As tube, rod, or flat strap.

Also, YO runs on DOS, so only permits 8 letter filenames. Then they become "number~1".

I can’t see how to turn on the resistive losses, but I may have a different version.

YO version 5.10 from the link I posted.

 

[Guineafowl]

I have some 15mm copper pipe lying around, and some 3/16”(4.7mm) cupronickel brake pipe for the folded dipole. Also a bagful of 15mm pipe clips.

In both of our YO calculations, the elements are around 100mm long, but the half-wavelength of 641.4 MHz is around 233mm. The yagi designer gives elements sized more like the latter figure, as well. Which is right?

 

[Baluncore]

Avoid expensive new materials when prototyping. I use scrap aluminium sheet, cut into strips with a guillotine. For this design, I would use 12 mm strips that will pop rivet onto the boom. The folded dipole is then easy to fold.

Which is right?

Double the YO element length to get the actual dipole length to cut.
It is mentioned in the documentation, and comes from the legacy file formats of previous versions. The NEC software takes advantage of symmetry about the boom.

 

[Averagesupernova]

Avoid expensive new materials when prototyping.

I'd agree here, but I'll take it a step farther. If a person likes tinkering with antenna building you might be wise to get some small diameter brass tubing. I've used some that is about the size of a brass welding rod. Where I come from that would be spec'd in inches, specifically .125 inches. Two different sizes will telescope, one into the other. Give one a slight pinch to grip the other. Lengths can then be played with and adjusted. I used the appropriate size copper wire at the base of the element that had a wavy bend or two so the brass tube would grip the wire. I did not use this scheme for a beam so I wasn't attaching anything part way up the element for a matching network such as a gamma or Tee match. Not sure what you'll be using at 600 plus MHz but the same idea could be applied. Various sizes of tubing that slide one over the other. The brass tubing I bought came from a hardware store here in the USA. I suspect such a thing can be found at a hobby store also.

 

[Baluncore]

Various sizes of tubing that slide one over the other. The brass tubing I bought came from a hardware store here in the USA.

Telescoping brass tube comes in 1/64" increments. But there is a problem. It must be soldered, or nickle plated, to make sure a good sliding contact is made at the external sleeve step. Without that conductivity guarantee, the oxide layer forms a bazooka-balun of the sleeve joint, or inserts a capacitor into the element.

 

[Guineafowl]

Avoid expensive new materials when prototyping. I use scrap aluminium sheet, cut into strips with a guillotine. For this design, I would use 12 mm strips that will pop rivet onto the boom. The folded dipole is then easy to fold.

Double the YO element length to get the actual dipole length to cut.
It is mentioned in the documentation, and comes from the legacy file formats of previous versions. The NEC software takes advantage of symmetry about the boom.

Thanks. It’s scrap copper pipe, old imperial size, so no great loss.

I did wonder if the values were halved in the YO software.

 

[Baluncore]

Thanks. It’s scrap copper pipe, old imperial size, so no great loss.

The scrap price of Al and Cu are similar per weight, but they have different densities. I would trade thicker Cu tube for an equal weight of Al sheet.
Old road signs are a good source of antenna element material when it is cut into strips, it is lighter and has lower windage.

Alternatively.
Use a pair of scissors to cut a helix from a used aluminium can. Mind your fingers. Unroll the helix to make a long strip. Bend the strip along the centre line to give it length-ways rigidity from the angle. Attach cut lengths of the strip to a wooden boom with rubber bands or staples.

Practice plating aluminium with solder through vegetable oil or turpentine, by scraping the oxidised surface with a blade until it is bright, then scratching with solder and a hot soldering iron. Once plated, you can solder it again without the oil.

 

[Averagesupernova]

Telescoping brass tube comes in 1/64" increments. But there is a problem. It must be soldered, or nickle plated, to make sure a good sliding contact is made at the external sleeve step. Without that conductivity guarantee, the oxide layer forms a bazooka-balun of the sleeve joint, or inserts a capacitor into the element.

The idea is not a permanent antenna with the sliding tubes. They're meant to be used to prototype antennas. Many different antennas can be experimented with this way using the same tubes. Once a person is happy with the results a more permanent construction technique is used. I've used this scheme with two vertical antennas in a time difference of arrival scheme. I adust the antennas to the exact frequency first, then go about tracking down the transmitter.

 

[Baluncore]

The idea is not a permanent antenna with the sliding tubes. They're meant to be used to prototype antennas.

Nickle plated brass is available, preassembled, and it costs less than telescoping brass tube.
Each six section unit, makes two or three adjustable directors for UHF.
5X Replacement 49cm 19.3" 6 Sections Telescopic Antenna
https://www.ebay.com.au/itm/394138218259?

 

[Guineafowl]

A quick update: I’ve built a prototype using the 15mm and 3/16” copper pipe that I have. I’ll come back with some details and VNA plots when I have some more time, and have built the balun as specified by the design software.
In the meantime, I’ve been reading through the ARRL chapters on the subject.

 

[Baluncore]

A quick update: I’ve built a prototype using the 15mm and 3/16” copper pipe that I have.

The software for Yagi design gives realistic and accurate dimensions for the array of passive elements and the far-field pattern.

Once the Yagi antenna array of dipoles has been designed, only the driven element length should be adjusted, to optimise the impedance matching. All other elements should remain in place and with the specified lengths, as any other strategy will rapidly lead to mayhem and confusion.

 

[Guineafowl]

The software for Yagi design gives realistic and accurate dimensions for the array of passive elements and the far-field pattern.

Once the Yagi antenna array of dipoles has been designed, only the driven element length should be adjusted, to optimise the impedance matching. All other elements should remain in place and with the specified lengths, as any other strategy will rapidly lead to mayhem and confusion.

Yes, it’s all built and screwed together using the surprisingly accurate import DRO on my milling machine. I’ve found some ally strip that might make a neater and more adjustable dipole.

 

[Guineafowl]

Right, some progress, but some confusion.

Standard copper pipe standoffs allow the elements to be raised into the plane of the dipole, and have a useful centre rib for alignment.

Balun cobbled together as per the design software.

I expect there will be some comments on this, and how the VNA is connected to the assembly. I can’t find guidance on how the $50\Omega$ SMA leads of the VNA should connect to the $75\Omega$ coax.

VNA plot, which looks reasonable but varies greatly when moving the balun around, and also depends on how the calibration is done.