Home - About AR - Learning Material - Exams - Clubs - Posters

VHF & UHF Mobile Verticals

Vertically polarised antennas to mount on motor vehicles are fairly easy to make if you access to a metal turning lathe, such as at school, work, a friend's, or a shared workshop / Hacker Space.

Most CB antenna bases have a 5/16 inch stud, with a 26 TPI thread, also called Brass or BSB (British Standard Brass).

I also had a budget CB antenna which used an M6 × 1mm thread.

A lathe-free option is mentioned below.

Turning

This assumes you know how to use a metal lathe safely, or are under direct supervision of an experienced user; and are using all appropriate safety equipment and PPE.

A base or ferrule is turned from hexagonal brass rod, with appropriate holes drilled. 5/8" or 16 mm across the faces is about right. Use around 30 mm of this rod, determined by the length needed to accommodate the threaded stud, and the 10 mm or so for the element.

Firstly taper it so the top is around 8 mm in diameter. Use a Jacob's chuck to drill a hole to accommodate the brazing rod element. This is likely to be 1/8", but check with a micrometer.

Turn the hex section around, and drill a 6.9 mm hole. This must be several millimetres longer than the length of the stud on the base.

Replace the drill with the hand tap. Rotate the chuck by hand, while carefully introducing the tap to the drilled hole. Periodically reverse the chuck a little to clear the chips. The two options are just to use an Intermediate tap, allowing extra length in the hole or using both an Intermediate and a Bottoming tap. If you also have a Taper, start with this.

Clean the threaded hole, then try the ferrule you have made on a base.

Assembly

Cut the brazing rod element so that the overall length will be a quarter-wave long.

Next, mount the ferrule in a metal vice or similar clamp, with as little of the metal in contact as possible. Heat the ferrule with a gas torch, and also the element. Tin both with plumbing or larger diameter electronics solder, then insert the rod into the ferrule. Avoid getting solder into the threaded area, perhaps by sloping the tip of the ferrule slightly downwards. If you get solder in the threads, you can clear it using a tap in a tap handle.

The alternative to soldering is to have cross-drilled the top of the hex rod, and tapped it such than a hex Allen grub-screw (set screw), of 4 or 5 mm diameter can retain the element. I'd personally go for two such holes and screws, for security. Once this is done, cut the taper.

Testing

In an area without excessive metal around, place the antenna on a base mounted on your vehicle. Using the radio and an SWR meter, an antenna analyser, or a nanaVNA, determine whether the SWR is acceptable at the desired transmit frequencies.

If it is poor, the analysers will tell you the frequency of the lowest SWR, while if you are using a radio you will need to manually check and record SWR at various points across the band. Brief testing beyond the band edges may also help understand the antenna.

If the low SWR point is below the desired frequency, remove a few millimetres from the top of the brazing rod, by filing of cutting. If the frequency is higher you can try to heat the ferrule, and slide the element out slightly, or otherwise replace it with a marginally longer element, and keep the one for future projects, for a higher band. You could also thread the top with a die, and fit a threaded spacer.

Lengths

to get the length we divide 299.7 by the frequency in MHz, to get the wavelength, then multiply by 0.25 (divide by 4) to get the quarter wave. Again, multiplying by 0.95 takes end-effect into account. This last step is the same as subtracting 5%. To get a figure for the top of the division 299.7 × 0.25 × 0.95 = 71.17875.

Thus L = 71.17875 / f.

For 146.520 MHz, the US call frequency 71.17875 / 146.52 = 0.4857954545 metres. This translates to 486 mm. If the the ferrule is 30 mm long, and the hole 10 mm deep, subtract 20 mm, so use 466 mm of rod, maybe a few mm more for safety.

Note that approaching 1 metre, the rod will likely be too floppy, so a different material might be used, or make a ferrule from a longer section of 3/4" or 19 mm hex, and use 5 mm rod, although this may place a fair bit of load on the mounting. For upper UHF you may wish to use a 1/2" (12.7 mm) rod, and make it as small as possible. You could also use round bar.

I've included a few frequencies, such as 156.8 MHz in the Marine band, as at some point an emergency support group you are in might be asked to facilitate something like an agricultural agency taking to a river patrol agency to enforce check-points preventing movement of potentially infected livestock.

Other frequencies

You may have access to licensed frequencies as part of a communications group, or to GMRS, MURS, PMR, or other services (UHF CB is already shown above at 477 MHz).

Assuming 123.45 MHz, type: 71.17875 ÷ 123.45 = which will give something like 0.5765795868733, meaning 576 (or 577) millimetres.

You can copy the value below, then use the online calculator below, or one on your PC or device.

Online calculator: RPN Calculator.

If using the above calculator, paste the figure above (Note: if you didn't get a copy-confirmed pop-up, copy this: 71.17875 ) into the lowest blank field, press the Enter button or key, enter the frequency in MHz, then click the / button or key. The length will be presented in metres.

RPN, Reverse Polish Notation, or Postfix (invented by Jan Łukasiewicz) uses a data entry order where the two values are entered, then the operation, such as 57 ENTER 23 +, giving 80.

You could use these calculations for other quarter-wave antennas, or each side of a half-wave dipole.

For SO-239 bases

From around 2 metres and up, an alternative for those with an SO-239 style base might be to use thinner rod inserted into a (true) PL-259 plug, designed for RG-8. Fill the back of the plug with epoxy resin, etc, noting that any holes under the threaded ring should be covered with tape first.

The calculated length above should be the length exposed beyond the top of the plug.

You can find links to training material on the Learning Material page.

These pages have taken a fair bit of work to write, so if you have found this useful, there is a "tip jar" below.

Written by Julian Sortland, VK2YJS & AG6LE, September 2022.

Tip Jar: A Jefferson (US$2), A$3, 3 Swiss Francs or another amount or currency. Thanks!