Easiest way for the ocfd - especially good for testing prior to committment with a sturdier pipe-build:
Radio Shack #15-1253 300:75 ohm transformer. (See below for better isolated transformer). 18 inches of wire (gauge not critical) to one of the screws. 48 inches to the other screw. Use a female - female F connector. A small run of say 12 foot pre-connectorized RG-6 to scanner. Will need an F-to-BNC adapter at the scanner. Hang vertically, and ideally run the coax away from the wires horizontally for a few feet.
Antenna is broadband in matching, but actually varies from 100 to 400 ohms or so depending on frequency. Transformer lowers the impedance to get somewhat near 50-75 ohms plus or minus, and definitely not perfect, but doesn't need to be for general purpose duties. So exact impedance of coax used is not critical. Previous descriptions of project show that due to construction, while it has a relatively good impedance matching capability (although not ideal), the look-angles get very high angle at UHF and above.
This may or may not be an impediment when indoors, or in cramped conditions where you don't have a good line of sight anyway, and may depend on reflections instead of perfect vertical polarization especially at UHF or above.
This is a compromise antenna that needs empirical testing at your site to see if it picks up what you want, as compared to a more specialized antenna. In other words, don't overthink or go nuts trying to optimize what can't really be optimized. You can however experiment with tilted or other odd mounting / hanging schemes since the reception lobes are not perfect to start with and never will be. It is however a HUGE improvement over a rubber duck usually. Note that you will NEVER see one of these in commercial service.
You can use anything to hang the thing vertically, even taping it to glass when traveling with nothing more than blue painter's-tape for a week or so. Tape it to a section of pvc if you like, but still you have to run the coax away from it perpendicularly (horizontally more or less) for a few feet. Not recommended to tape it next to drywall or any other rf absorptive surface just like any other antenna. In this case, hanging with a thumbtack in the celing with the lightweight transformer version may be a better bet.
For a sturdier build, and a little bit better transformer since it is
isolated, use a
Radio Shack #15-1230 or 15-1140. This is the one with twinlead leads. You don't want to stretch the twinlead too far apart - try to keep them parallel to each other for the whole length. Use a two-position barrier strip (or whatever) for attachment to the wires. I had a 4 position #274-658 laying around and two of the positions go unused. The reason is that you don't want to spread the twinlead section too far apart as it will then become part of the antenna elements, and your wire lengths will then be of the wrong dimensions.
Of course if you commit to tubing for the elements, then a direct attachment from the transformer twinlead to the *ends* of the tubing sections is ok - the key thing to avoid is spreading the twinlead too far apart for the same reason that the tubing lengths are now the wrong length with the spread-out twinlead actually becoming part of the overall antenna. But this is NOT caliper-measured critical - common construction methods ok with a little bit of sloppiness. Try the wire versions first before building one with tubing as you may be very disappointed depending on circumstances.
Tubing diameter is also not critical - just use what gives you the necessary stiffness for your application. What fat tubing does is broaden the initial 100-400 ohm impedance swings as it crosses the spectrum, but the offset feed and it's longwire-like high angle reception lobes at uhf and above will be more critical than matching is.
Warning - with the dimensions listed (48 inches one side and 18 inches the other), the primary resonance is in the FM broadcast band, so FM broadcaster overload *could* become an issue. We merely offset-feed (note that this is NOT a classical 1/3 offset, but tweaked) this halfwave to achieve wideband matching, but the reception lobes suffer in a purist environment. But in a poor location, this may not matter and the end-user will have to test it to give it a big thumbs-up, or immediately decide to throw it in the trash. I have done both depending on location
