The only possible problem with this design is feeding a balanced antenna (the folded dipole) with unbalanced feed. This will lead to some amount of feedline radiation; yes, it is using coax which is shielded but the unwanted RF current in this situation flows on the _outside_ of the coax, on the shield.
Yes, this is a commercially successful design, but I've got to wonder... I think a better way would be to use a half-wave section of 50 coax as a combination 4:1 impedance transformer _and_ balun. Details in almost any ARRL Antenna Handbook. The only catch to this approach is that the folded dipole element must be designed to yield 200 ohms rather than the more common 300 ohms impedance, so that the 4:1 transform yields 50 ohms. Again, older versions of the ARRL Ant. Hdbk have details on doing this; it has to do with the element diameter & element spacing. However, mounting of the antenna on a tower leg or equiv. may reduce the impedance from 300 ohms enough all by itself. When scaling antennas to other frequencies, pay close attention to the physical scaling. On 1.2 GHz, RF "things" can get quirky. Do not cut specific-length lines (e.g. phasing lines) using a ruler & the cable's alleged velocity factor, that can be quite a way off. I use a noise bridge instead or network analyzer. --John --- On Thu, 8/27/09, Burt Lang <[email protected]> wrote: From: Burt Lang <[email protected]> Subject: Re: [Repeater-Builder] 2 meter folded dipoles for multi bay design information needed To: [email protected] Date: Thursday, August 27, 2009, 7:21 AM The technique to match the natural 300 ohm impedance of a folded dipole to 50 ohms simply uses matching section of an electrical 1/4 wavelength of 125 ohm coax (RG-63B). It is usually installing inside the dipole in order to be able to weatherproof the dipole easily at the feedpoint but I imagine it could be installed external as well. I have a page with a pictorial of the design I used some years ago. I made a number of arrays for VHF and 220 MHz using this design some time ago. http://www.gorum. ca/sinc_ant. html (ignore the links at the bottom - I didn't finish the web page set) RG-63B is very hard to find and it is not cheap ($3/ft) but I do have a quantity left over from when I was making the dipoles 15-20 years ago. Burt VE2BMQ wd4nmq_1 wrote: > I am looking at building a 2M four bay antenna, But, I have a > question > > My question is the design and construction of folded dipole antennas > used as individual elements in bay type antennas. In doing research > into their construction I came upon a catch. All design info I found > in books, ARRL Antenna Book, etc, says a two element folded dipole > has a nominal input impedance of 300 ohms. But, all sources, DB > Products, Benelec, etc, I see say the nominal input impedance for > each folded dipole element is 50 ohms, 300. I reference WB2EDV's > article on a 440 antenna he built. > > But, the bottom line is can anybody point me to where I can find the > info on designing each element for 50 ohm impedance? > > Jeff wd4nmq
