John Sehring wrote: > > > 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...
Don't knock success. There are thousands of such antennas in operation and nobody complains about RF currant on the outside of the coax. > > 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. But can you weatherproof it for commercial usage? > > 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. From my experience, the folded dipole impedance only starts to drop when you get closer than 1/4 wavelength from the mounting mast. 1/4 or 1/2 wave makes no significant difference on the impedance. Sinclair's 110C1 low band antenna is however mounted about 1/8 wave from the support and has a different matching coax internally (less than 100ohm I think). > > 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. The design that I showed on the web page was for 2m and 220. I tried making a UHF array with the same 3/4 in diameter tubing and it was really flakey. I found it easier to work with 3/8in tubing at UHF. I did think about making 900 or higher dipoles with copper tubing type hardline like 0.141 in stuff used for microwave lines. I could not figure out how to change the impedance with the normally available 50 ohm stuff. It never got beyond the thinking stage. Burt VE2BMQ > > > --- On *Thu, 8/27/09, Burt Lang /<b...@gorum.ca>/* wrote: > > > From: Burt Lang <b...@gorum.ca> > Subject: Re: [Repeater-Builder] 2 meter folded dipoles for multi bay > design information needed > To: Repeater-Builder@yahoogroups.com > 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 <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 > > >
