That's correct. The folded dipole impedance is 300 ohms. The 1/4 wavelength of 125 ohm coax transforms that down close to 50 ohms. This 1/4 wavelength matching section is completely inside the dipole itself. The transition to 50 ohm cable occurs somewhere near the top of the folded dipole, so we see the 50 ohm cable exiting the dipole.
In my dipoles the 125 ohm cable is RG-63B/U which, owing to its partly air dielectric, no doubt has a higher velocity factor than solid dielectric coax. So the section is somewhat longer than 13.5 inches. I'm still trying to find a reference to the exact velocity factor of RG-63B/U. It sounds like you have some very interesting (and rare) Sinclair documentation there! Paul N1BUG Larry Horlick wrote: > > > On the drawing it does not show any 125 ohm cable, but I think what you > are saying is that > from the feedpoint of the folded dipole, inside the tubing there is a > 1/4 wavelength piece of > 125 ohm cable (about 13.5 inches at 2m) that is joined to 50 ohm cable. > What we see exiting > the tube (opposite the feedpoint) is the 50 ohm stuff. If this is > correct it fully explains a drawing > on the previous page showing a cross section of a single element folder > dipole. \ > > lh > > On 5/18/10, *N1BUG* <p...@n1bug.com <mailto:p...@n1bug.com>> wrote: > > > > Thanks. That makes sense and should work out quite well for a > harness external to the mast. Of course the quarter wave of 125 ohm > coax will still be required inside each dipole, but the use of all > 50 ohm coax beyond that point simplifies construction. > > Apparently Sinclair had different ways of doing it, perhaps > depending on the exact model. Or maybe they changed the harness > design at some point. > > Paul N1BUG > > Larry Horlick wrote: > > > > > > I have a drawing from Sinclair that shows 4 stacked folded dipoles > (it > > does not indicate an > > antenna model) using all 50 ohm cable. So using the 210C4 harness > > picture from the link > > below as a template, this is how it's done: > > > > Feedlines from dipole A, B, C, and D are any length, but identical. A > > and B go to a tee, > > C and D go to another tee. The feedlines from the output (if I am > > allowed to use that rather > > crude term!) of these tees are any odd 1/4 wavelength (but do not > have > > to be the same) and go > > to a 3rd tee. The output of this tee is 50 ohms. I suspect that the > > harness does not affect the > > pattern, but rather it is the dipole to mast spacing. > > > > lh > > > ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Repeater-Builder/join (Yahoo! ID required) <*> To change settings via email: repeater-builder-dig...@yahoogroups.com repeater-builder-fullfeatu...@yahoogroups.com <*> To unsubscribe from this group, send an email to: repeater-builder-unsubscr...@yahoogroups.com <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
