You may find this link helpful when looking at the matching harness for these dipoles: http://www.w4dex.com/ant.htm
It's appeared here before but will be helpful to see it again. 73, Tony W4ZT At 01:52 AM 4/18/2004, you wrote: >--- Chuck Kelsey <[EMAIL PROTECTED]> wrote: > > Just curious... how did you determine that each > > element was 50 ohms? > > > > I was always of the understanding that the Decibel > > design, each element was > > 100 ohms. Also, that the later versions of Decibel > > arrays used 50 ohm and 35 > > ohm cable, no 75 ohm stuff. > > > > Chuck > > WB2EDV > > > > > > > > ----- Original Message ----- > > From: "Al Wolfe" <[EMAIL PROTECTED]> > > To: <[email protected]> > > Sent: Saturday, April 17, 2004 6:20 PM > > Subject: [Repeater-Builder] Re: Decibel repeater > > antenna question...... > > > > > > > I've worked on a great many of these types of > > antenna arrays in the > > last > > > 35+ years and every one had an impedance of 50 > > ohms at the individual > > > element. An odd multiple of a 1/4 wavelength of 75 > > ohm coax takes it to > > 100 > > > ohms. When stacking elements two 100 ohm loads in > > parallel are 50. Then do > > > it again for four bays, again for eight, etc. > > > > > > In free space their impedance would be higher, > > but they are designed > > to > > > work only a few inched from a mast pipe and > > normally the elements are > > fairly > > > fat in terms of diameter to length ratio Hence > > the nominal 50 ohm > > > impedance. > > > > > > Another scheme was to use two bays, make the > > feed line from each bay a > > > piece of 50 ohm cable, the length being > > unimportant other than being > > equal, > > > and tying them together for 25 ohms. Then a > > special 35 ohm 1/4 wave piece > > of > > > line brought it back to 50 ohms. Two pieces of 75 > > ohm cable in parallel > > > would do the same transformation but can be messy > > to fabricate. > > > > > > YMMV, though, > > > > > > 73, Al K9SI > > > > > > > > > <snip> > > > Judging from the cable and the lengths listed, > > each > > > dipole must present a 100 ohm impedence, not 50, > > > assuming the data is correct. > > > <snip> > > > > > > > > > >IF the lengths and types of coax are correctly >represented by the web article (and I don't know that >for a fact) THEN the impedence would work out to 100 >ohms per bay. 50 ohm impedence per bay does not work >out correctly, given the info from the web site. > >http://www.kc5dgc.net/db224.htm > >Taking their measurements, all coax sections are "Q" >sections, or impedence transformers. A 75 ohm Q >section will transform 50 to 100 ohs OR 100 to 50 >ohms. A 35 ohm q section will transform 50 ohms to 25 >ohms OR 25 ohm to 50 ohms. > >Now, you can start from either end if you know the >impedence. Since we know this antenna is 50 ohms at >the feed point, and from there it goes through a 35 >ohm Q section, the impedence at the first tee would be >25 ohms. Since 2 cables are in parallel at this tee, >each cable (at that point) must represent 50 ohms (2 >50 ohm resistors paralleled give 25 ohms). Now you can >take either leg at this point, since the top pair and >the bottom pair are identical. Going through the 35 >ohm Q section transforms our 50 ohms to 25 ohms. This >would be at the upper or lower tee. At this tee, to >have 25 ohms, we must have a pair of 50 ohm impedences >meeting at the tee. Now we go through a 75 ohm Q >section, which raises the impedence to 100 ohms, which >should be the impedence of each dipole, if we want the >thing to match. > >Going backwards, start at the dipole and assume 100 >ohms. The 75 ohm Q section changes this to 50 ohms. >Two 50 ohms in parallel is 25 ohms (at the top or >bottom tee). The 35 ohm Q section changes this to 50 >ohms, again in parallel with the bottom 2 bays which >would also be 50 ohms, which gives 25 ohms. The final >35 ohm Q section transforms the 25 ohms to 50 ohms, to >feed our coax of any length. > >Now, if we assume each dipole is 50 ohms, here's what >happens. The first 75 ohm Q section will increase the >impedence to 100 ohms. 2 100 ohms in parallel will >give 50 ohms. The 35 ohm Q section will transform that >to 25 ohms. Now we have two 25 ohms in parallel, >giving us 12.5 ohms going into the last Q section. By >using Q section calculations, this 35 ohm section >would transform the impedence to 100 ohms, or a 2 to 1 >match for our 50 ohm coaxial line. Although 2 to 1 >will work, it's generally not acceptable for repeater >use, where we like to see 1.2 to one or so, with 1.5 >being an outside margin. > >IF all the Q sections (except the first) were 75 ohm >cable, then you would have 50 ohms at each dipole. >This is the usual way of phasing 4 50 ohm antennas, >but these antennas must present a different impedence >for them to do what they do. > >Joe > > > > >__________________________________ >Do you Yahoo!? >Yahoo! Photos: High-quality 4x6 digital prints for 25� >http://photos.yahoo.com/ph/print_splash > > > > > >Yahoo! Groups Links > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
