Hi All Thanks to all for the many responses to my questions.
I see many different ideas on the feed impedance of the dB224 and dB420 stacked dipoles. I have seen that the inpedance of a standard folded dipole (300 Ohm ) lowered when brought in close proximity to the mast. What is the recommended spacing between the uhf and vhf to the mast to go on the 100 Ohm impedence area ? Has antone done a antenna shootout between these antennas and other stacked arrays ? Regards Bradley Glen ZS5WT --- Tony King - W4ZT <[EMAIL PROTECTED]> wrote: > 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 > > > > > === message truncated === __________________________________ Do you Yahoo!? Yahoo! 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