Jeff, As some have stated the coax coming off the DB212-2 dipoles is 50 Ohms. These two are connected to a T connector and these 2 will be in parallel making 25 Ohms.
The 35 Ohm coax, special for this application, forms what is called a coaxial transformer. Transformers can be used to convert to other impedances and coax can be used for this in RF. So a piece of 35 Ohm piece of coax at 1/4 electrical wavelength converts the 25 Ohms to 50 Ohms. This is the coaxial transformer. Hope this helps. 73, ron, n9ee/r >From: Jeff Kincaid <[EMAIL PROTECTED]> >Date: 2008/03/06 Thu PM 04:39:40 CST >To: [email protected] >Subject: [Repeater-Builder] Re: DB 212-2 Harness info. > >So the elements are 50 Ohms? It does help to know that. :) > >But you seem to be calling for some kind of magical 35 Ohm coaxial >transformer. > >Jeff > >--- In [email protected], Ron Wright <[EMAIL PROTECTED]> wrote: >> >> JK, >> >> The coaxial sections coming off each antenna element is 35 Ohm coax >at odd multiples of 1/4 wave length. >> >> What this does with antenna element at 50 Ohms then 1/4 wl down the >35 Ohm cable it looks close to 100 Ohms looking into this cable. Then >you take 2 of these ant elements/35 Ohm coax in parallel and you get >50 Ohms at this junction. If have 4 elements do the same for the >other 2 and then make another to join these 2 again using 35 Ohm 1/4 >wl coax. >> >> The length of the 50 Ohm coax going from the tx/rx to this antenna >input does not matter for SWR or impedance. Only concern would be >length and loss of 50 Ohm cable. >> >> 73, ron, n9ee/r >> >> >> >> >> >> >From: Jeff Kincaid <[EMAIL PROTECTED]> >> >Date: 2008/03/05 Wed PM 11:24:35 CST >> >To: [email protected] >> >Subject: [Repeater-Builder] Re: DB 212-2 Harness info. >> >> > >> >OK John, just making sure. I'm not familiar with the configuration of >> >the antenna, but what you are saying makes sense. I'm confused by the >> >idea that the length doesn't matter, because that would imply that >> >everything is 50 Ohms throughout, which doesn't seem to be the case. >> > >> >'JK >> > >> >--- In [email protected], "John Everson" >> ><johneverson@> wrote: >> >> >> >> --- In [email protected], "Jeff Kincaid" <jeffk@> >> >> wrote: >> >> > >> >> > I think this is just sloppy word choice, rather than a lack of >> >> > understanding, but I've seen it a couple of times now so let me >> >> clear >> >> > it up in any case. Regarding velocity factor, it's a factor, >so you >> >> > want to multiply it. Subtraction is not a part of the equation. >> >> > >> >> > Say your free space length is 100 inches, and your velocity factor >> >> is >> >> > 88%. Your length in cable is then 88 inches: >> >> > >> >> > 100 x 0.88 = 88 >> >> > >> >> > Regards, >> >> > Jeff >> >> > >> >> > --- In [email protected], "John Everson" >> >> > <johneverson@> wrote: >> >> > > >> >> > > So if I understand correctly, the length of the 50 ohm sections >> >> is >> >> > > irrelevant, as long as they are equal? Also, is my previous >> >> > > assumption that the matching stub is just made of 35 ohm cable >> >> cut to >> >> > > 1/4 wavelength at the operating frequency minus the vel. factor >> >> > > correct? >> >> > > >> >> > > Thans again for the help. >> >> > > John >> >> > >> >> Hello Jeff. >> >> >> >> You are correct. It was a poor choice of words on my part. I >> >> understand the theory of velocity factor. I am still not sure about >> >> the lengths involved with the harness. If the 50 ohm sections from >> >> the loops to the "tee" can be any length, (as long as they are the >> >> same) doesn't that affect the impedance at the tee? I will assume >the >> >> impedance at the feedpoint of the loop is 200 ohms. If we use half >> >> wavelengths of coax between the loops, the feedpoint impedance will >> >> be repeated at the end of the cable to the tee. Is this correct? If >> >> so, when the loops are connected to the tee, we should see 100 ohms, >> >> then, the matching section brings us to about 50 ohms. >> >> >> >> Is this correct? Or am I out to lunch? >> >> >> >> Is the matching section a quarter wave? >> >> >> >> I need to dig out the antenna handbook and refresh. That is for >> >> certain. >> >> >> >> Thanks. John >> >> >> > >> > >> >> >> Ron Wright, N9EE >> 727-376-6575 >> MICRO COMPUTER CONCEPTS >> Owner 146.64 repeater Tampa Bay, FL >> No tone, all are welcome. >> > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
