s., This is true about the folded dipoles on some of these antennas. I've seen some remove them from the mast that came with them, re-mount on the leg of a tower and the SWR went wild. More so on UHF versions.
73, ron, n9ee/r >From: skipp025 <[EMAIL PROTECTED]> >Date: 2008/03/06 Thu PM 07:06:22 CST >To: [email protected] >Subject: [Repeater-Builder] Decibel and Telewave - Sinclair type Folded Dipole >Antennas > >> So the elements are 50 Ohms? It does help to know that. :) > >I've heard a number of stories regarding the actual feedpoint >impedance of the single Decibel Dipole. I've not actually confirmed >it to be near the 50 ohm value. What I will say is the key issue >to be aware of is dipole is next to a pole, which changes everything >from a free-space dipole of approx 200 ohms. > >So keep in mind the Decibel Brand of Dipoles have been reported >by various people to be a number of different impedance values >with the lastest value mention of 50 ohms. These dipoles are >adjacent to a pole where as other brands (Sinclair and Telewave) >are not. In operation the Decibel Dipole depends on the adjacent >pole. > >So for example the Sinclair Dipoles are approx 200 ohm and matched >using special sections of coax actually inside the antenna dipole. >The Sinclair type dipole are mounted on a mast or pole spaced 1/4 >or 1/2 wave distant and they should also operate freespace on a wood >mast (without a metal pole mount). > >The key difference to the person paying attention is the usable >bandwidth. > >The Decibel type dipole layout is a fairly straight-forward >percentage of the design frequency, where you see examples of >450-470 MHz operation specified. > >The Sinclair dipole design is much more broad in performance with >typical operation from 410 to 490 as an example. > >Also note the physical size differences in same band dipole between >the brands... which play a key in operation. > >Just some casual information to observe... > >cheers, >s. > >> >> But you seem to be calling for some kind of magical 35 Ohm coaxial >> transformer. >> >> Jeff >> >> --- In [email protected], Ron Wright <mccrpt@> 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 <jeffk@> >> > >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.
