The feedpoint for any of these antennas will be one value in free space, another near a conductive mounting pole, another when mounted near another resonant dipole, and yet another when fed in a phased array with that nearby resonant dipole.
The center two dipoles of a four-element colinear also behave quite differently from the outer ones in terms of impedance and current distribution when close-spaced. If you get to play with an antenna modeling program such as EZNEC, you'll be amazed at the subtle differences. (And frustrated if you're a perfectionist!) 73, Paul, AE4KR ----- Original Message ----- From: skipp025 To: [email protected] Sent: Thursday, March 06, 2008 5:06 PM 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. > > >
