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: Repeater-Builder@yahoogroups.com
>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 Repeater-Builder@yahoogroups.com, Ron Wright 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
>> > >Date: 2008/03/05 Wed PM 11:24:35 CST
>> > >To: Repeater-Builder@yahoogroups.com
>> > >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 Repeater-Builder@yahoogroups.com, "John Everson"
>> > > wrote:
>> > >>
>> > >> --- In Repeater-Builder@yahoogroups.com, "Jeff Kincaid"
>> > >> 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 Repeater-Builder@yahoogroups.com, "John Everson"
>> > >> > 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
