I'm doing this from memory - I have the docs at home and can verify later.
The DB lowband dipoles are 50 ohm feed Z due to the close spacing to the
tower leg.
1 dipole - fed directly with 50 ohm coax (VB-8)
2 dipoles - fed with equal legs of 50 ohm coax (VB-8) to a tee, match 25
ohms from tee to 50 ohm feedline with quarter-wave transformer (35 ohm
VB-83)
3 dipoles - fed with equal legs of 50 ohm coax (VB-8) to two mated tees (two
mated tees give you four ports - three to bays, one for input) yielding 17
ohms. First transform 17 ohms to 72 ohms via a quarter-wave of 35 ohm
VB-83. Then transform 72 ohms to 50 ohms with a 'twelfth-wave' transformer
(1/12 wave of 50 ohm cable then 1/12 wave of 72/75 ohm cable) to result in
50 ohms to feedline.
4 dipoles - same as 2 dipole case, but add another tee, two more
equal-length 50 ohm cables from the added tee to the 35 ohm matching
sections on the bay pairs described above, and another final 35 ohm Q
section from the new tee to the feedline
These dipoles couple a lot of energy to the tower - you'll likely need even
more vertical isolation than what free-space curves might otherwise predict.
--- Jeff WN3A
> -----Original Message-----
> From: Repeater-Builder@yahoogroups.com
> [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Chuck Kelsey
> Sent: Monday, August 30, 2010 2:35 PM
> To: Repeater-Builder@yahoogroups.com
> Subject: Re: [Repeater-Builder] DB212-3
>
>
>
> Doug -
>
> Do you know how the phasing harness was constructed for the
> three-element
> version? I don't, and that's why I suggested to Norm that he
> go with four -
> the phasing harness is easy.
>
> Or, he could use two elements for transmit and one for
> receive. I don't know
> how much isolation he'll need, but he might just get away
> without a duplexer
> if there's enough tower.
>
> Chuck
> WB2EDV
>
> ----- Original Message -----
> From: "Doug Rehman" <d...@k4ac.com <mailto:doug%40k4ac.com> >
> To: <Repeater-Builder@yahoogroups.com
> <mailto:Repeater-Builder%40yahoogroups.com> >
> Sent: Monday, August 30, 2010 2:28 PM
> Subject: RE: [Repeater-Builder] DB212-3
>
> > In a previous life I managed the communications for a state
> police agency.
> > We used 45 MHz for our main system and had forty some odd
> tower sites,
> > almost all running DB212-3 antennas.
> >
> > Two of the sites were on 1000+ towers and used a single
> DB-212 element due
> > to the large tower face and the great height. One was a
> repeater using a
> > receive antenna at 1450' and a transmit antenna at 1350'.
> The other was a
> > remote base station with the single loop at about 850'.
> >
> > As we were an investigative agency, almost all of the
> mobiles were using
> > AM/FM disguise antennas. (Yeah, I know, but we were stuck
> with the band
> > that the State Division of Communications had dictated...)
> Despite the
> > radiating dummy load antennas, we had excellent mobile coverage in
> > virtually all of the state.
> >
> > A consideration for DB212 antennas is that lining them up
> on one leg can
> > make them pretty directional.
> >
> > For towers that were very close to the coast, I would put all three
> > elements on a single leg, but skew them so that only one
> was pointed
> > directly off of the leg. This seemed to give me a somewhat cardioid
> > pattern, but with a little better pattern to the back than
> if all three
> > elements were in line.
> >
> > Another consideration is that they were designed to be used on Rohn
> > 45/55/65 sized tower. If you put them all on one leg, a
> larger tower face
> > doesn't matter much except that the rearward pattern will
> likely have a
> > larger null. Mounting them on all three legs of a larger
> face tower will
> > result in reduced gain and a pretty messed up pattern.
> >
> > I don't know if I'd worry a whole lot about adding a fourth
> element- the
> > three element antenna will deliver excellent results.
> >
> > Doug
> > K4AC
> > (Running for ARRL Southeastern Division Director- please
> check out my
> > website at www.k4ac.com)
>
>
>
>
>
