On Nov 23, 2007, at 3:55 PM, Derek wrote:
I've used the DB-408 antenna and am happy with it's performance, but
am
wondering about significant difference in using a DB-420 for future
repeaters. Also considering the RFS 1151 (Tessco # 435830) fiberglass
antenna. It is tuned for 440-450 MHz and has 8dB gain, but I've heard
some say fiberglass is not the way to go for repeaters.
In reality, whatever works is the answer for you.
But in my *opinion*:
I won't use fiberglass sticks (unless I get them for free)... too much
lightning out here, and what's left of them after a direct strike will
fill a hefty bag nicely, as one local repeater builder type said
about his that he found scattered all over a mountain-top.
If I do use them, they're always side-mounted and always with a top
stay arm to keep them from destroying themselves with vibration in the
wind, and we try to make a good guess at what the side mounting will
do to the pattern and adjust for it accordingly.
Obviously I'm a fan of the folded-dipole style antennas. They
massively broad-banded when the element size is nice and fat, and
therefore, I really like the Sinclair stuff. The harness is inside
the mast on the good models, out of the elements where it belongs, not
out in the sunlight (UV rays up here will eat a harness alive, since
the sites are always above 9000' MSL), and even the regular ones are
very heavy-duty. I've seen idiots climbing towers step on them,
putting their feet in the wrong place, and they don't give. Those
same folks wouldn't think of stepping on a 420 or 408, it wouldn't
hold them, and they'd know it.
Sinclair also makes heavy duty ones that are even more beefy. They
also make low IMD ones for sites where passive intermod and mixing are
a problem. You can at least claim you've done all you can to not add
to the insanity.
With that said, and with Sinclairs on almost all of our systems now --
I have heard systems that sounded great on both the DB-420 and the
DB-408. I won't deny that.
But...
I don't think the comments about where you want to put your signal
apply as much as some folks would have you believe. Even though the
408 pushes more gain to the horizon, it still is rated for something
like 7 degrees of vertical beamwidth to the 3 dB down points.
During a local discussion here recently someone also pointed out that
the 408 and that other one with 10 dipoles (sheesh!) that they make
now, have a lot of really odd-ball types of coax in their harnesses
for the phasing, and there's a LOT of it. How much energy is really
making it to the dipoles in a 408, and how much is just radiated from
all that relatively lossy coax in the phasing harness itself? We'll
all never know, I guess. But there *has* to be some loss there. I'd
be curious what others think about that, since it was just a local
discussion over a beer after hanging another Sinclair. (GRIN)
Back to the angle of the dangle question -- unless trigonometry has
suddenly failed or the manufacturer's specifications aren't really
that accurate, I'd say the signal is going to cover both close-in and
far away just fine for any repeater below about 5000' above average
terrain. Up that high, a degree or two of electrical down-tilt would
be nice to cover in close.
Other's experiences with the 408 and other high-gain antennas not
reaching close-in stations very well make me wonder if something else
was going on. Multipath and various forms of fading can really be a
bear in urban or hilly environments.
7 degrees down with 3 dB of difference in signal, right in close,
shouldn't make that much of a difference, since you're closer to the
repeater. I'll admit though, again, that the Sinclair 4-bay's numbers
for vertical beamwidth sure look a lot nicer for a building-top system
than the DB's... it has something like 9 more degrees of vertical
beamwidth to it's 3dB points. Yeah, that means some of our signal is
going up, where it's not needed... but with gain numbers similar to
the DB's and a wider beamwidth, doesn't that say something about the
antenna itself? Just my opinion...
(That 7 degree number for the 408 is from memory, but you can get the
specs and do the triangle math yourself, pretty easily... fire up the
old pythagorean theorem and do some engineering... then decide. I ran
the numbers for the Sinclair 4-bay at our 11,440' MSL site that's over
5000' HAAT, to see if down-tilt was needed. It had a MUCH larger
vertical beamwidth over the DB's, and the answer was, not needed at
all. 1 degree of electrical down-tilt would have been nice,
perhaps... to push just a tiny bit more signal into Denver... but not
really necessary at all.)
The one time I saw a 408 smoke everything else, the repeater was about
1500' HAAT and it was set up to push all of its signal to one side.
It was mounted upside-down under a platform on an old ATT
