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 AT&T microwave tower, so the platform may have been acting like a bit of a reflector and a bit of an upside-down ground plane over the top of the antenna. Most of the users were about 15 miles away from the tower. That thing rocked. It was great until the tower took a lightning strike, and the antenna was arc-welded together (the two sections never came apart again) and two holes were burnt through its mast where the lightning jumped back to the tower. The second 408 after the strike up there also walked the dog. Great site, great antenna for that particular site. Too bad the repeater's not there anymore. Worked well. So really -- "Whatever works!" -- but I like the Sinclairs. :-) -- Nate Duehr, WY0X [EMAIL PROTECTED]
