Well said. Thank you Jeff. Chuck WB2EDV
----- Original Message ----- From: "Jeff DePolo" <[EMAIL PROTECTED]> To: <Repeater-Builder@yahoogroups.com> Sent: Saturday, June 07, 2008 9:25 AM Subject: RE: [Repeater-Builder] Re: antenna suggestions for 440mhz > >> First off you will definitely loose some of the gain from the original >> "rated specs". About 1.5 - 2 Dbd. is what I'd expect...Maybe a little >> worse depending on where the antenna was originally "cut for"... For >> example, If the antenna was originally built for say a 454.xxx freq >> (center freq) the loss would be less at 442.xxx than one that had been >> built for 467.xxx. The antenna typically has a 20Mhz. bandwidth, +/- >> 10Mhz each side of center freq so you can see that one cut for the >> upper 460-ish range would be a little worse than the 450-ish freq. > > DB404/408/420 antennas were never "cut to frequency" - they were sold in > frequency ranges. For example, a DB408-A is 406-420, DB408-B is 450-470, > etc. So the closest one to 440 would be a DB4xx-B. If you ordered an > antenna for 454.575, you would get a -B series antenna. In the wayback > days, sometimes they would even stamp the label with the exact frequency > you > ordered, but the antenna wasn't "cut" for that frequency, they just marked > it to identify the requested frequency. Nowadays they don't even bother > to > do that, the sticker will just say "450-470 MHz" and it will come with a > return loss sweep showing its performance across the entire band. > > The more bays, the worse the VSWR will be (speaking in very general terms > here) as you operate these types of antennas out of band. For example, a > 408-B will likely have better return loss than a 420-B when used at 440 > MHz. > >> The next thing that would change would be the "downtilt" rating. > > No, parallel-fed antennas do NOT suffer uptilt/downtilt as frequency is > varied unless the harness was special-ordered for factory downtilt. If > the > antenna wasn't ordered with downtilt, all of the elements are fed in > phase, > and they will always be in phase regardless of frequency. The peak gain > will be reduced as you operate them away from their design range as more > power ends up in the usually-undesired sidelobes, but the main lobe will > still be on the horizon. > > Series-fed antennas (like Stationmasters) will have the elevation pattern > (downtilt/uptilt) affected as frequency changes, because the further up > the > antenna you go, the more and more the radiating elements end up being out > of > phase compared to the lower ones, thereby creating the uptilt/downtilt. > > This issue comes up so many times, and is so misunderstood, that there > should probably be a FAQ about it on the web site... > >> > I like the DB's but unsure how bad the 450mhz matches when trying to >> > use it down at 440.000mhz. > > I have 404's, 408's, 411's, 413's, and 420's in stock, all in the B > version, > a few A version, and a few "S-440-450" ham-band versions which they don't > make any more. I can sweep one if you want (please don't make me drag all > of them out of the warehouse to test). Email direct if you're interested. > > --- Jeff WN3A > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
