The PD220 has 5.25dB gain and 18 degrees vertical beamwidth. The DB222 has 3dB gain in the omni configuration, and 36 degrees vertical beamwidth.
Vertical beamwidth is measured at the 3dB points on main lobe of the horizontal axis plot, so divide the published number in half and you can easily plot the main lobe pattern on graph paper using the installed height on the tower, and see what might or might not be an issue. |________________ INSTALLED HEIGHT |\ | \ ANGLE | \ | \ | \ _|_____\___________ GROUND The above is a very simplified idea of what you will be plotting. The vertical component is the tower, pick a convienient scale for height. The horizantal component is the ground elevation, be sure to keep the scale the same and add in any hills, valleys etc. as shown on a topo map. ANGLE is 1/2 of the published vertical beamwidth. INSTALLED HEIGHT is an imaginary horizontal line at the base of the antenna (or the center, or the top, it's just a reference line and the ANGLE is measured from it toward GROUND). The point where the line representing ANGLE meets the line representing GROUND is the approximate point where the -3dB point of the main lobe of the antenna will meet the ground. Other minor lobes, reflections etc. will give more close in fill coverage, but for planning and comparison purposes this will give you a good idea of what you might expect. Using the above antenna numbers, the PD220 main lobe will be 9 degrees below the imaginary horizontal line at the antenna base, and the DB-222 will be 18 degrees below the imaginary horizontal line at the antenna base. Therefore, the main lobe of the PD220 will touch down further away from the tower than the main lobe of the DB-222. Depending on the geography this may or may not make a difference in the coverage, it's just one of the many factors to consider. In general, the lower the gain of the antenna, the greater the vertical beamwidth. Just for fun try plotting a quarterwave antenna. The DB-201 ground plane (0dB gain) is speced at 78 degrees vertical beamwidth, thus the main lobe is 39 degrees below the imaginary horizontal line at the antenna base. ;-) BTW using a topo map and plotting radials around the repeater site can be a good way to get to know what things look like from your site. Of course buildings are not shown, but finding the hills and valleys can give you a new perspective on why things do or don't work. Once I had to try and figure out why an 800 MHz control station was not properly getting into a distant repeater. After getting the topo out and locating the repeater and control station and adding in the tower height at both ends of the path, I found that the direct, straight line path was through two hilltops. The station was getting by (poorly) on the indirect reflected path(s) that existed. The only way to solve the problem would have been to relocate the base to the top of the nearest hill. Good luck Milt N3LTQ ----- Original Message ----- From: "Tony L." <[EMAIL PROTECTED]> To: <Repeater-Builder@yahoogroups.com> Sent: Tuesday, November 27, 2007 11:47 AM Subject: [Repeater-Builder] VHF Repeater Antenna Choice > Any comments on the difference between expected antenna patterns and > coverage on a DB222 vs. Celwave/RFS PD220? > > I'm using a PD220 and find the local coverage to be somewhat spotty, > but coverage 20 miles away to be excellent. > > Is this typical of the PD220? Would a DB222 fill local areas any > better? > > --- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> > wrote: >> >> Dipole phasing is not easy even for the Decibel 420 type antennas. >> It is common for very sharp nearfield nulls and cancel areas >> expecially near and underneath a mountain top mounted antenna. >> I label the effect something similar to what I call unwanted >> nearfield/local re-entrant energy. >> >> Less of a similar antenna in the case of the DB-408 would have >> less gain but less close-in and below problematic areas. It >> would also have a different vertical radiation angle. >> >> Using at least one of all the Decibel DB-408 and DB-420 type >> antennas from a mountain top repeater site... I can tell you >> first hand there is quite a bit of difference in portable and >> distant in-building coverage using the higher gain Decibel DB-420 >> antenna. There is also something to be said for what I call the >> antenna capture area, which is the shear amount of dipole surface >> area (metal) spaced up and down many wave-lengths on the tower. >> >> In most cases there should never be "too much antenna" but there >> can be the wrong antenna for an application and location. >> >> One sidebar I noticed in your post... you weren't using a Decibel >> DB-420 Brand Antenna. The Signals Brand Antenna first used in your >> system is a different animal indeed. >> >> cheers, >> s. >> >> >> >> > "Chuck Kelsey" <kelsey@> wrote: >> > >> > We had a DB-420 style antenna (actually it was made by Signals, > but >> it was folded-dipole design) on our UHF repeater at work. We were >> constantly having difficulty with portables being able to hit and > hold >> the repeater and they were no more than 1/2 mile out. The local >> M/A-Com shop kept saying "too much antenna." We changed it out to a >> DB-408 and the problem was corrected. We are in rolling hills and > the >> antenna was about 70' above ground level at a water tank. I plotted >> the antenna pattern against topographic map data and discovered that >> the portables were in some deep nulls with the higher-gain antenna. >> > >> > In another instance, a UHF ham repeater on a pretty decent site > was >> using a DB-420 style antenna (I believe it was actually an Antenna >> Specialists version). It worked great out at the horizon, but closer >> in mobiles would become noisy and portables were tough. It got > changed >> to a Sinclair 4-element folded dipole, and the improvement was >> substantial. Slight loss out at the extremes of the coverage area. >> > >> > I'm convinced that bigger isn't always better. You need to use the >> right antenna for the intended coverage. If all of your users are > out >> at the extremes of where your repeater is located, the highest gain >> antenna might make more sense. I'd dare say that this usually isn't >> the case. >> > >> > Chuck >> > WB2EDV >> > >> > >> > >> > ----- Original Message ----- >> > From: Keith, KB7M >> > To: Repeater-Builder@yahoogroups.com >> > Sent: Monday, November 26, 2007 11:31 AM >> > Subject: Re: [Repeater-Builder] Re: Repeater Antenna Choice >> > >> > >> > The area served by many of our radio sites (we are in Central >> Utah), sit at approximately a 12 degree downtilt from the sites. > Most >> of these sites are at 3000-4000' AGL. In some cases, we have opted >> for lower gain antennas to cover close in areas better. We > designate >> repeaters as local or wide area coverage to account for this. Wide >> area repeaters get high gain antennas to aim for the horizon (about >> 50-100 miles out), and local area repeaters get lower gain antennas >> for about 5-20 miles out. In some cases we opt for directional >> antennas such as corner reflectors or dipole arrays with all > elements >> on one side of the mast when we want to cover the populated areas >> better at the expense of "the back country". >> > >> > -- >> > Keith McQueen >> > kb7m@ >> > 801-224-9460 >> > >> > > > > > > > > Yahoo! Groups Links > > >
