> Same gain - same coverage. Fiberglass encased or exposed > dipole - 5.8 dBd = 5.8 dBd - this is about the max you can > expect from a 2M antenna before the law of diminishing > returns kicks in. More Gain for $$$$BUCKS
An Amen is heard from the crowd... > - look at a DB228 - 40 some odd ft long exposed dipole array > and an offset pattern. Aside from the shear size and weight, the 228 tis a monster working antenna if you can get one put up. But my god it's also a real physical monster to handle. > Some of the numbers I have seen published for antennas > are pure bull. Let the buyer be ware. You mean people tell stories? > Fiberglass encased antennas get blown to kingdom come more > often than exposed dipole arrays. If you take a direct hit, you're probably going to have to work on the entire antenna system (and the radio gear) regardless of the type antenna you have in place. That is unless you're deeply in bed with incredible grounding and Polyphasor (or some other similar product). > Exposed dipole arrays suffer loose hardware and noise when > you run in duplex Which is why you take great care to ensure everything is tight, sealed and locked down to ensure you don't suffer an unwanted gremlin (aka the grunge monster). > learn about isolated TEEs and effective sensitivity before > you go any further. Which can also make you feel somewhat helpless after you find out how much desense your site actually has. > Ron, N9EE makes a lot of sense on this one - easy to get > to and change - use the diamond or comet job - also hustler. Only if you can easily get the antenna changed without a lot of grief. Diamond and Comet antennas tend to get internally beat to hell in strong windy areas. I expect a commercial quality antenna to last at least a decade. In a strong windy area I'd maybe trust the Diamond - Comet Antennas for a few years or otherwise place an RF Sample unit into the feed to ensure the reflected power is under control. Don't believe me... pull the fiberglass radome off a Diamond or Comet Antenna and see for yourself. I'm not putting a Diamond or Comet Antenna at the top of a really high commercial tower (in a bad area) when the owner requires an expensive tower-person to mount it. > Nate hit the nail on the head about Height...Why have a > 100 plus watt repeater in a non-voted system with 2 to > 4 watt talkies trying to talk in.... Simple... Because hearing a strong repeater output is a real positive reinforcement to many people. Not everyone runs a walkie talkie (portable) radio. The strong repeater more often than not encourages lower power users to try different antennas, small amplifiers and locations that might otherwise not be of obvious interest. It can be very positive thing to some folks and a real pain in the a$$ to others. > Don't make much sense and won't buy you any coverage - Depends on ones opinion... and I would probably say it does add a measure of coverage. If you're looking at hard on paper numbers... maybe not. But typical repeater radio systems always seem to be very non-linear operations... > now one way paging, or tall tower with split send and rec > antennas and TTA - makes a whole new game plan. The antenna combiner and split receive layout can be a real rocket ship (a good thing).... but I've also seen some really, really amazing things done on single duplexed and diplexed antennas. Things I would have never tried until I saw (and heard) it demonstrated in action. A few friends of mine have tried to buck the "no free lunch" rule and actually seemed to have gotten away with it. > Best option - birds nest on the ground - find a tall building > WITHOUT radio tenants - A congested site that cannot receive > is close to useless unless you have a separate receive site, > or voting receivers. Sometimes you have to take what you can get... but a numb radio site is a real bummer. Especially if there's little other choice of available other options. > You can also reach a point of diminishing returns on antenna > height - remember 96.6 plus 20 times the common log of the > distance in miles plus 20 times the common log of the frequency > in Ghz will yield path loss. Start with a 4 watt talkie in the > clear (deduct 20 dB for wearing the talkie on your belt in a > car), and calculate to space loss, antenna gain, line and > duplexer loss, and see how much over say -120 dBm actually > gets to the receiver. Most receivers get 12 dB sinad at > around 118 to 122 dBm at the antenna. Don't forget effective > sensitivity - just because the receive can hear -119 from a > generator don't mean the noise from a paging or fm broadcast > station won't stifle the receiver. Also cavity filters don't > necessarily block out of band signals. Anyhow, the point > is that when the subscriber unit lacks enough RF to get to > the receiver due to path loss, antenna height won't help. True in hard numbers... but how many people are talking on portables from their belts? ... or on belts in a car? With proper care and feeding of the receiver (through filters or duplexer and pre-amplifiers) and a little planning enough usable signal can make the trip inbound... > 50 ft - I would probably use 1/2" heliax, and first use > whatever antenna I have laying around. If I have to reach > in my pocket and buy the thing, then the $200 job looks > pretty good (because at 50 ft it is easy to change out > when it craps out) - Don't forget proper grounding, > grounding kits on the transmission line, polyphasers, etc. I agree... if 1/2" heliax is out of the budget... then some of the better types of flexible coax products would work fairly well. Just stay away from LMR-400 type lines... > Another thing - is the site in the middle of the desired > coverage area? If not, an offset pattern antenna may be > in order (no need to pump signal into an area where there > are no users. If the 50 ft tower is on a big mountain > overlooking the town, then both an offset pattern AND > depressed pattern may be in order. So many people miss the boat in regards to smart antenna system planning. A number of local gents run distant mountain repeaters on yagi antennas with great results. > Anyhow that is how I would handle the question for my own > use. As always I may be wrong, your mileage may vary, etc. > Best 73, > Steve NU5D I don't know... everything you said makes sense. I come from the "lots of power where possible is a good thing" school and after talking with a number of RB Group folks in person I'm also sure you can find a lot of people who sit on the other side of the fence regarding high power level repeaters. But I certainly can't help you figure out who to vote for in November... Cheers Steve, skipp
