Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!accuvax.nwu.edu!nucsrl!telecom-request From: bongo!julian@apple.com (Julian Macassey) Newsgroups: comp.dcom.telecom Subject: Re: Information Needed About Cellular Antennas Message-ID: <15462@accuvax.nwu.edu> Date: 17 Dec 90 14:21:00 GMT Sender: news@accuvax.nwu.edu Reply-To: Julian Macassey Organization: Politically Correct Thought Division U.S.A. Lines: 88 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 10, Issue 886, Message 1 of 4 In article <15427@accuvax.nwu.edu> srm@dimacs.rutgers.edu (Scott R. Myers) writes: X-Telecom-Digest: Volume 10, Issue 883, Message 2 of 9 >I need an education on antennas. I'm purchasing a transportable and >want to install an antenna on my car for greater signal strength when >driving. I have a fair understanding of the different types (roof >mount, trunk mount, glass mount, etc.). What I want to know is the >layman's explanation of Db's. I see ads for 3db trunk mounts and 5db >glass mounts. The impression I'm given is the 5db glass mounts are >very acceptable for signal strength. How do they compare to roof and >trunk mounts? Just what are Db's anyhow? First of all, 'Db' is a Deci-Bell. Simply speaking 3dB represents a doubling of power. When discussing antennas (or anything else for that matter) you need to know "dB over what?" A half decent antenna can be 12dB better than a wet noodle. So to put it another way, a three watt cellular phone has 3dB more output. So yes, with 3dB more antenna, you could get away with a 1.5 watt output radio. But now consider that antenna gain (what the Db thing is discussing) applies to reception too. So a gain antenna will hear the site better too. Same goes for coax loss which is also measured in dB. If I had some coax that had a loss of 3dB per hundred foot at 800 Mhz (Cellular frequencies), then if I pumped three watts in one end, only 1.5 watts would come out the other. Alas coax loss is much higher than those figures which are just an illustration. I assume that the cellular industry judges its antennas as dB over a 1/4 wave ground plane antenna. But there are already several "fudge factors" in here. First of all, there is loss caused by coupling an antenna to the feed coax through a glass window, that is worth a few dB. Next there is the feeder loss (long run of RG-58. Then crummy connector assembly can loose some more. But the biggy is antenna position. If you put the antenna on the trunk, especially with a wimpy "no holes" mount, and the cell site is in front of the car, you are going too loose even more dB (power loss). The stickum on the glass antennas are often below the roof line of the car and so also exhibit loss. What I am leading up to is this: If you want maximum signal out of your antenna, bite the bullet, drill a hole in the middle of the car roof and put a real antenna in there. That way, you will have an antenna that does not have its own vehicle shadow it. You will also have the most height so it will see the cell site better. I realise you may be shunned by yuppies for not having a trendy stickum on the glass antenna, but that is the price you pay for performance. You may have to go to a real two-way radio shop to get this done. Many of the "cellular to go" shops don't have the tools or expertise to do this. They will waffle and lie telling you that their 10dB licky sticky special is much much better than a real antenna in the middle of the roof. >I have a second part to this question. I have read about the cabling >used in antenna installs (RG-58). Based on what I'm reading, there is >significant signal loss with that type of coax run long distances (ie. >trunk to front passenger side seat.). Would using another gauge of >coax reduce the amount of signal loss from the cable? Any other >recommendations to cut down on signal loss as well as the best antenna >configurations. Thanks in advance. Yes, coax matters, but usually in a car you do not have long runs. The guts of the radio are usually in the trunk - that is just the control head up front - so a run to the roof is six to ten feet. Also cable with better loss characteristics is thicker and stiffer so harder to route to the antenna. Yes, you can check it out, look at RG-8 and Belden 9913. Belden 9913 is like a garden hose full of ice. RG-8 can bend to a radius of maybe nine inches - the specs are available. One final thing: The cell site will adjust the power it sends to you depending on its received signal strength. So you with your 100% super duper install may be doing no better into a nearby cell site than the guy using a hand held unit next to you. Where you will notice the difference is in the fringe areas. It is because of the power adjustment circuitry and density of cell sites that so many poor installations work "good enough". How bad can they get? A friend had an installation done where the antenna connector was shorted, he put up with it for months. What told him that something was wrong was poor performance in the suburbs. Sorry it rambles, there is much to say on this subject, I have tried to be brief. I wish we could get questions like this in rec.ham-radio. Julian Macassey, n6are julian@bongo.info.com ucla-an!denwa!bongo!julian N6ARE@N6YN (Packet Radio) n6are.ampr.org [44.16.0.81] voice (213) 653-4495