srm@dimacs.rutgers.edu (Scott R. Myers) (12/16/90)
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?
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 guage 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.
Scott R. Myers
Snail: 26 Stiles Street Phone:(201)352-4162
Apartment 18 Elizabeth, NJ 07201
Arpa: srm@dimacs.rutgers.edu Uucp: ..!dimacs!srm
[Moderator's Note: In any form of radio service, antennas are 'where
it is at' when discussing the overall performance of the radio.
Antennas are, IMHO, 75 percent of the radio's performance. The least
expensive radio will talk like a million dollars when the antenna is
properly tuned. In long-ago days, when I ran, again IMHO, a *very
good* CB site involved with Northern Illinois REACT, I fretted about
the antenna constantly. With cellular though, and the saturation of
cell-sites in metro areas, I dunno how much you need to worry. I had a
5db antenna on my handheld (which could easily be used on the bag
phone also) and it poked me in the ribs constantly, and was always
getting bent and banged around. I swapped it for a tiny little 1/8
wave antenna which is barely noticeable. The difference is miniscule,
but that is the Chicago cellular scene. Where you're at may be a lot
different. Reader comments? PAT]julian@apple.com (Julian Macassey) (12/17/90)
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
Tad.Cook@cs.washington.edu (12/18/90)
In article <15427@accuvax.nwu.edu>, srm@dimacs.rutgers.edu (Scott R. Myers) writes: > 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? 'Dbs' are decibels. It is an expression of a ratio of two power levels. The db gain figure is against a reference antenna, which is probably a 1/4 wave (about three inches long at cellular freqs) ground plane. Decibels are on a logarithmic scale. For power, you can figure it as decibels = ten times the log of the ratio of the two powers. So 3 db gain is the same as doubling your power output. 5 db is the same as multiplying your power by about 3.16. But remember that antenna gain also helps the received signal. There is quite a bit of loss through the glass on the glass mounted type, particularly if it is near any defrosting elements. The best mount is in the center of the roof with a hole drilled in the roof. The worst is when you use a glass mount on one of those side windows on a van, so that the antenna is below the roof line. I like the looks of these trunk mount ones that are on a long sleeve, that puts them high above the trunk of the car. > 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 guage 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. RG58 has quite a bit of loss at 900 MHz. RG8/U would be better, although it is larger and harder to install. There is a mil spec grade of RG8 that is best, although with just a few feet of line it may not make much difference. At 900 MHz there can be a lot of loss in improperly installed fittings and antennas, so best to use a professional installer. I wonder about some of these deals that I see for phones "$200, installed" at the local auto supply. Tad Cook Seattle, WA Packet: KT7H @ N7HFZ.WA.USA.NA Phone: 206/527-4089 MCI Mail: 3288544 Telex: 6503288544 MCI UW USENET:...uw-beaver!sumax!amc-gw!ssc!tad or, tad@ssc.UUCP
dalyb@ncar.ucar.edu (Brian Daly) (12/19/90)
In article <15427@accuvax.nwu.edu>, srm@dimacs.rutgers.edu (Scott R. Myers) writes: > 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? > 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 guage of > coax reduce the amount of signal loss from the cable? First, let's tackle the dB question: Say I have two radio transmitters. Transmitter A has an output power of ten watts, and transmitter B has an output power of one watt. One way of representing the ratio of transmitter A 's power to transmitter B's is using the decibel. The decibel is defined as: db = 10 log (P2/P1) So, in my example, the ratio of transmitter A's power to B's is: db = 10 log (10/1) = 10 db (note that the log is to the base 10) So, a 10db ratio represents a 10:1 power ratio. Now suppose transmitter A had a power output of two watts; now the db power ratio is 10 log (2/1) or 3db. thus a 2:1 power ratio represents a 3db power ratio. If I do this for other values of transmitter A power, I find that every time I double transmitter A's power (keeping B constant), I increase the db ratio by 3 db. Thus, doubling the power is the same as a 3db increase. Now, how does this apply to antennas? A measure of antenna efficiency is the gain (which is a power quantity). To identify the gain of a particular antenna, you have to have something to compare it to; as the above example demonstrated, the db is a measure of power ratios. Usually we refer to the gain of an antenna as compared to an isotropic source (an isotropic source is a lossless antenna that radiates power uniformly in all directions). The isotropic source has a unity power gain, or 0db. This is the reference. Looking at the two antennas you mentioned, a 3 db trunk mount and a 5 db window mount -- these measurements are against the same reference. From the above discussion, the 3 db antenna has twice the gain as the reference, and the 5 db has 3.16 times the gain. How are db's used? Let's say your transmitter (cellular phone) has a 5db power output, and you have 2 db loss in the coax running from the phone to the antenna. With the 3 db antenna, your overall system performance is: 5db - 2db + 3 db = 6 db. With the 5 db antenna: 5 db - 2 db + 5 db = 8 db. So, keeping everything else constant, the 5 db antenna will give better performance than the 3 db. Another important factor with the antenna is the placement of the antenna on the car. This will have an effect on the antenna pattern -- the pattern is a "picture" of how the electromagnetic energy is transmitted. An antenna will not radiate uniformly in all directions; the antenna might transmit better in one direction, and worse in another. This pattern is affected by the metal body of a car. To get the best pattern, I've usually had success with placing the antenna directly in the center of the roof. Placing the antenna on the rear window or trunk will change the pattern. On the subject of coax, you need 50 ohm cable for your cellular system. RG58 is by far the most common, and least expensive. It has an attenuation of about 20db per 100 feet, which is not great. However, the distance from the trunk to the front passanger side of a car is less that ten to fifteen feet, so you should be OK. There are better cables available, but these are usually larger in diameter which might not be good for automotive installations. I'd recommend RG-58A/U (specify this type -- it contains not only a copper braid, but an aluminum foil shield). At 900MHz, it has a loss of 13.8 db per 100 feet. There is one possible flaw in this however. I assume that both antennas in question were measured against an isotropic source. However, as one of my colleagues here at AGCS pointed out, you need to make certain of that fact.... > Looking at the two antennas you mentioned, a 3 db trunk mount and a > 5 db window mount -- these measurements are against the same > reference. From the above discussion, the 3 db antenna has twice the > gain as the reference, and the 5 db has 3.16 times the gain. Ah, but therein lies the rub - sometimes these AREN'T measured against the same reference. Sometimes dBi (dB gain relative to an isotropic point source) are used, and sometimes it's dBd (dB gain relative to a dipole at that frequency). I believe a dipole has about 1 dB gain over a point source. So it's more attractive for manufacturers to quote dBi if others quote dBd. I've seen this with ham and CB antennas, don't know if the cellular folks are doing it or not. Thus, when you read antenna specifications, make certain you are comparing apples to apples! Brian K. Daly WB7OML @ AG Communication Systems, Phoenix, Arizona UUCP: {...!ames!ncar!noao!asuvax | uunet!zardoz!hrc | att}!gtephx!dalyb Phone: (602) 582-7644 FAX: (602) 582-7111
skaggs@nsslsun.gcn.uoknor.edu (Gary Skaggs) (12/20/90)
In article <15517@accuvax.nwu.edu> asuvax!godzilla!dalyb@ncar.ucar.edu (Brian Daly) writes: >In article <15427@accuvax.nwu.edu>, srm@dimacs.rutgers.edu (Scott R. >Myers) writes: >> 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 guage of >> coax reduce the amount of signal loss from the cable? >First, let's tackle the dB question: An excellent discussion of db by Brian deleted ... Brian metions the use of RG-58A/U cable in the installation. In fact, the Larsen 800-900 Mhz antennas use a double copper braid in their coax. If you were going to do it yourself, I would recommend Belden 9311. It has a layer of "Duobond(r) II" (aluminum foil) with a braid of coax around it. Its loss is much less compared to standard RG-58A/U. My Belden book says: RG-58A/U (Tinned copper braid, 96% shield coverage) 20.0 db/100ft @900 Mhz. This is Belden code 8259: RG-58A/U (Duobond, etc, 100% shield) 12.5 db/100ft @900 Mhz. One caveat: 9311 is cellular polyethylene. Be careful with heat, do your soldering hot and FAST. And don't run it through door seals, trunk lip seals, etc. It will compress more easily than standard and cause an impendance 'bump' at that point. Gary Skaggs - WB5ULK skaggs@nssl.gcn.uoknor.edu DOC/NOAA/ERL/NSSL