| Author |
Message |
    
Snakedr
Junior Member
Username: Snakedr
Post Number: 12
Registered: 11-2005
| | Posted on Thursday, December 15, 2005 - 10:10 am: | 
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Good Mornin'Guys,
I have been thinking about setting up a base station and have been doing some research on antennas. What is DB Gain and Power Multiplication and how are they determined? Thanks in advance for your help.
Mike |
Bruce
Senior Member
Username: Bruce
Post Number: 3316
Registered: 9-2003
| | Posted on Thursday, December 15, 2005 - 1:08 pm: |
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This is a GOOD question!
Ok I'll keep it simple ......
gain in db -----
3DB = 2X power, 6DB = 4X power, 9DB = 8X power, 10DB = 10X power, 13 DB = 20X power and so on ....
Gain is base on the CHANGE in signal based on either isotropic or Dipole referance.
A isotropic sorce DOES NOT EXSIST but if it did would be 2.1 DB UNDER a dipole.
To get gain you must compress the pattern from it's starting point to a narrower beam pointed at the station you want to talk too..... like a flashlight beam gets briter you signal gets stronger.
I hope this helps answer your question.
Common gain ...... ( about )REAL gains .....
Skylab and A-99 under 2 DB
3 elm beam about 6 DB
5 elm beam about 8 db
2 meter antennas ......
Ringo ranger about 5 DB
4 bay stacked dipole omi is about 6 db offset 9 db
Bruce |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1057
Registered: 7-2004
| | Posted on Thursday, December 15, 2005 - 1:46 pm: |
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First of all do not go by advertised gain
figures on antennas. Most often they are
way to high and not correct. Some Companies use these tactics to sell antennas.
Omni antennas consist of the 1/4 wave groundplane,1/2 wave, 5/8 wave, .64 wave and
even larger. After .64 wave you begin to see
them actually become ineffective in certain ways. The 5/8 wave and .64 wave antennas are your best choice in my opinion for a Omni Antenna.
Db gain of an antenna can be measured over
a simple dipole antenna or over a Isotropic Plane.
Best thing to do is search the Web for related
articles on this subject or purchase books to
read. Maybe someone can recommend some good
reading for you.
I'll write alittle on beam antennas later as i'm in alot of pain right now.
JIM/PA/CEF 375
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Wildrat
Intermediate Member
Username: Wildrat
Post Number: 488
Registered: 12-2003
| | Posted on Thursday, December 15, 2005 - 8:01 pm: |
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Sorry to hear your hurting, I'm not just took my methadone a little while ago. What did you do to yourself RW. Did it have anything to do with stuborness? Or was it an accident? Hope ya get better, if not comeon down we can sit and watch my radio together, since I don't have an antenna in the air yet. I do have 30' of tower in the air though. The little one will be over tomorrow so maybe it will go up. I still have not made all my new jumpers yet. I did make some delicious peanut butter cookies today, and yes Istayed at a Holiday Inn Express last night.
WildRat
P.S. Snake Doctor I almost use a I-Max 2000 for my omni.When it goes in the air I will use it. It's marketed as a 5/8th wave but according to this web-site it's actually a .64 wave antenna. Here is one link you can read:
http://www.copperelectronics.com/cgi-bin/discus4/board-auth.cgi?lm=1009557886&file=/7750/9224.html
here's another:
http://www.copperelectronics.com/cgi-bin/discus4/board-auth.cgi?lm=1011570399&file=/7750/10782.html
and another:
http://www.copperelectronics.com/cgi-bin/discus4/board-auth.cgi?lm=1134158800&file=/7750/33226.html
Hope this helps.
WildRat |
Tech291
Moderator
Username: Tech291
Post Number: 324
Registered: 11-2004
| | Posted on Thursday, December 15, 2005 - 8:32 pm: |
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Bruce,
Good answer but(you knew this was comming)you left out the"JoGunn Gain Factor"
tech291
CEF#291
kc8zpj |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1058
Registered: 7-2004
| | Posted on Thursday, December 15, 2005 - 9:28 pm: |
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Yes Bruce,
You really need to explain about the Jo Gunn
Gain Factor. How it's DB gain is rated over
a butterknife and all that good audio gain
stuff.
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Wildrat
Intermediate Member
Username: Wildrat
Post Number: 489
Registered: 12-2003
| | Posted on Thursday, December 15, 2005 - 9:55 pm: |
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Isn't the Jo Gunn Gain Factor used on Cobra 29s with outputs above 80 watts? or am I mistaken?
WildRat |
Bruce
Senior Member
Username: Bruce
Post Number: 3317
Registered: 9-2003
| | Posted on Thursday, December 15, 2005 - 9:59 pm: |
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GUYS ...........
DONT GET ME STARTED ON AUDIO GAIN !!!!!!!!! |
Bruce
Senior Member
Username: Bruce
Post Number: 3318
Registered: 9-2003
| | Posted on Thursday, December 15, 2005 - 10:41 pm: |
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This is what you need to get 10 DBD real gain
 |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 2059
Registered: 7-2004
| | Posted on Friday, December 16, 2005 - 3:25 pm: |
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'This is what you need to get 10 DBD real gain'-do you know how BIG that thing is on 27mHz??????
go with the JO
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Wildrat
Advanced Member
Username: Wildrat
Post Number: 502
Registered: 12-2003
| | Posted on Friday, December 16, 2005 - 3:46 pm: |
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What kind of antenna is that Bruce? It's not CB band.
Wildrat |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1063
Registered: 7-2004
| | Posted on Friday, December 16, 2005 - 4:04 pm: |
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That's a great picture to include in
the posts that ask about rotor set-ups also.
Nice pic Bruce.
Is that a Jo Gunn mounted? LOL (Teasin..) |
Bruce
Senior Member
Username: Bruce
Post Number: 3319
Registered: 9-2003
| | Posted on Friday, December 16, 2005 - 6:30 pm: |
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ARROW stacked 4 elm 2 meter beams good stuff very well made |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 2067
Registered: 7-2004
| | Posted on Friday, December 16, 2005 - 7:15 pm: |
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'stacked 4 elm 2 meter beams...'-but the SWR's are TERRIBLE @ 27.205 mHz  |
Bronzepony
Junior Member
Username: Bronzepony
Post Number: 13
Registered: 12-2005
| | Posted on Monday, December 19, 2005 - 11:36 pm: |
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It takes 6 dg to gain one s unit every 15 feet of height is 6 db if you go from a 2 element pdl to a three element it is 2 s-units or 12 db |
Bruce
Senior Member
Username: Bruce
Post Number: 3329
Registered: 9-2003
| | Posted on Tuesday, December 20, 2005 - 9:22 am: |
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"It takes 6 dg to gain one s unit every 15 feet of height is 6 db if you go from a 2 element pdl to a three element it is 2 s-units or 12 db"
what?
Where did you get THAT from? TRUE there is hight gain because you have less objects to get in the way but its NOT 6 db per 15 foot. As for going from a 2 to 3 elem array you would gain 3DB. to get another 3 DB you would have to add 3 more directors. |
Astar
New member
Username: Astar
Post Number: 1
Registered: 9-2005
| | Posted on Saturday, January 28, 2006 - 4:38 am: |
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Im going to post some useful information here for you guys because you won't find the truth on the internet anywhere. Hopefully this will give you all something to think about.
In actual practice the db gain you get by the added height is as follows.
(6db = 1 s unit)
10 ft high = 0 db gain or 0 s unit increase
20 ft high = 6 db gain or 1 s unit increase
30 ft high = 10 db gain or 1.67 s unit increase
37 ft high = 11.4 db gain or 1.9 s unit increase
50 ft high = 15 db gain or 2.5 s unit increase
100 ft high = 20 db gain or 3.33 s unit increase
400 ft high = 30 db gain or 5 s unit increase
It would not be worthwhile to go over 80 feet. Keep in mind the FCC limits antenna height to 60 feet. 37 feet is as high as you can go with an IMAX and be legally at 60 feet. (on a tower or mast) Roughly every 5 feet of additional height gets you 1.2 miles of added range.
_________________________________________________
NOW some facts about DB gain.
(To get DBd gain subract 2.15 from DBi)
If you go through the trouble and expense to stack beams you get a lousy 3 DBd gain. (1/2 S unit) Stacking will help for skip however.
S unit increases (decreases) of some antennas.
Antron 99 .75 DBd
Imax 2000 1.2 DBd
M400 (starduster copy) -3.0 DBd negative gain
2 element beam 6.9 DBd 1.2s over imax
3 element beam 8.3 DBd 1.4s over imax
4 element beam 10.8 DBd 0.6s over 2-el
5 element beam 12.1 DBd 2.0s over imax
6 element beam 12.2 DBd not practical
7 element beam 12.3 DBd not practical
8 element beam 12.4 DBd not practical
Anything over a 5 element is a waste. stacking two 5 elements gets you 15.1 DBd which is 1/2 s unit more.
________________________________________________
POWER OUTPUT
Every time you double your power you get 3 DBd gain or 1/2 S unit increase.
e.g.
WATTS/S UNIT INCREASE
1/ 0
2/ 1/2
4/ 1
8/ 1 1/2
16/ 2
32/ 2 1/2
64/ 3
128/ 3 1/2
256/ 4
512/ 4 1/2
1,024/ 5
2,048/ 5 1/2
4,096/ 6
8,192/ 6 1/2
16,384/ 7
You have to go from 256 watts to 1,024 watts to get 1 s unit of signal increase. 4 watts to 256 watts gets you 3 s units of signal increase. The FCC limits your wattage output to 4 watts AM.
When Antennas state that they multiply your power by 64 times this simply means that you will have a 3 s unit increase, if a beam could produce such an outragous multiplication. A 16x multiplication will yield 2 s units. A 4x multiplication will yield 1 s unit. A 2-element beam would have a 4x multiplication at best, while a 5-element would multiply 16x at best.
(ERP = Effective Radiated Power or watts output)
.8x Imax 4watts into antenna = 3.2watts ERP
4x 2-EL 4watts into antenna = 16watts ERP
16x 5-EL 4watts into antenna = 64watts ERP
*some ERP watts would be lost by coax/swr.
________________________________________________
COAX
Most coax has 2db or less loss at 100 feet on 27 megahertz. Remember if this was 3 db loss you're only talking 1/2 s unit lost! SO DON"T SWEAT IT!
Hope this helps :-)
Astar
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Bruce
Senior Member
Username: Bruce
Post Number: 3473
Registered: 9-2003
| | Posted on Saturday, January 28, 2006 - 6:45 pm: |
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Where did you get THIS ????????
10 ft high = 0 db gain or 0 s unit increase
20 ft high = 6 db gain or 1 s unit increase
30 ft high = 10 db gain or 1.67 s unit increase
37 ft high = 11.4 db gain or 1.9 s unit increase
50 ft high = 15 db gain or 2.5 s unit increase
100 ft high = 20 db gain or 3.33 s unit increase
400 ft high = 30 db gain or 5 s unit increase
M400 (starduster copy) -3.0 DBd negative gain ???
( OVER DBD ????? It IS a dypole ! ) |
Tech833
Moderator
Username: Tech833
Post Number: 1176
Registered: 8-2002
| | Posted on Saturday, January 28, 2006 - 7:57 pm: |
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You cannot put firm numbers together for height vs. gain. Sorry, it is not as repeatable as that. Now, if you want to discuss takeoff angle vs. elevation, you can do some generalization and get pretty close. Each antenna is going to be different and each installation is different. WAY too many variables to give static numbers like that.
For instance- If you were to put the Top One against your height vs. gain chart, you would be so very wrong! The Top One works the same 20 feet above ground as 80. |
