2 meter "whip" question

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NWI_Scanner_Guy

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Okay, I'm about to show my total lack of antenna knowledge, so please bear with me....

I'm wanting to get a 2 meter mobile antenna so I can use my Yaesu FT2800-M in my car (just using it as a base right now hooked up to a j-pole), and have been looking on e-bay at some of the antennas, and I've seen in more than one description where it says that the gain is 2.5 dbd on UHF (for those antennas that are dual band) and "unity gain" on VHF. What in the world is "unity gain?" Even a Google search didn't really help.

Any info provided will be greatly appreciated. Many thanks in advance.

SSSG

:)
 

SCPD

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Basically it means no gain - the antenna to which gain antennas are referenced to.
 
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reedeb

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Got to HRO [Ham Radio Outlet] site and look at some of their antennas or order a catalog from em and buy a new one [many or NOT that expensive].
 

n5ims

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Okay, I'm about to show my total lack of antenna knowledge, so please bear with me....

I'm wanting to get a 2 meter mobile antenna so I can use my Yaesu FT2800-M in my car (just using it as a base right now hooked up to a j-pole), and have been looking on e-bay at some of the antennas, and I've seen in more than one description where it says that the gain is 2.5 dbd on UHF (for those antennas that are dual band) and "unity gain" on VHF. What in the world is "unity gain?" Even a Google search didn't really help.

Any info provided will be greatly appreciated. Many thanks in advance.

SSSG

:)
"gain is 2.5 dbd" - Translates to 2.5 dB gain over a standard dipole (based on the final "d" on dBd). Since the reference antenna is a dipole, a "unity gain" antenna would have the same gain (or loss) as a standard dipole antenna. What you're probably seeing are simple 1/4 wave verticals on 2 VHF, which can work on UHF as well (not great, but they do work OK) and due to the extra length on UHF, you get a small amount of gain. This is the most basic and least expensive antenna that can be made so the price should be pretty low.

Note, dBi uses an "isotropic" antenna as its reference point and is 2.15 dB more than if the same antenna used dBd as its reference point (since a standard dipole antenna has 2.15 dBi of gain). These two reference points make it difficult to judge any gain figures posted by an antenna seller unless they specifically state what the reference is. Assume dBi if they don't specify (if you're wrong, you simply get an extra 2.15 dB of "gain" over what you were expecting).
 

popnokick

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If you're looking for gain in a mobile 2M antenna, check out 5/8 wave antennas. They are longer than the 1/4 wave unity gain 19" whips, but you didn't say it had to be a short antenna. Typically they will have a loading coil at the base and come in magnet, trunk lip, and body mount styles. Larsen, Diamond, and Comet are some of the better quality antenna brands you'll see.
 

LtDoc

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And just to keep things in perspective, that 19" 2 meter 1/4 wave antenna is not a unity gain antenna, it has a negative gain. It's only half of a 1/2 wave antenna which is the industry standard so has less gain (negative number). That doesn't mean it isn't any good, that it doesn't work, because it certainly can and does. It isn't a simple matter of '+' or '-', but of how much '+' or '-' there is. How something is described can make a huge difference.
- 'Doc

(You can name your cat Rover, but it still ain't gonna bark.)
 

n5ims

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And just to keep things in perspective, that 19" 2 meter 1/4 wave antenna is not a unity gain antenna, it has a negative gain. It's only half of a 1/2 wave antenna which is the industry standard so has less gain (negative number). That doesn't mean it isn't any good, that it doesn't work, because it certainly can and does. It isn't a simple matter of '+' or '-', but of how much '+' or '-' there is. How something is described can make a huge difference.
- 'Doc

(You can name your cat Rover, but it still ain't gonna bark.)
While a 1/4 wave antenna is only half of a 1/2 wave antenna (basically a dipole), once you place it on the car or other ground plane, it becomes a 1/2 wave antenna and provides that unity gain. The car (or other ground plane) provides the other half of the 1/2 wave antenna.

Remember that an antenna system isn't just an antenna, it's the antenna, the mount, and the surroundings. The key word is system, which is why an antenna will never work out exactly as modeled in theory. There are just too many variables to correctly model correctly (did you take into account that aircraft flying in your signal path?).
 
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