What is the gain of the Comet BNC24 at 440 MHz? 3.5 or 5.3 dbi?

[ILikeHatchChili]

I've seen both gains for this antenna at different web sites. I really want the 5.3 figure. Is there an HT antenna with a 5.3 dbi gain at 440 MHz?

Thanks in advance.

 

[JPSan]

DO NOT trust an antenna rated by a dBi gain figure!!!

DO NOT ever trust a antenna manufacturer who RATES there antenna's by dBi ratings. If you follow the following you will get an idea why.
I once saw a guy on a website bad mouthing an antenna on his website that he was comparing to an another antenna, a basic dipole design because the antenna he favored had a 2.5 dBi rating and as far as he was concerned it was a GAIN antenna, THOUGH in his review the antenna DID NOT improve his receive capabilities over a basic whip antenna. He blamed it on "conditions" of the air at the time. LOL.

dBd (dB dipole)
The gain an antenna has over a dipole antenna at the same frequency.
A dipole antenna is the smallest, least gain practical antenna that can be made. The term dBd (sometimes just called dB) generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz).

The reason why the gain of many antennas, especially VHF/UHF antennas, is measured in dBd is because antenna manufacturers calibrate their equipment using a simple dipole antenna as the standard. Then they replace it with the antenna they are testing. The difference in gain (in dB) is reference to the signal from the dipole.

dBi (dB isotropic)
The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and system performance.

The gain of Microwave antennas (above 1 GHz) is usually given in dBi.

A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna.

If an antenna gain is given in dBd, not dBi, add 2.15 ( I have seen this number actually vary by different authors, TOO) to it to get the dBi rating.

Example, if an omni antenna has 5 dBd gain, it would have 5 + 2.15 = 7.15 dBi gain.

Note: If an antenna gain is just specified in dB from a manufacturer, be sure to ask if it is dBi or dBd. If they cannot tell you or do not know the difference, then you should consider buying from another vendor! This can lead to over blown antenna performance ratings and expectations

I hope this was easy to follow and grasp.

 

[n5ims]

I've seen both gains for this antenna at different web sites. I really want the 5.3 figure. Is there an HT antenna with a 5.3 dbi gain at 440 MHz?

Thanks in advance.

The Comet BNC24 is a standard 1/4 wave antenna on 2 and 5/8 wave on 440. The gain difference is non-existant since they're the same antenna but the numbers are different since they're being reported in different units (somewhat like meters vs feet). The 3.5 gain figure is being reported in dBd while the 5.3 gain figure is being reported in dBi. What you have are folks saying that this 2.54 cm antenna is longer than the other guy's 1 inch antenna since 2.54 is larger than 1. Put them side by side and they're both the same length.

 

[ILikeHatchChili]

Thank you.

Is there an HT antenna that can give me some gain in the UHF band other than a piece of wire around 7" tall? I plan to put the antenna on a BNC mag mount on top of a metal cabinet (for ground plane) in the house. Doesn't matter if the antenna is tall and bulky. Would the Radio Shack 20-006 have any appreciable gain over a 7" inch tall wire "stick" in my band of interest?

I will use the antenna for my BCD 996XT and not for transmitting (I would use the stock antenna for my HT transceiver outside of the house).

Thank you again.

 

[n5ims]

Normal handheld antennas are generally not good performers since portability is formost in their design over performance. Since you're looking to mount your antenna on a metal cabinet, you can easily find one that will outperform your typical rubber duck (even ones with gain). Just look for one that covers the UHF or a UHF/VHF-Hi dual band antenna (easiest to find and afford would normally be ham band antennas). Below are some places to look. You'll probably want your antenna to be an "NMO" mount and get an NMO mag mount to stick on your cabinet (only the cable end needs to match your scanner, not the antenna end since you're not physically attaching the antenna to your scanner).

http://www.larsen-antennas.com/docfiles/LarsenAmateurCatalogVol1.pdf (starting on page 22, also note that they list the antenna gain in both dBd and dBi to make it easy to compare to those that only list one or the other)
Diamond® Antenna ~ Mobile Antennas
NCG Company::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Once you find the antenna you like, search in Google (or your favorite search tool) to find folks that sell it and shop around based on price, support, shipping, etc.

 

[prcguy]

There is nothing not to trust if a mfr rates their antenna gain in dBi, in fact it should evoke trust that they offered a known reference.

dBD gain was adopted many years ago by the land mobile antenna mfrs but dBi is the industry standard for commercial, military, satellite and measurement antennas. When you send a reference antenna to an antenna range to be calibrated and spend thousands of $$ the data will in dBi.

Unfortunately most companies don't send their antennas to a range and spend the $$ and just use theoretical numbers. Then there are those companies that don't know any better and use the highest number ever published for a particular antenna, which could be bogus to start with.

With my very limited antenna knowledge I would say the the 3.5dBD or even dBi gain rating for the antenna in question is completely fictitious.
prcguy

DO NOT ever trust a antenna manufacturer who RATES there antenna's by dBi ratings. If you follow the following you will get an idea why.
I once saw a guy on a website bad mouthing an antenna on his website that he was comparing to an another antenna, a basic dipole design because the antenna he favored had a 2.5 dBi rating and as far as he was concerned it was a GAIN antenna, THOUGH in his review the antenna DID NOT improve his receive capabilities over a basic whip antenna. He blamed it on "conditions" of the air at the time. LOL.

dBd (dB dipole)
The gain an antenna has over a dipole antenna at the same frequency.
A dipole antenna is the smallest, least gain practical antenna that can be made. The term dBd (sometimes just called dB) generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz).

The reason why the gain of many antennas, especially VHF/UHF antennas, is measured in dBd is because antenna manufacturers calibrate their equipment using a simple dipole antenna as the standard. Then they replace it with the antenna they are testing. The difference in gain (in dB) is reference to the signal from the dipole.

dBi (dB isotropic)
The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and system performance.

The gain of Microwave antennas (above 1 GHz) is usually given in dBi.

A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna.

If an antenna gain is given in dBd, not dBi, add 2.15 ( I have seen this number actually vary by different authors, TOO) to it to get the dBi rating.

Example, if an omni antenna has 5 dBd gain, it would have 5 + 2.15 = 7.15 dBi gain.

Note: If an antenna gain is just specified in dB from a manufacturer, be sure to ask if it is dBi or dBd. If they cannot tell you or do not know the difference, then you should consider buying from another vendor! This can lead to over blown antenna performance ratings and expectations

I hope this was easy to follow and grasp.

 

[gewecke]

Thank you.

Is there an HT antenna that can give me some gain in the UHF band other than a piece of wire around 7" tall? I plan to put the antenna on a BNC mag mount on top of a metal cabinet (for ground plane) in the house. Doesn't matter if the antenna is tall and bulky. Would the Radio Shack 20-006 have any appreciable gain over a 7" inch tall wire "stick" in my band of interest?

I will use the antenna for my BCD 996XT and not for transmitting (I would use the stock antenna for my HT transceiver outside of the house).

Thank you again.

YES! www.smileyantenna.com

n9zas

 

[ILikeHatchChili]

Thank you, all !

 

[jackj]

prcguy is 100% correct. From Wikipedia: "The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level." So the gain rating of an antenna must be referenced to a base. A gain of 2.5 dBi is just as valid as a gain of 0 dBd and these two values are exactly the same. Just be sure you understand what is used as the reference.