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Antenna db rating questions

Turbo T

Turbo T

Certified CB Rambo
Feb 2, 2011
963
142
53
Pardon me if this might sound like a silly question, but I searched and didn't find what I was looking for.

I am wanting to know, in laymens terms, what it means when you see an amateur radio antenna advertised as having "5.5 db" gain while another is advertised as having a "7.5 db" gain.

I am presuming it means the antenna with the 7.5 gain can RX better than the antenna with the 5.5 gain....am I right? And if so, does it also mean the antenna with the 7.5 gain can TX farther than the antenna with the 5.5 gain?

And for my next question...one antenna I have had my eye on is a Comet GP-9 which is a 2 m/70 cm base antenna. On the website that is selling them, it states:

Gain & Wave:
146MHz 8.5dBi 5/8 wave x 3
446MHz 11.9dBi
5/8 wave x 8
Click to expand...

What does it mean to have a 5/8 wave x 3? And how relevant is this information?

Thanks in advance.
 

Turbo T said:
Pardon me if this might sound like a silly question, but I searched and didn't find what I was looking for.

I am wanting to know, in laymens terms, what it means when you see an amateur radio antenna advertised as having "5.5 db" gain while another is advertised as having a "7.5 db" gain.

I am presuming it means the antenna with the 7.5 gain can RX better than the antenna with the 5.5 gain....am I right? YES And if so, does it also mean the antenna with the 7.5 gain can TX farther than the antenna with the 5.5 gain? YES

And for my next question...one antenna I have had my eye on is a Comet GP-9 which is a 2 m/70 cm base antenna. On the website that is selling them, it states:



What does it mean to have a 5/8 wave x 3? And how relevant is this information? That means that over it's length, the antenna is three co-phased 5/8 wave antennas in one. Three will give more gain than two and two will give more gain than one.

Thanks in advance.
Click to expand...

Also know that while comparison shopping, there are different ways of rating gain. db/dbd or dbi. Many advertise in "dbi", however some don't. 10dbi = roughly 8db/dbd.

That GP9 is a great omni BTW...
 
What does it mean to have a 5/8 wave x 3? And how relevant is this information?
Click to expand...


What they are saying is that the length of the antenna ( the length of the wire inside the antenna and built-in matching load coil and capacitance couplers ) is 3- 5/8 long, or that the length is like 3 separate 5/8 wave lengths long ( co-phased).
Same thing on 440 the antenna ( the length of the wire inside the antenna and built-in matching load coil and capacitance couplers ) is a 5/8 wave x 8 , or that the length is like 8 separate 5/8 wave lengths long ( co-phased ).


Acid rod already answered while I was typing this i see...lol


73
Jeff
 
All gain figures are comparisons with another antenna. There's a 'standard' antenna that's used to make comparisons between antennas more 'standardized', you have a reasonable expectation to expect antennas to be relatively 'this much' better/worse than another when both are compared to a 'standard' antenna. (That made sense to me when I thought it, not too sure it does when I typed it. Oh well.)
That standard antenna is a 1/2 wave dipole. So when that comparison's gain is written it's stated as 'dBd', the ending 'd' denotes a dipole comparison. An isotropic, perfect antenna in aperfect environment, type comparison is written as 'dBi'. That isotropic type comparison doesn't take into account all the things that can affect an antenna in the 'real world', so is a theoretical type comparison. As long as you stick to the same type comparison, not mixing dBd and dBi gain figures, you should be able to make reasonably accurate comparisons.
Then you get into how well/accurate a comparison was done, that the author of those gain figures didn't 'fudge' a little here and there. That's a lot more common than you might think.
There are several aspects that can be expressed as gain, and they are only vaguely about the same things. You have to know what's being compared and how and why. It just isn't as 'simple' as a bare-bones gain figure. It can get very complicated very quickly (and I'm not the one who can explain all of it, sorry).
- 'Doc
 
I know this much when you buy a Antenna look for proven ones. The Antenna CO's dont always shoot you strate on DB Gain. So i would into it and do some research befor buying. God Bless.
 
There are so many ways of calculating gain using dBd, dBi, modelling in free space, over real ground etc that most of it can be taken with a pinch of salt.

The truth is really quite simple:

Shortened antennas are crap - sometimes as much as 20dB down on a full size quarter wave vertical.
Verticals are slightly less crap - typically 3-5dB down on a dipole - and are good for distance work >3000 miles and local work <100 miles.
Dipoles at a half wavelength high or more are the reference and when looking at gain, you should look at dBd which means its in reference to a dipole. Dipoles are poor for local work.
Beams, even 2 element ones, have gain over a dipole however gain isn't the most important figure to look at when buying one. The FB or Front to Back ratio, also expressed in dB, is the indicator of performance with a beam. You buy a beam to eliminate unwanted signals from behind and to the side more than to send more power.

And finally remember that on a properly calibrated S meter, 3dB is half a S point even though the CBers would have you believe its at least 10.
 
I ran across this thread and would just like to add my $.02. I just got my license and was using a 1/4 wave at 20' with RG59. I finally got things set up here with a Comet GP9 & LMR400 at 35' up. YOWZA is this thing on FIRE! I'm in central MI and can pick up Canada. When conditions are not bad I can get a large portion of the state. I can listen to ambulances calling hospitals 40 miles away! (and that's out of band supposedly).
 
Turbo T said:
Yeah that's one thing I have read on the GP-9, they have some ears to them.
Click to expand...

No matter how good the antenna is you need decent feedline and height to take advantage of that antenna's quality. An otherwise excellent antenna fed with 75 feet of RG-58 and mounted 20 feet high will not perform as good as a simple 1/4 wave at 50 feet and fed with LMR400 or LDF4 heliax cable.
 

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