Question about gain Db ratings on yagi antennas.

[SvenMarbles]

So I recently set up a yagi for GMRS that is rated for 7Dbi gain. 
 

Compared to a vertical omni directional that might have the same gain rating, am I correct in assuming that there is additional range benefit from the radiation pattern lobe being concentrated into a single direction? 
 

Dbi ratings being equal omni vs directional, the directional is concentrating the same power in the same way that you might put your thumb on a garden hose, correct? 
 

Or,.. is that rating done with the idea that the furthest point of the radiation lobe achieved a (non-symmetrically) distance representative of 7Dbi?
 

 

[kidphc]

So with that being said, what would be the advantage of a yagi with 7Dbi that is directional vs an omnidirectional vertical that is 7Dbi?

Exactly as Wrxb215 said.

Basically, since the cone is narrower then the omni directional antenna's pattern, you can steer the yagi in the direction you want to hear. Effectively nullify any RF interference (ham call it qrm) or noise, well at least lowering the level. If you radio fox hunt generally a yagi, even low gain ones are helpful in pointing you in the direction. They are often coupled with attenuators, you will often see the guys flipping the yagi on the side to attenuate it further. Opposite to the suspects polizaration. To make reception weaker and isolate to a finer degree heading.

Yagis generally have a higher gain level than their vertical/horizontal counterparts due to the squishing of the pattern. You generally switch from length of the antenna space to the swing space a yagi needs. Plus you can stack yagis onto arrays, Which will increase their gain levels.

So comes down to do you need spray gun (coverage) or an ultra fine pinstriping paint brush (detailed control).

Each have their pros and cons.

Just an fyi: the amateur (hams) that are into eme and moon bounce (considered weak signal due to the distance). Use stacked arrays with high gain levels and 1500 or so watts of power. To get as much signal to where they want it instead of scattering it around them.

Same with the guys that work satellite.






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[SteveShannon]

14 minutes ago, WSAK691 said:

So I recently set up a yagi for GMRS that is rated for 7Dbi gain. 
 

Compared to a vertical omni directional that might have the same gain rating, am I correct in assuming that there is additional range benefit from the radiation pattern lobe being concentrated into a single direction? 
 

Dbi ratings being equal omni vs directional, the directional is concentrating the same power in the same way that you might put your thumb on a garden hose, correct? 
 

Or,.. is that rating done with the idea that the furthest point of the radiation lobe achieved a (non-symmetrically) distance representative of 7Dbi?
 

 

No.
A Yagi that is rated at 7 dBi has 7 dBi in its strongest direction.

A vertical Omni rated at 7 dBi has 7 dBi in its strongest direction. 
 

Your last paragraph is correct:

Or,.. is that rating done with the idea that the furthest point of the radiation lobe achieved a (non-symmetrically) distance representative of 7Dbi?

[kidphc]

I like to think of antennas like and adjustable flashlight.

Omni kinda like a donut from the side 3d wise. It would be like having the open without a shroud like a latern.



As you you increase gain. The beam becomes flatter and flatter into a disk.

When you speak of yagis you have put the shroud on and focusing a beam. A beam that shoots more light out the front but spits some out the rear. Hence why yagis have a front to back ratio for gain. There is a technical expectation, and some design that focus more forward? But I am skipping that.

All antennas create lobes so generally, they measure the strongest lobes.

The attached with the donut is omni directional. The phallic looking graph is the yagi.

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[SvenMarbles]

So with that being said, what would be the advantage of a yagi with 7Dbi that is directional vs an omnidirectional vertical that is 7Dbi?

[WRXB215]

You might have a reason to limit the radiation to the sides and rear.

[UpperBucks]

On 1/31/2024 at 4:24 PM, WSAK691 said:

So with that being said, what would be the advantage of a yagi with 7Dbi that is directional vs an omnidirectional vertical that is 7Dbi?

So many excellent answers, but for me, the analogy always comes down to a garden hose nozzle.

If you set the nozzle to a wide "Omnidirectional" pattern, you'll spray all the water in a wide area close to your feet.
If you set the nozzle to a narrow, directional patters (that's what a Yagi does), you'll spray all the water a long distance, but in a small area. 

In radio, it's both ways - if you want to have your signal reach (and hear) a distant point, you use a directional antenna (a Yagi) - at the expense of wide area coverage. 

A good use of a Yagi might be if you have a home-based GMRS radio and you want to reach a distant repeater that's not in range with an omni-directional antenna. You can point the Yagi at the location of the repeater's antenna and you'll be able to connect to that repeater. HOWEVER you might/will no longer be able to reach other repeaters that were more local and in range of an Omnidirectional antenna.  In my Ham radio universe, when I was active, I lived in a place where all the 144 Mhz repeaters were too far for an Omni antenna, so I installed a Yagi with an antenna rotator - so I could point at a particular repeater and successfully communicate.