3dBi v. 5.5 dB... mobile antenna

[WRZM243]

Pulse Larsen Antenna for GMRS, mobile hood mount on pickup... 3dBi v. 5.5 dB... operating in flat open valley area... much of a difference?  What would your choice be?

Edited January 14 by WRZM243
Clarification

[Guest]

31 minutes ago, WRZM243 said:

Pulse Larsen Antenna for GMRS, mobile hood mount on pickup... 3dBi v. 5.5 dB... operating in flat open valley area... much of a difference?

Quick upfront question: Are both antennas measure gain the same way? - dBi vs dBd - If they use the same unit, the higher number is clearly achieving more gain.

... However, the concept of gain is not that straight forward (pun intended) but rather directional.

You achieve gain through "focus" and now theory and reality are drifting apart.

If you are using a stationary narrow beam antenna to reach a specific stationary receiver, gain is somewhat easy: More gain, better performance! - Easy because of the simple radiation pattern.

But, if you put an "omnidirectional" high gain antenna onto a pickup-truck, you are introducing a lot of variables!

Suddenly, the specific radiation pattern of the specific antenna has to deal with the specific mounting point and the metal around that and interference and ...

Again, in theory, higher gain is desirable. In the real world, the outcome cannot be easily predicted by numbers on the box.

 

[WRKC935]

repost

 

[WRKC935]

2 minutes ago, WRXD372 said:

Quick upfront question: Are both antennas measure gain the same way? - dBi vs dBd - If they use the same unit, the higher number is clearly achieving more gain.

... However, the concept of gain is not that straight forward (pun intended) but rather directional.

You achieve gain through "focus" and now theory and reality are drifting apart.

If you are using a stationary narrow beam antenna to reach a specific stationary receiver, gain is somewhat easy: More gain, better performance! - Easy because of the simple radiation pattern.

But, if you put an "omnidirectional" high gain antenna onto a pickup-truck, you are introducing a lot of variables!

Suddenly, the specific radiation pattern of the specific antenna has to deal with the specific mounting point and the metal around that and interference and ...

Again, in theory, higher gain is desirable. In the real world, the outcome cannot be easily predicted by numbers on the box.

 

It's funny you posted this as I was typing out the same thing. 

Great minds...

 

[WRKC935]

Well is it 3.5dBi vs 5dBi or 3.5dBi vs 5Dbd or something else?

The dBi gain number is based off an isotropic radiator.  Which is an antenna that only exists in theory.  It's a 'paper antenna' based on a radiation pattern that is a sphere.  Equal signal radiation in all directions.  Even a standard dipole will have gain over an isotropic radiator which is the dBd gain number. 

So in short they could be the same, or the 5dB antenna could actually be worse depending on what the gain number is based on. 

Now with that being said, Larsen typically has reasonable numbers attached to their products.  And they are based on actual testing in a RF free chamber so the numbers are reasonably accurate.  Some of the other 'no name' brands could be basing their gain on anything including a dummy load to pump up their gain numbers. 

One thing to understand here is what GAIN actually is.  And what it's not.  Amplifiers have active gain.  You put 1 watt in and get 10 watts out.  That's a 10dB gain number.  See that there was no i, d or anything else after the dB.  And dB is a measurement of CHANGE or difference between two things.  In this case signal.  Now power can be measured in watts, which is what everyone typically see's. But it can also be measured in dBm which is based on a 0dBm or 1milliwatt.  So 30dBm is 1 watt, and 40dBm is 10 watts,  which is a 10dB increase in signal. 

[WRYZ926]

Delete

[SteveShannon]

Let’s use correct terminology.  It’s not true to say that dBi is always greater than dB. A dB is a relative unit of measurement.  It’s never correct to say that an antenna has X dB of gain unless you also specify what that gain is relative to.  That’s usually done by expressing gain as dBi, dBd.  

dBi is relative to an isentropic antenna, a theoretical antenna that consists of a single point radiating exactly the same in every direction.  

dBd is relative to a dipole, which is a very real antenna.  A dipole antenna has 2.15 dB greater gain in certain directions, than an isentropic antenna.  That also means it radiates less in other directions.  

There’s also the term dBm, which is not used to characterize passive antennas but to characterize electrical sensitivity, such as receivers.

[WRUW493]

I agree with the responses above and add the following:  Any end fed antenna (like we are certainly talking about here) is really just the top part of the actual radiating structure. The bottom part is the "ground plane", or in practice that metal stuff that the base of the antenna is connected to. If you put the antenna in the middle of the trunk lid of a sedan, or on the roof with a large metal roof, then, and only then, will you get close to the "gain" and radiation pattern that the manufacturer states. But this is rarely a practical way of mounting the antenna to the vehicle. Further, consider that any phase delay, read length of metal, between the base of the antenna and the "large" metal surface of the vehicle only serves to mess up the phase relationship between the whip and the ground, thus messing up the radiation pattern. I've seen lots of installations where people mount the antenna to the edge of the front hood of a pickup truck, but the mounting bracket is inches long between the antenna base and it's connection to the hood. This is a phase delay, see previous sentence. 

This is the reason that you'll see a lot of police cars with the antenna mounted via NMO in the center of the roof or trunk lid. Is the trunk lid lower to the ground than the roof? yes, does it matter? no. The improved radiation angle and direction symmetry benefit out weighs any height loss and even "blockage" by the passenger compartment.

Hope this helps.