Antenna Gain (dBi, dBd, dBq, Unity)

[C2]

I'm trying to verify empirical antenna gain figures and in the context of microwave frequencies, where I think ground effects are of less concern that typical at HF.

I come across a HT "duck" antenna (5.4 GHz) that states it as a quarter wave with unity gain. How would this be expressed as dBi? Can I first assume unity here means 0 dBq? I wonder because I've also seen unity to imply equal to an isotropic, and also to a free space dipole...?

How does dBq then relate to dBi? I read the Wikipedia article that states a quarter wave antenna has 3 dBd gain. I've read that elsewhere too, but I've also read that a quarter wave whip has loss relative to a reference dipole. ?

The antenna and its image form a

dipole that radiates only in the upper half of space.
The quarter-wave monopole antenna is a single-element antenna fed at one end, that behaves as a dipole antenna. It is formed by a conductor

in length, fed in the lower end, which is near a conductive surface which works as a reflector (see effect of ground) and is an example of a Marconi antenna. The current in the reflected image has the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together form a half-wave dipole that radiates only in the upper half of space.

In this upper side of space, the emitted field has the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be half of the series impedance of a half-wave dipole. As the reactive part is also divided by 2, the impedance of a quarter-wave antenna is

ohms. Since the fields above ground are the same as for the dipole, but only half the power is applied, the gain is twice (3 dB over) that of a half-wave dipole (

), that is, 5.14 dBi.

https://en.wikipedia.org/wiki/Dipole_antenna

The bolded statement seems to contradict the entire section, however. I also wonder of the validity of this in the context of this "duck" antenna, since there may well be near perfect ground. So is it even possible to make any associations to relative gain figures here? I've read that quarter wave antennas have basically equal gain as a dipole.

It is already well established that 2.15 dBi = 0 dBd, at least I am not considering ground effects and that a dipole actually has ~8 dBi gain...

 

[Captain Kilowatt]

It's all relative. In order to state gain you must also state what the reference is. Unity gain simply means it has a gain of zero dB over the reference whatever that reference is. It could be an isotropic radiator, dBi or it could be a dipole, dBd or another antenna altogether.

 

[The DB]

dBq, don't see that reference mentioned very often... I think its been years since I've seen anyone mention it...

The bolded statement seems to contradict the entire section, however. I also wonder of the validity of this in the context of this "duck" antenna, since there may well be near perfect ground. So is it even possible to make any associations to relative gain figures here? I've read that quarter wave antennas have basically equal gain as a dipole.

Unless you are over a body of salt water, the ground in your area is nothing like a perfect ground.

When referring to vertical antennas, a 1/4 wavelength antenna that has the same tip height as a longer antenna, say 1/2 or 5/8 wavelength, will have very close to the same amount of gain. There are exceptions of course, but this is true in most cases.

I wouldn't dwell on the figures you see for dBd and dBq. Yes, they have 2.15 and 0.15 dBi gain, but only in freespace, which doesn't exist on earth short of being inside an anionic chamber... Once an earth is present, it dominates the gain and radiation pattern of all antennas.


The DB

 

[C2]

Thanks for chiming in. The DB, do you mean to state the dBd and dBq are respectively 2.15 and 0.15 dBi? Since I am just presenting empirical data, I would take these in reference to free space.

 

[Moleculo]

Thanks for chiming in. The DB, do you mean to state the dBd and dBq are respectively 2.15 and 0.15 dBi? Since I am just presenting empirical data, I would take these in reference to free space.

Yep, that's correct.

 

[C2]

I knew this was going to happen. I have wiki, WWRF, and two real RF engineers supplying info, and I have four different answers.

I was thinking a 1/4 wave has equivalent gain to a dipole since you can bend a dipole 90* to make a 1/4 wave vertical.

 

[Beetle]

I knew this was going to happen. I have wiki, WWRF, and two real RF engineers supplying info, and I have four different answers.

I was thinking a 1/4 wave has equivalent gain to a dipole since you can bend a dipole 90* to make a 1/4 wave vertical.

Find an area that's clear and level for several wavelengths in all directions and do some experimenting with different antennas at varying heights. There are so many variables that one definitive answer is going to be very hard to come by. Especially if the marketing folks at antenna manufacturers have any input.

 

[n8fgb]

I knew this was going to happen. I have wiki, WWRF, and two real RF engineers supplying info, and I have four different answers.

I was thinking a 1/4 wave has equivalent gain to a dipole since you can bend a dipole 90* to make a 1/4 wave vertical.

A dipole gets its gain b being bidirectional, to the broadside of the antenna. when you make it bent a 90 degrees, it is no longer bidirectional, inverted "V", and loses its gain.

 

[C2]

I was thinking of vertical dipole: | vs L but also extending the idea of the L to a ┴ and then further to 4 radials, to infinite radials.

 

[The DB]

do you mean to state the dBd and dBq are respectively 2.15 and 0.15 dBi? Since I am just presenting empirical data, I would take these in reference to free space.

Yes, these gain figures for these antennas are only accurate in freespace.

I knew this was going to happen. I have wiki, WWRF, and two real RF engineers supplying info, and I have four different answers.

This happens all the time. Its not that any of your sources are necessarily wrong, but how one antenna functions in one environment or orientation will vary if the same antenna is put in another orientation/environment.

I was thinking a 1/4 wave has equivalent gain to a dipole since you can bend a dipole 90* to make a 1/4 wave vertical.

If you take a 1/4 wavelength vertical antenna, and angle its radials down, the more the radials are angled down the more the antenna acts like a dipole. In freespace said 1/4 and 1/2 wavelength antennas will act very differently to each other, and from how they each act in the presence of an earth. In the presence of said earth, the radiation pattern is dominated by the earth's characteristics, as well as how high the antenna is above the earth.

If you truly want to understand antenna theory, I highly suggest getting a copy of the ARRL Antenna Book, I hear a new version is due out, and they are all very good at explaining how antennas work, and they are written with the average ham in mind, not engineers... The newest editions of the book are imho the best.


The DB