I've got a bunch of antennas with varying VSWRs and dBi ratings, and a couple of standard SMA headers from various 2.4Ghz radios. I would like to learn how antenna selection will affect RF power and range.
Hi Mark,
The gain of the antenna expressed in dBi is the amount of power transmitted in the boresight (in other words, the direction where it radiates the maximum) relative to what it would be if the 0dBm you put into was radiated isotropically (equally in all directions). The power received at the other end is governed by the Friis Equation:
Pr/Pt = Gt*Gr*(lambda/4*pi*R)^2
where
Pr = received power
Pt = transmitted power (0dBm in your example)
Gt = transmit antenna gain (dBi)
Gr = receive antenna gain (dBi)
lambda = wavelength
R = transmission distance
See also http://en.wikipedia.org/wiki/Friis_transmission_equation
Range is mainly depend up on radiation pattern of that particular antenna that you you have not mentioned in your question.
You might to take a look at this:
http://www.antenna-theory.com/definitions/decibels.php
The dBi measure means dB relative to isotropic, or omnidirectional pattern.
The range depends on the transmitter's output power, this transmit antenna's gain or loss, depending if the antenna is aimed right at the receiver or not, then it depends on the propagation loss, the gain or loss at the receive antenna, and on the sensitivity of the receiver. These are all dB values, either added or subtracted, starting with that 0 dBm figure you cited for the (1 mW) transmitter. Oh, and you might also throw in the loss in dB for the lengths of RG-6 or other antenna leads, at the transmit and receive sides.
Propagation loss varies depending on your environment. In simple line-of-site cases, you can use the free space propagation loss model:
Loss(dB) = 32.45 + 20logbase10(freq in MHz) + 20logbase10(dist in Km)
But if you have walls in the way, or hills, or buildings, or what have you, that ideal free space equation will not hold. It might give overly optimistic results. And to make things even more interesting, sometimes reception is only possible because the signal is bouncing off a wall or other object, from transmitter to receiver.
An antenna with "gain" only has gain in one or a few directions. In other directions, you get signal loss instead. You don't get something for nothing. Using antenna gain figures allows the propagation loss model to be based only on range and frequency.
So, take the case of a pencil beam, super tight, similar to a laser. You might think, why does the same RF loss model work for a tight beam like that, and an omni beam, same equation? And that's the reason. Antenna gain is the trick used to compensate for how the power is actually concentrated throughout its path. Instead of creating a new propagation path loss model for each type of antenna, you use the same propagation loss model and pretend that the antenna gave your transmitter more power.
Another question that comes up is, why the frequency dependency? Well, that's also a bit of a trick. In free space, frequency should not matter for propagation path loss, one would think. And it really doesn't. What matters is how many photons the transmitter and transmit antenna are pushing your way, and how many photons the receive antenna can gather up, per unit time.
Propagation loss equations include frequency only because they are all based on an antenna gain of 1. For equal antenna gain, antenna size is proportional to the wavelength. So an antenna with a gain of 1 at 60 GHz is way, way smaller than an antenna with gain of 1 at 100 MHz. And as you might expect, small antenna "aperture" results in fewer photons transmitted and then gathered up, in that beam. If you instead consider equal size antennas, regardless of frequency, such as large dish antennas for example, now you can compensate for the supposed range loss suggested by the propagation loss model at the higher frequencies.
VSWR is a measure which conveys how well matched the impedance of the antenna is with free space, at a given carrier frequency. Lower VSWR indicates that more of the power flowing to the antenna, through the antenna lead, finds its way beyond the antenna into free space. And less power is instead reflected back, due to impedance mismatch. The lower VSWR is preferable. You need to use the antenna best suited to the RF carrier frequency you want to transmit.
Primarily, I understood your confusion in three area. 1) VSWR, 2) dBi and 3) SMA connector
The VSWR conveys us how much power will reflect back.To the extent of reflected power radiation will be less, hence better VSWR antenna (Quantitatively less VSWR) to be selected
Gain in dBi conveys more radiated power towards particular direction. If application is directive antenna, higher dBi gain is better
SMA connector is better suitable for microwave frequency as it has very good match and low loss at your frequency of operation
Hope this clarify your doubt
Don't forget that range depends also on Rx sensibility.
Hello Mark,
In simple words, to increase the range, you have to decide whether you want an omni-directional range or the LoS (Line-of-Sight). Assuming you have taken care or less obstacles in your environment, in an omni directional scene, you can only increase the range by increasing the power of your Tx. When you use LoS applications, you can use a directional antenna. Note that antenna gain does not increase your power from transmitter. As our experts like Mathew and Albert have stated, antenna gain is a relative figure measured against an Isotropic Radiator. Therefore, a directional antenna directs or concentrates the energy in a particular direction given by its directional pattern that effectively maximizes signal in the specified direction. Now you decide what type of antenna you have to use.
Coming back to the SMA connector, note that it is your choice as what type of antenna you want to use - ob-board or external. The connector will only help you to connect an external antenna through a suitable cable. This has nothing to do with your power and range achievements!
Hope this help you in reaching to your goals. Wish you success in your work!
Thanks,
- Badges
- Science topic