Antennas 101:
A Course in RF Basics
August, 2012
Antenna Basics Agenda:
In today’s training, we will go over a brief summary of the following topics at a basic level:
• Electromagnetic Waves • Frequency and Wavelength
• Ground Planes
• Gain
• VSWR/Return Loss
• Antenna Types
Electromagnetic Waves
•
•
•
An electric field moving through space in a wave shaped path creates an electromagnetic wave. This path is shown below and from this you can begin to see the wavelength
All of the information an antenna sends or receives is on these waves
Wavelength
Frequency and Wavelength
•
A radio sends out a signal (a wave as shown in the previous slide) that has a specific frequency. •
Frequency: How often an Electromagnetic Wave will go through one cycle or period. •
The unit used to describe frequency is Hertz (Hz)
•
1 Hertz = 1 period per second
•
These waves travel a constant speed
•
Since we know the speed at which they travel, we know that a higher frequency can only be a function of a shorter wave Frequency and Wavelength
In the frequencies we generally deal with, you will most likely see units of:
• MHz (Megahertz – 1,000,000 Hertz) and •
GHz (Gigahertz – 1,000,000,000 Hertz or 1,000MHz) •
Wavelength (λ) related to frequency:
•
300/F (Frequency in MHz) = λ (wavelength in meters)
•
984/F (Frequency in MHz) = λ (wavelength in feet)
Frequency and Wavelength
•
Wavelength and frequency are fundamental to understanding antennas. •
The simplest forms of antennas are referred to by what portion of a wavelength they are
• ¼ Wave • ½ Wave
• ⅝ Wave
• ½ Wave over a ¼ Wave (Collinear array.) •
Amateur radio (Ham Radio) users refer to their frequencies of operation by wavelength (m) • 20 meter
• (300/F=20) F~15MHz
• 10 meter
• (300/F=10) F~30MHz Frequency and Wavelength:
Ground Planes
•
Wavelength of an antenna determines whether a Ground Plane is necessary
• ½ Wave does not require a ground plane
• ¼ Wave does require a ground plane
•
Wavelength determines the minimum Ground Plane required for any ground plane dependent antenna
•
Ground planes have nothing to do with a DC ground
•
¼ wave antenna requires AT THE VERY LEAST a ¼ wavelength diameter ground plane
•
½ wavelength diameter ground plane is the minimum we recommend
•
More is better!
Gain – What is it?
•
Gain is the measure of directionality in an antenna •
It is a multiplier of power in one given direction
•
You don’t get more power in one direction without taking it from another •
Gain is measured in decibels (dB) which is a logarithmic scale that can only be measured in relation to something else
•
Adding 3dB is doubling
• 6 is 3dB more than 3
•
Adding 10dB is multiplying by 10
• 100 is 10dB more than 10
Top View of Omnidirectional and directional antenna together – Excellent representation of gain
Gain – Standard Points of Reference
The standard points of reference for antennas:
• The ½ wave dipole (dBd ‐ decibels in relation to a dipole)
•
Isotropic Radiator ‐ radiates in a perfect sphere (dBi (decibels in relation to an isotrope)
0 dBd = 2.15 dBi Gain – Mobile Antennas •
•
•
Mobile antenna gain (unless marked in dBi or dBd) is measured in dB without saying a reference point. The reference point used as the industry standard is the ¼ wave antenna on a ground plane
The gain is measured at the horizon because that is the gain users are interested in. =
=
0 dB ¼ Wave
=
3dB (½ Wave over ¼ Wave)
5dB (½ Wave over ½ Wave over ¼ Wave)
(V)SWR/Return Loss
•
•
•
•
•
•
•
•
SWR and VSWR are essentially the same SWR = Standing Wave Ratio
VSWR = Voltage Standing Wave Ratio
It is a way to tell how much of the power you put into the antenna reflects back into the radio. This is also represented by a term known as Return Loss (RL) Return Loss shows the drop in dB of power from what you put into the cable and antenna (AKA forward power) to what comes back into the radio (AKA reflected power.)
If 10% coming back to the radio, the RL is 10dB SWR and RL are directly convertible from one to the other
(V)SWR/Return Loss
RL (dB)
VSWR
% Reflected
% Transmitted
RL (dB)
VSWR
% Reflected
% Transmitted
32
1.052:1
0.06%
99.94%
16
1.377:1
2.51%
97.49%
31
1.058:1
0.08%
99.92%
15
1.433:1
3.16%
96.84%
30
1.065:1
0.10%
99.90%
14
1.499:1
3.98%
96.02%
29
1.074:1
0.13%
99.87%
13
1.577:1
5.01%
94.99%
28
1.083:1
0.16%
99.84%
12
1.671:1
6.31%
93.69%
27
1.094:1
0.20%
99.80%
11
1.785:1
7.94%
92.06%
26
1.106:1
0.25%
99.75%
10
1.925:1
10.00%
90.00%
25
1.119:1
0.32%
99.68%
9
2.100:1
12.59%
87.41%
24
1.135:1
0.40%
99.60%
8
2.323:1
15.85%
84.15%
23
1.152:1
0.50%
99.50%
7
2.615:1
19.95%
80.05%
22
1.173:1
0.63%
99.37%
6
3.010:1
25.12%
74.88%
21
1.196:1
0.79%
99.21%
5
3.570:1
31.62%
68.38%
20
1.222:1
1.00%
99.00%
4
4.419:1
39.81%
60.19%
19
1.253:1
1.26%
98.74%
3
5.848:1
50.12%
49.88%
18
1.288:1
1.58%
98.42%
2
8.724:1
63.10%
36.90%
17
1.329:1
2.00%
98.00%
1
17.391:1
79.43%
20.57%
Antenna Types – Directional
There are many types of directional antennas, in day to day life you will see (but perhaps not notice) many of them. Here are the ones that PCTEL produces:
• Yagi • Panel • Parabolic Dish (Satellite Dish)
• Sector Panel
• Patch • Corner Reflector Questions and Comments:
If you have any other questions or comments, feel free to call or email me and I will get answers to you right away.
Eric Singer (630) 233‐8071
eric.singer@pctel.com
Thank You!