Welcome To my WHY 3dB? Homepage

I started this website for myself as a web portal to store information on my on-going RF learning and as well as a sharing to the community. I would probably keep the information in my homepage as simple as possible to understand. The main content of the page are mostly extracted from other people’s journals/website which I found interesting and the things we should know in the competitive world of RF field. Some maybe out of the topic, but its no harm to read as far as I’m concern. However, you know, I might be lazy, and it’s the best excuse if there is not much of updates and maintenance. Ok, my name? people called me YT ~ you know the famous You Tube ? :D

So you may ask, why named it “Why 3dB?” well I come across the below enquiries from someone in the forum which led for my own thinking.

“To determine the low and high cutoff frequency for the bandwidth, why we are taking 3dB as the limits for upper cut frequencies and lower cut off frequencies? Any particular reasons?

I know that -3dB is equivalent to half of the power ratio. But why?”

ELI the ICE man

Capacitance is the ability of the cable to hold a charge. The larger the capacitance value, the longer it will takes for a signal to reach full amplitude within the cable. There are 3 passive components (passive components dissipate or store energy) that are used in impedance circuits. These are the resister, the inductor and the capacitor.

For an ideal inductor, the voltage leads the current by 90⁰

For an ideal capacitor, the voltage lags the current by 90⁰

Skin Effect

Skin depth decreases with increasing frequency.

At high frequencies, the AC current avoids travel through most of the conductor’s cross-sectional area. For the purpose of conducting current, the wire might as well be hollow!

In some radio applications (antennas, most notably) this effect is exploited. Since radio frequency “RF” AC currents would not travel through the middle of a conductor anyway. Most antenna structures and RF power conductors are made of hollow metal tubes to save weight and cost.

Reflection Coefficient

Reflection coefficient is the ratio of the amplitude of the reflected wave and the amplitude of the incident wave.

For NWA, the cable impedance is either 50Ω or 75Ω Transmission Line (TL). It is well to note that a lower cable impedance has a greater effect on the reflection coefficient than a high impedance.

Take example of 50Ω reference to 50±25 Ω

Γ= (Z_L – Zo) / (Z_L + Zo)

Γ₁ = (75 -50) / (75 +50)

= 25 / 125

= 0.2

Γ₂ = (25 – 50) / (25 + 50)

= -25 / 75

= -0.33

Comparing with 75Ω reference to 75±25Ω

Γ₃ = (100 -75) / (100 +75)

= 25 / 175

= 0.14

Γ₄ = (50 – 75) / (50 + 75)

= -25 / 125

= -0.2

As we can see from above, (75 + 25) provides the lowest value of 0.14. Hence the Reflection Loss (RL) is

20 log | 0.14 | = -17dB

Thus it is more difficult to maintain low Γ (Reflection coefficient) on 50Ω cable than it is on a 75Ω cable.

Two friends (Low Frequency) and (High Frequency) in the high rise building city

In general, signal attenuation from absorption increases with increasing frequency. Thus, lower frequencies such as very low frequency (VLF) and medium frequency (MF) bands will penetrate building material and even earth to a considerable distance, while 800MHz ultra high frequencies (UHF) will not.

Lower frequencies propagate further than higher frequencies, both through the air and into the ground. As for the air, the spectrum can be divided into

• Surface: The longest electromagnetic waves, below 500 kHz, tend to follow the curvature of the Earth, guided between the Earth and the ionized layers of the upper atmosphere (i.e. the ionosphere).
• Sky: 3 – 30MHz are reflected by the ionosphere and are capable of propagation around the Earth.
• Line of sight: Above 1GHz, electromagnetic waves behave much like light propagating through a clear, reasonably uniform atmosphere. When originating from a point source, they propagate in all directions.

Despite propagation advantages, a disadvantage of the lower frequency bands is the limited amount of available spectrum. At VLF, the available spectrum for public safety radio is only 27 kHz. This band is primarily used for application that only required low-speed data transmission. Ironically, the super high frequency (SHF) band has 27GHz of spectrum. However, it suffers from poor propagation characteristics. Additionally, the antennas for the lower frequencies will be large and cumbersome to be practical to certain users.

Between 500 kHz to 3MHz and 30MHz to 1GHz are the bands where the propagation characteristics in the atmosphere are more complex.