What Are the Advantages and Disadvantages of VHF and HF?
HF stands for high frequency, while VHF means very high frequency. We’ve seen a shift from lower frequency antennas to high frequency antennas for a variety of reasons. What are the advantages and disadvantages of VHF and HF? Why would you want to use high frequencies instead of lower ones?
What Are the Advantages of VHF and HF?
The size of the antenna element is based on a fraction of the wavelength. Higher frequency antennas receive smaller wavelengths, and they are therefore smaller. Receiving antennas will be smaller than the transmitting antennas, but the receiving HF antennas would still be smaller than receiving microwave antennas. Smaller antennas require less material, are easier to mount, and are more discrete.
Higher frequencies provide more bandwidth; you could send far more data in less time with a VHF antenna than a lower frequency antenna. That explains the prevalence of 2.4 GHz and 5.8 GHz antennas for wifi over the older 900 MHz antennas. The 5.8 GHz antenna can deliver more data in the same time frame than the 2.4 GHz. When you’re dealing with a crowded field like a road full of cars broadcasting their locations, you have to work at higher frequencies if the RFID system is to simultaneously handle all the signals without interference.
A side benefit of VHF is that there are fewer devices working in this range, so they don’t suffer as much interference in general. For example, the 900 MHz range is already used for RFID systems and cordless phones, so 900 MHz communication devices can suffer from interference with any of these.
And as you increase the frequency, you increase the transmission quality as long as you’re dealing with high-definition signals. An additional advantage of VHF is that it is less subject to interference from background radiation such as an electrical system interfering with a wireless phone’s reception.
What Are the Disadvantages of VHF and HF?
Lower frequencies provide deeper penetration through solid materials. If you want a radio or TV signal to pass through walls to rabbit-ears on top of the family TV, you’d prefer the broadcast to be on a lower frequency. VHF and HF are also more likely to be affected by weather events, sea water and natural conditions. Conversely, a high frequency antenna probably has to be in line-of-sight to send or receive a signal.
High frequency generally equals shorter range. A 2.4 GHz antenna will have almost a third of the range of a 900 MHz antenna. Furthermore, a 2.4 GHz antenna can send and receive over greater distances than a 5.8 GHz antenna. This has a major impact when you’re designing a network to provide full coverage for an area. With HF and VHF, you’ll need more antennas to provide full coverage for a given area.
Let’s use cell phone networks as an example. The higher frequency antennas going in to support 5G networks will need to be present at several times the density of 4G cell phone towers. Instead of everyone connecting to a single tall tower, intermediate antennas will need to be placed along roads, in public places and inside buildings to provide sufficient bandwidth and be close enough to the end users. Your live streaming via Periscope may then be routed to a VHF antenna a few meters overhead while a text message can travel 300 meters to a cell phone tower.
If you’re using HF frequencies for skywave transmissions, the quality of the transmission depends on the ionospheric conditions. That forces operators to change frequencies twice a day or more. If you’re working at lower frequencies, this is less of an issue.
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