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Vhf Communication

Is there really any difference between the upper HF and the lower VHF frequencies in voice communication?

In regards to a wireless system.Range, interference, penetration of structures, weather?

HF is below 30 MHz, where VHF is above 30 MHz. Typical voice operating modes are AM and SSB on HF, and FM and SSB on VHF. AM, FM and SSB are all used near 30 MHz. SSB is by far the best communications, with the greatest range, followed by AM, then FM.

Range depends on antenna height, antenna gain, transmit power and receiver sensitivity. There is not much difference in range bewteen upper HF and lower VHF in this respect. Antenna size decreases as frequency increases. The smallest antenna is typically 1/4 wavelength long. At 20 MHz this is 12.3 feet, and at 50 MHz this is 4.9 feet. This means you can have more antenna in less space as frequency increases. Power output tends to be more difficult above 30 MHz, as many modules are designed to go up to 30 MHz. That's not to say there aren't Low VHF modules too, but they are slightly more expensive, and more complex.

Interference depends on the sunspot cycle (currently we are near the 11 year low). During times of high sunspot activity, the atmosphere is energized, and allows longer communications known as skip. This typically starts at lower frequencies first, and increases. This is known as the MUF (Maximum Useable Frequency). During these openings communications over long distances are open, as if they were next door. The problem with these, is they are very unpredictable. For this reason a higher frequency is desired, but skip can occur in the high VHF range (150 MHz) as well, just less often.

Static interference (Lightning) increases as frequency decreases. FM is not affected by lightning, where AM and SSB are.

Structure penetration increases as frequency increases, but without more specifics here it's hard to say if there would be much difference. At UHF frequencies structure penetration is noticeable better than at high VHF. 50 Mhz tends not to be asbsorbed by foliage as much as 30 MHz.

Weather affects both of these bands. My perception is that weather affects low frequencies more, but that's difficult to quantify. Local propagation is affected by temperature, humidity, cloud cover and such, but I haven't been able to understand exactly why.

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STS-131 Shuttle Launch VHF Communication



What is the highest channel that UHF broadcast channels go up to?

For instance, VHF goes up to 1-13, so UHF would start at 14 but end where?

**Majoring in Communications***

In the U.S., since the digital conversion in June 2009, the available broadcast channels are primarily 7-13 VHF and 14-51 UHF. U.S. channel frequencies above 51 were transferred to other communications services.

There are a very few stations still transmitting on low VHF, ch. 2-6, but they are a rare exception to the rule in the U.S.

Note that the channel a station is transmitting on is not necessarily the channel number that it's commonly known by. For example, my local channel "3" is actually transmitting on ch. 35, and my channel "6" is really transmitting on ch. 9. This is the mysterious "actual" and "virtual" channel process adopted by U.S. stations with the digital conversion. For example, channel "3" has been known by that number for over 50 years. With the new channel assignment plan, they could no longer operate on ch. 3, so they switched to one of the permitted channels, 35. But they kept "3" as their identity, and they send a digital signal on their broadcast that makes TV tuners display "3" when the signal is actually coming in on ch. 35.

Did you notice that with this process, one needs a UHF antenna to receive channel "3" ? No wonder we have hundreds of DTV questions here every week.

This is exactly what most over-the-air stations in the U.S. are now doing.

Tags: antenna, ham, radio, reference, vhf, vhf communication, vhf communication in aircraft, vhf communication procedure, vhf communication system, vhf communications magazine

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