My DTV is like HF??

Originally Posted by Ed Foster

Hi, guys. This might be simple for somebody here so here goes. I've set up my old analogs with the converter boxes and did the autoprogram. I'm using a Phillips outdoor amplified antenna mounted in various spots on a pole. I live on Melrose Mt. in Tryon, about 2300' elevation but heavily wooded. At first it seemed like I hooked up to a dish, showing about 50 channels and most were godlike. However, as nighttime rolls around some of our favorite channels lose signal strength like crazy and come and go. It reminds me of my years on HF, shortwave, when daytime signals faded and you had to change freqs.

Somebody at RS told me that some stations are not at full power yet on HD and also reduce power at night.

Any ideas?

It's probably more common to have improved reception on some channels at night because of atmospheric temperature inversions.

UHF advantages

Fringe or DX UHF TV reception has unique technical challenges, which leave little room for error. Let's consider some of the advantages for UHF verses VHF TV reception:

1. Man-made and external ambient noise levels at UHF are at least 20dB less than VHF band I (45-70 MHz).
2. Low noise masthead preamplifiers are very beneficial at UHF.
3. UHF antennas are smaller and compact, hence relatively easy to produce high gain aerials.
4. Due to the small wavelength size, it is more practical to stack high gain UHF antennas, but with manageable physical space.
5. Height gain is significant at UHF. Usually one obtains about 0.5dB gain for every foot heigher, for heights between 25 and 35ft. In other words, the same aerial will provide about 5dB more signal at 35ft than at 25ft.
6. UHF transmitter powers are very high, typically 500kW to 1000kW.

UHF disadvantages

1. UHF TV tuner noise figures are around 10dB (typical). VHF tuner noise figures are typically 6dB. This means UHF tuners are 4-6dB less sensitive than VHF tuners.
2. Signal path loss is much greater at UHF.
3. Trees, foliage, buildings, etc, greatly absorb UHF signals.
4. Transmission coax cable line losses are far greater at UHF compared to VHF band I.
5. UHF TV aerials need to be positioned higher than VHF aerials.
6. The voltage developed by a dipole element decreases inversely with frequency. For example, a band I dipole will develope from a given field strength about 15 times as much signal voltage as a high band V UHF dipole.

To help compensate for UHF disadvantages, we need to observe the following points:

1. Use high gain (13-18dB) 'deep fringe' type UHF only TV aerial(s) (see list below).
2. Use a quality low noise (2dB or less) masthead UHF preamplifier.
3. Use low loss 75 ohm coax cable (see table below). For example, RG-11 coax. Never use RG-75U coax!
4. Position the UHF antenna at least 30 ft above ground level. Within constructional limits, the higher the better.