If the earth is flat why is it that ham radio operators need repeaters to transmit their signals more than 12-14 miles?

• Indirect propagation: Radio waves can reach points beyond the line-of-sight by diffraction and reflection.^[7] Diffraction allows a radio wave to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as cell phones, cordless phones, walkie-talkies, and wireless networks. A drawback of this mode is multipath propagation, in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves interfere, often causing fading and other reception problems.
• Ground waves: At lower frequencies below 2 MHz, in the medium wave and longwave bands, due to diffraction vertically polarized radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling as surface waves which follow the contour of the Earth. This allows mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, out to hundreds of miles. As the frequency drops, the losses decrease and the achievable range increases. Military very low frequency (VLF) and extremely low frequency (ELF) communication systems can communicate over most of the Earth, and with submarines hundreds of feet underwater.
• Skywaves: At medium wave and shortwave wavelengths, radio waves reflect off conductive layers of charged particles (ions) in a part of the atmosphere called the ionosphere. So radio waves directed at an angle into the sky can return to Earth beyond the horizon; this is called "skip" or "skywave" propagation. By using multiple skips communication at intercontinental distances can be achieved. Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Widely used during the first half of the 20th century, due to its unreliability skywave communication has mostly been abandoned. Remaining uses are by military over-the-horizon (OTH) radar systems, by some automated systems, by radio amateurs, and by shortwave broadcasting stations to broadcast to other countries.

Marlelar, happenby is attempting to blow smoke in your face... seems though that she is doing a little research on how radio waves behave... so bravo for that.

The problem with her response is... it doesn't answer your question. What she demonstrates is the very type of radio frequencys needed to overcome the issue of the horizon, aka curvature. What she's putting forth accurately describes LF radio. But, the most commonly used frequencies used day-to-day is VHF (very high frequency) and UHF (ultra high frequency.) These will be covered here below from a description that I retrieved from Wikipedia.


Propagation characteristicsEdit
Radio waves in the VHF band propagate mainly by line-of-sight and ground-bounce paths; unlike in the HF band there is only some reflection at lower frequencies from theionosphere (skywave propagation).^[2] They do not follow the contour of the Earth as ground waves and so are blocked by hills and mountains, although because they are weakly refracted (bent) by the atmosphere they can travel somewhat beyond the visual horizonout to about 160 km (100 miles). These characteristics allow the same VHF frequencies to be used by different users in neighboring geographical areas without interference (frequency reuse). They can penetrate building walls and be received indoors, although in urban areas reflections from buildings cause multipath propagation, which can interfere with television reception. Atmospheric radio noise and interference (RFI) from electrical equipment is less of a problem in the band than at lower frequencies. The VHF band is the first band at which efficient transmitting antennas are small enough that they can be mounted on vehicles and portable devices, so the band is used for two-way land mobile radio systems, such as walkie-talkies. Occasionally, when conditions are right, VHF waves can travel long distances by tropospheric ducting due to refraction by temperature gradients in the atmosphere.


And also this...


Line-of-sight calculationEdit

"Rabbit-ears" VHF television antenna(the small loop is a separate UHF antenna).
For analog TV, VHF transmission range is a function of transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a near line-of-sightphenomenon. The distance to the radio horizon is slightly extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere.
An approximation to calculate the line-of-sight horizon distance (on Earth) is:

These approximations are only valid for antennas at heights that are small compared to the radius of the Earth. They may not necessarily be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves.
In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station.^{[citation needed]}
The accuracy of these calculations for digital TV signals is being debated.^[3]

So the earth is curved, demonstrated by the fact that radio waves travel in a straight line but will not travel from San Francisco to Tokyo unless bounced off the ionosphere?  If the earth was flat then the signal would just shoot straight across the Pacific and not need a bounce?

Common sense is a wonderful thing... ain't it?

Ok, weather I get, but what are your “lot of other reasons”, what obstructions lie between San Francisco and Tokyo?  What do you mean by “etc”, what else would there be?