radio telephony at 700 nm above

[Shabez]

an a/c is flying at a range of 750 nm on 320 fL which frequency should be used during night for r/t vhf,hf,mf,lf


it cant be vhf its line of sight

hf is used for long range but its max range in ndb's is 200nm

at mf and lf too much attenuation/poor signal to communicate in radio telephony

so i cant figure this one out any help appreciated

[galaxy flyer]

Once beyond VHF range, about 240nm, all comms are either HF (2-18Mhz) or datalink using SATCOM.

GF

[sevenstrokeroll]

don't forget two dixie cups and a string...albeit a long one.

[Tarq57]

HF is capable of quite long range, longer than you suggest (NDB's are usually in the MF range) as the radio signals bounce of the ionosphere.

Quality/range varies, depending on the location, the frequency used, and the weather on the day. (Or night.)

The list of frequencies that each oceanic control centre uses are available in the relevant AIP for the area of the world concerned.

[FullOppositeRudder]

For a very basic outline of what happens on HF, scroll down this page to the "Radio Propagation at HF frequencies" section.

ACMA - Radio propagation fact sheet (http://www.acma.gov.au/WEB/STANDARD/pc=PC_1691)

As I said - basic, but this may be helpful. HF propagation can also be affected by the conditions of the ionosphere which takes us into yet another set of considerations (We won't go there this time).

In the real world, the earlier comments by Tarq57 can be followed with confidence, especially with reference to the published lists of ATC frequencies used by control centres.

FOR

[NZScion]

Who says that the maximum range of an NDB is 200 NM? I thought that the max range of a NDB was whatever the rated coverage of that particular beacon was, dependent on transmission strength, frequency, terrain, environmental conditions, etc.

I've seen NDBs with a rated coverage of 400 NM under certain conditions, just as I've seen some with a rated coverage of 30 NM...

[Green Guard]

True,
especially for some sea port NDBs etc.

it cant be vhf its line of sight


still it can even on distances over 700 NM, but if that distance is over a sea level, and other station is on an island etc...

:ouch:

[GarageYears]

VHF/UHF radio comms are line-of-sight but that does not mean visible or optical LOS.... rather RADIO FREQUENCY LINE-OF-SIGHT.

VHF and higher radio frequencies do not bounce off the atmosphere unlike HF for example, but travel in 'straight-lines', so line-of-sight really means NOT BLOCKED by something large and solid - like the Earth itself.

If transmitter and receiver are at ground level, then curvature of the Earth will limit the 'line-of-sight' to perhaps 25 miles or so for a VHF radio (ignoring any Fresnel diffraction...).

However if one of the radios happens to be at 20,000 feet, the Earth's horizon is 173 miles away, so you might get transmission at least that far.

If two aircraft are at 20000 feet then.... a lot further. The range limit will switch from being the horizon to being the transmitted power and sensitivity of the receiving antenna/receiver combination. Radio waves effectively follow the 1/r squared rule, so every doubling of distance results in a reduction of the received signal by 6dB...

Military radios in the UHF band (ARC-164) appear to have a typical range at 20000+ feet of around 200-250 miles based on pilot comments I have worked with.

-GY :8