| Author |
Message |
    
Weatherman49
Member
Username: Weatherman49
Post Number: 70
Registered: 2-2005
| | Posted on Wednesday, December 07, 2011 - 2:55 pm: | 
|
well whats the deal fill me in how will this so called take off angle affect short distance/long distance comms? will hieght affect take off angle? |
Tech237
Moderator
Username: Tech237
Post Number: 1501
Registered: 4-2004
| | Posted on Thursday, December 08, 2011 - 9:15 am: |
|
The lower ther take of angle (to a point) the better the signal for skip. Draw an half circle to represent the earth, then draw a large one above it to represent the ionosphere. Pick a point on the earth to be your antenna, then draw straight lines from that point, at various angles, until they hit the ionosphere. At that point draw the reflected line (eqhal angle) until they hit the earth again.
The lower the angle the further away it returns to the earth. Now antennas themselves will have different angles of radiation - quarter wave antennas are around 45 degrees for example.
As for the height, there is a specific height for each antenna and frequency that will give the lowest angle of radiation. Now 833 may recall better, but I believe that it is around a halfwave above the "true" ground - ground may not be the physical ground, but maybe a metal roof, or even below ground level.
THis is better explained in both the ARRL Handbook, and their Antenna Handbook.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
|
Barracuda
Intermediate Member
Username: Barracuda
Post Number: 348
Registered: 3-2003
| | Posted on Friday, December 09, 2011 - 7:16 pm: |
|
The thing that many forget is that an antenna placed between the points of take-off and the point where the wave finally makes it back to earth will hear nothing unless it happens to be within ground wave distance of the transmitting antenna or high enough above the earth to intersect the decending wave. They don't call this region the SKIP zone for nothing. Given the relatively high frequencies of 11m (27mhz) you really can't vary the take-off angle too high or the ionosphere will not reflect the signal back to earth at all. However, much lower frequencies like 40m and 75m can be used at "cloud warmer" angles (nearly vertical) to permit regional communications that fill in the distance between the groundwave signals and the skip zone that would have been created by lower takeoff angles. This style of propagation is called Near Vertical Incident Skywave (NVIS). It is great for regional comms, say for emergency communications situations. Unfortunately, as I said before, this is totally useless on 11m or really anything over 40m these days. When we are in the thick of the solar cycle maybe 30m will be OK, but then only during the daytime. |
Tech833
Moderator
Username: Tech833
Post Number: 2079
Registered: 8-2002
| | Posted on Tuesday, December 13, 2011 - 12:52 pm: |
|
For local communications, you want a low takeoff angle so your signal goes to those around you, and not over the top of them. You get a low takeoff angle by raising your antenna higher above ground and obstructions.
For long distance skip, you also want a low takeoff angle for the reasons stated already. For short distance skip, you want a higher takeoff angle. You achieve this by lowering your antenna.
For some of the amateur contest stations, you'll see several beam antennas mounted on very tall towers. The lower antennas are selected when doing closer-in DX and the higher antennas selected for longer DX (like overseas).
There is no one perfect formula to calculate takeoff angle vs. skip zone because the ionosphere is always changing. The takeoff angle that works today might not work at all tomorrow.
Large shortwave broadcast stations have curtain antenna arrays with variable takeoff angle. That is so the programming can be beamed into exactly the area wanted, and can be adjusted on the fly as conditions change.
Your radio 'Mythbuster' since 1998
|
Weatherman49
Member
Username: Weatherman49
Post Number: 71
Registered: 2-2005
| | Posted on Friday, December 16, 2011 - 11:14 am: |
|
Thanx guys! |
Henry_hpsd
Junior Member
Username: Henry_hpsd
Post Number: 11
Registered: 2-2007
| | Posted on Sunday, December 18, 2011 - 4:08 pm: |
|
http://www.dx-antennas.com/Height%20versus%20take%20off%20angle.htm
Kind regards,
Henry |
|
