height above mean sea level?

[bobdavcav]

Why is this number important? I can see why the height above average terrain and the height above ground level, but why is the height above sea level important? Is it just for calculating the average terrain?

 

[TomT]

Yes. Plugs into the FCC's own AHAAT calculator:

http://transition.fcc.gov/mb/audio/bickel/haat_calculator.html

BTW, ( and you find this if you read the notes), AHAAT for full power stations is determined by 8 standard radials. If you run more radials, you will get a different figure.

 

[bobdavcav]

Hmm, that's pretty cool. What I don't see in that program though is how high I would have to build an antenna to get the desired haat. It seems to me like they have all the terrain data for these areas, so it seems like the height above mean sea level should be replaced with height above ground, shouldn't it? I just checked with an app on my phone and it looks like I was entering height above ground level of about 15 meters, but if I didn't know the elevation, how would I calculate how high the antenna should be?

 

[bilco]

Height above sea level is also significant when comparing possible conflicts when two fairly distant stations are being compared. HAAT of one may have little refernce to the other. Height above sea level is constant for both.

 

[TomT]

You can pull up your antenna site on a Topozone topographic map:

http://www.trails.com/maps.aspx

Topographic maps are also on-line at the USGS, you can download the entire 7.5 minute series topographic as a pdf:

You will need to know the name of the map you are looking for, usually listed as the "quad" when you zoom in on the Topozone site.

Once you find the site it is a simple matter to find a marked contour line to determine the elevation above sea level at your site. Make sure you check to see if the map's contours are in meters or feet (should be obvious); FCC applications want all numbers in metric form.