How to point a yagi antenna in the line of site
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How to point a yagi antenna in the line of site
I would like to use a yagi antenna to point to a vhf station 50 miles from my home, what would be a good meted to get it in the right direction (line of site), I have both Latitude/Longitude I just need it should by aligned.
From the site you are attempting to hear, you can transmit a weak carrier on channel, and then aim your antenna to the highest signal level.
That's the simplest way to get spot on +/- a degree or so.
If you can hear the weak signal on a hand held, you sure will be able to peak on a yagi with directional gain.
Compass bearings work also, using the main and reciprocal of the two, or 180 degree difference.
Remember, in certain areas of the country, you will have a geomagnetic disturbance and the readings will be off by as much as 7 degrees depending on actual location.
High iron content soil can cause false readings if the ore deposit is large enough and distributed over a vast disatnce under ground, as in the U.P of Michigan.
Magnetic north and true north are not the same reading either.
just food for thought....
That's the simplest way to get spot on +/- a degree or so.
If you can hear the weak signal on a hand held, you sure will be able to peak on a yagi with directional gain.
Compass bearings work also, using the main and reciprocal of the two, or 180 degree difference.
Remember, in certain areas of the country, you will have a geomagnetic disturbance and the readings will be off by as much as 7 degrees depending on actual location.
High iron content soil can cause false readings if the ore deposit is large enough and distributed over a vast disatnce under ground, as in the U.P of Michigan.
Magnetic north and true north are not the same reading either.
just food for thought....
A couple of nits:
First, if you derive a bearing between two lat/lons, be sure you know whether the resultant bearing is based on True North or Magnetic North. If the former and if you want to use a magetic compass to align the Yagi, you'll have to convert the true bearing to a magnetic bearing. In New England, magnetic bearings are 16 degrees greater than true bearings; I don't know offhand what the deviation is for other parts of the country.
Second, if you try to use a small magnetic compass on the roof of a building or the platform of a tower, surrounded by metal objects, the result is going to be pretty crude. There is a better way, based on "local apparent noon," which is going to sound complicated but really isn't.
First, you have to know what time is local apparent noon for your longitude for any given day (it will vary by as much as 15-20 minutes either way over the course of the year).
Local apparent noon is equal to:
Zone Mean Noon (clock time; use 1200 for standard time and 1300 for daylight savings time),
Adjusted for Zone Offset (the difference between your exact longitude and the median longitude of your time zone), and
Further adjusted for the "Equation of Time" (which is that daily variation I mentioned above).
Median longitudes are as follows:
Eastern Zone = 75 degrees W. Lo.
Central Zone = 90 degrees W. Lo.
Mountain Zone = 105 degrees W. Lo.
Pacific Zone = 120 degrees W. Lo.
Local Apparent Noon moves west at the rate of 15 degrees of Lo. per hour, or one degree in 4 minutes. To calculate the difference between your longitude and your zone's median longitude, in decimal degrees; multiply that result by 4; and if you are east of the median longitude, subtract the result from 12:00 noon (zone time) (or 13:00, if on daylight savings time) or if you are west of the median longitude, add it to 12:00 (or 1300).
(For example, in Boston, at a longitude of 71 degrees W, local apparent noon is approximately 1144 during the winter and 1244 during the summer. 71 - 75 = -4; -4 * 4 = -16; 12:00 - 00:16 = 11:44.)
For most purposes, you can ignore the Equation of Time, which can be found in the Nautical Almanac for each day of the current year.
Now, go to your site a few minutes before local apparent noon. Put a pencil or other thin rod-like object vertical against a horizontal flat surface and wait; at the time of local apparent noon, draw a line over the shadow cast by the pencil on the horizontal flat surface.
That line represents True North/South at your location.
Use a protractor to find the precise true bearing toward which you want to point your Yagi.
First, if you derive a bearing between two lat/lons, be sure you know whether the resultant bearing is based on True North or Magnetic North. If the former and if you want to use a magetic compass to align the Yagi, you'll have to convert the true bearing to a magnetic bearing. In New England, magnetic bearings are 16 degrees greater than true bearings; I don't know offhand what the deviation is for other parts of the country.
Second, if you try to use a small magnetic compass on the roof of a building or the platform of a tower, surrounded by metal objects, the result is going to be pretty crude. There is a better way, based on "local apparent noon," which is going to sound complicated but really isn't.
First, you have to know what time is local apparent noon for your longitude for any given day (it will vary by as much as 15-20 minutes either way over the course of the year).
Local apparent noon is equal to:
Zone Mean Noon (clock time; use 1200 for standard time and 1300 for daylight savings time),
Adjusted for Zone Offset (the difference between your exact longitude and the median longitude of your time zone), and
Further adjusted for the "Equation of Time" (which is that daily variation I mentioned above).
Median longitudes are as follows:
Eastern Zone = 75 degrees W. Lo.
Central Zone = 90 degrees W. Lo.
Mountain Zone = 105 degrees W. Lo.
Pacific Zone = 120 degrees W. Lo.
Local Apparent Noon moves west at the rate of 15 degrees of Lo. per hour, or one degree in 4 minutes. To calculate the difference between your longitude and your zone's median longitude, in decimal degrees; multiply that result by 4; and if you are east of the median longitude, subtract the result from 12:00 noon (zone time) (or 13:00, if on daylight savings time) or if you are west of the median longitude, add it to 12:00 (or 1300).
(For example, in Boston, at a longitude of 71 degrees W, local apparent noon is approximately 1144 during the winter and 1244 during the summer. 71 - 75 = -4; -4 * 4 = -16; 12:00 - 00:16 = 11:44.)
For most purposes, you can ignore the Equation of Time, which can be found in the Nautical Almanac for each day of the current year.
Now, go to your site a few minutes before local apparent noon. Put a pencil or other thin rod-like object vertical against a horizontal flat surface and wait; at the time of local apparent noon, draw a line over the shadow cast by the pencil on the horizontal flat surface.
That line represents True North/South at your location.
Use a protractor to find the precise true bearing toward which you want to point your Yagi.