# Horizontal Fresnel

How important is Horizontal Fresnel Zone…?

Is it Ok if the LOS passes 10 to 50 meters horizontal away from Obstacle like small hill , whose height is higher than that of towers at both ptp location.

The Link distance is 36 kms & the obstacle is at a distance of 10 kms from one point where height is 100 m from MSL.

The cross section of the Fresnel zone is circular at all points. Calculate the Fresnel radius normally, using the appropriate frequency, link distance, and near-object (hill) distance. The radius calculated is applicable in all directions perpendicular to the direction of the link. Double the Fresnel radius and add it to the earth curvature at the point where the link crosses the hill.

Why double the Fresnel radius? The slope of the hill causes the hill to encroach a greater amount into the Fresnel zone. A 45-degree slope (100% grade) would mean the Fresnel clearance is only 70% (half of the square root of 2) of the vertical (earth curvature) clearance. The slope of the hill makes the accuracy of your topographic maps critical in all three dimensions: latitude, longitude, & elevation. Doubling the calculated Fresnel zone radius gives you some margin for error.

All this assumes, of course, that you’re doing these calculations manually. I don’t know if software for calculating RF links will typically take into account the slope of the terrain.

Ok. So this means that if LOS passes at a distance > Double the Fresnel zone width from the place from where hill upper slop start upwards in away direction & Ground Height is ok like the signal would pass through …am i correct…?

Also for the Horizontal Fresnel clearance , I just need to ensure for the Fresnel only right ? & not additional thing is t be added like earth curvature for vertical fresnel am i right…?

I suggested calculating the RF LOS normally by adding the Fresnel radius to the earth curvature at the critical point: the hill.

I presume the hill is sloping from left to right or right to left when looking from one end of the link to the other. To compensate for both a steep hill and inaccuracies with topo maps I also suggested doubling the Fresnel radius before adding it to the earth curvature. If your maps are accurate, however, the slope would have to be a 173% grade – rising 173 feet in 100 horizontal feet, a mountainous 60-degree angle – to justify doubling the Fresnel radius. You can likely get by even if the hill encroaches within double the Fresnel radius.

Your last post indicates you may be adding the Fresnel radius to a known visual LOS instead of calculating earth curvature. That will also work.

Yes. I went up the hill & checked the actual LOS.We here do not have MAP & GIS stuff handy.

Total Link Distance 36.5 km. End_1 has 90 m height , hill at 10.5 km from End_1 .Hill Height is 170 mtr.The place where LOS is crossed has 105 mtr of height. End_2 has 60 m height.The Other terrain is not exceeding 30 mtrs actual meters + 15 mtrs of Tree.

All the heights r MSL readings.

I feel adding 30 mtrs height at End_1 should clear hill obstacle.As the Fresnel at 10.5 kms comes to 15 mtrs @ Fresnel 1 & 9 mtr @Fresnel .6
So looks like doubling These to 30 mtr & 18 mtrs also makes LOS clear.

Horizontal height where the LOS is cut is does not have slop at this place & remains same for these width.

It’s very difficult to verify visual LOS from a midpoint; just because you can see both endpoints from the midpoint doesn’t mean the end points can see each other. You could instead climb one tower and try to see the other tower using binoculars or a spotting scope, but this is likely impractical due to haze or wind.

What I meant was that where the Hill Obstacle is there @ 10.5km from end_1 I measure the height & where LOS as per GPS mtr cuts LOS remains constant @100 mtr for a width allowance of arond 70 t 100 mtrs.

The other terrain is ok like .

I can not see the end_2 from Obstacle from Binoculars as it is around 26.5 kms. & as u say is not practical.

OK. i CHECKED up with Terrain data.as per the Radio Mobile Link would not go though…

So looks like tough work.Will check for BH-30