Topic Author
Posts: 2276
Joined: Thu Aug 05, 1999 1:42 pm

H-Class VORs

Wed Oct 04, 2000 1:46 am

I've been wondering how these work.

More specifically, I was wondering how they can guarantee that at 1,000' to 14,500' the radius of the signal extends to 40nm. Then at 14,500' to 18,000' the radius of the signal is 100 nm. Then from 18,000' to 45,000' it's 130nm. From 45,000' to 60,000' it drops back to 100 nm.

How do they make it work in this "upside-down wedding cake" fashion?

I would expect it to work in a fashion like a cone or like a flashlight, where the widest point is at the end. I really don't understand how they can make it work like this.

If there's anyone out there who can help, it would be appreciated and you're a better person than I am!  

- Neil Harrison
Posts: 1589
Joined: Tue Nov 09, 1999 6:20 pm

RE: H-Class VORs

Wed Oct 04, 2000 2:43 am

Think of it more as an ice cream cone than a flashlight. The widest part is almost at the top, but not quite, it's rounded off at the top and therefore narrows down as it approaches the top. The wedding cake shape is formed by the "guarenteed" ranges, however you can usually receive it beyond these ranges, it's just not guarenteed. The "range cone" does extend beyond the wedding cake borders, but the "guarenteed range" is defined by the cake  
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RE: H-Class VORs

Wed Oct 04, 2000 3:53 am


The most important phrase to remember here is: LINE OF SIGHT! Your flashlight analogy is a good one!

With that said, all VORs are in the VHF frequency range of 108--117.95 mHZ.

108--111.95 is reserved for localizer use (often paired with a glide slope channel). The lower ends of the alloted VOR spectrum are LOWER POWERED. Thus, in order to pick up the low powered signal (or to see your low powered pen light, at night) from the localizer, you have to be in close.

112--117.95 are the navigable VORs that you are thinking of. The low altitude VORs will produce a lower powered signal, and these stations will be placed where they cannot interfere or be interfered with by other stations. The signal on a low altitude VOR is akin to that "d" cell ever ready flashlight you had as a cub scout--it can be seen further out than the pen light, but it doesn't penetrate too far into the woods or too much into the darkness.

The surface of the earth is curved, so the closer you are flying to the surface, the closer you must be to the source of the signal to receive it. The earth is "flat", so they say, about 24 miles in all directions--terrain, obstacles, and trees, aside. Obstructions and atmospheric conditions will degrade the signal strength.

Now, the high altitude VHF OMNI RANGE station has a very high powered transmitter. Not only must this signal carry out through obstructions, it must also power itself upward and outward. Inside that cone shaped station is a powerful directionalized antenna which rotates 30 complete revolutions in a second. In order to be picked up from 1,000 to 14,500 ft, the VHF signal is thrust outward with enough power that it will penetrate through obstructions and reach you, as long as you are line of sight. VHF will even penetrate (somewhat) through the atmospheric D layer and skip off, thus guaranteeing to reach you, even if you are flying 1000 ft AGL. Using your flashlight analysis, this VOR is one of those Brinkman flashlights that the night security patrols at Target use. They put out a lot of power, and you can directionalize and focus the strength of the beam. The ability to directionalize and focus the beam is done in a VOR through the antenna and the power output.

From 18,000 to 45,000 you are, theoretically, line of sight from a long ways out. At 130nm, the low altitude VOR will probably not reach you, because it does not have the power (or long range throwing arm to reach you) although you can, theoretically still see it.

I think that at 45,000 and above, the ionization kicks in, and it becomes difficult for a signal to penetrate. In fact, HF signals will reflect and skip off this layer (D layer).

Here is another factor. At night, the D layer disappears and radio signals carry a long way. That's why you may be driving in the upper peninsula of Michigan and pick up a radio station from Detroit at night.

But, by and large, the ranges that you mention are guaranteed. From time to time, the VOR stations are calibrated and adjusted--through power output, antenna adjustment, frequency re-allocation, etc.

The differentiation is made between the VORs to avoid frequency interference. Hope this helps.

Topic Author
Posts: 2276
Joined: Thu Aug 05, 1999 1:42 pm

RE: H-Class VORs

Wed Oct 04, 2000 6:08 am

Thanks, DC-9CAPT and Ralgha! It's much appreciated.

I think I get it now!  

- Neil Harrison

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