Line of Sight Antenna Range Vs Frequency?

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Scott R. Archer:
Hi, I know this is probably a simple question that I should be able to find a simple answer for via a Google search, but I can't find a clear answer anywhere.  Basically, in a normal day, I am curious how to calculate max line of sight (normal ground wave) range at different frequencies..

Say you have a 6 meter, 2 meter, and a 440 omni ground plane on a 100' tower, what is the range difference?  I know there is a simple Line of Sight formula where you use something like the square root of the height above ground then add something like 15%, but shouldn't there be large variance for different frequency bands?  I know there must be some difference distance, as even a VHF/UHF TV antennas seem to receive VHF frequencies substancially further away..  This also seems to be true of 2 meter vs 440 repeaters.  Is this a directly proportional (or inversely proportional) formula (meaning at 50mhz my ground wave range should be twice as far than 100 mhz)?  Or is it a little more complex?

Again, I know there are special conditions as when the band is open range is increased and tropo/skywave, but I'm asking in regards to regular groundwave conditions at same antenna height, and same antenna design/gain.

Steve Katz:
There are dozens of on-line free space path loss calculators which might be helpful, like this one:

http://www.changpuak.ch/electronics/calc_10.html

The problem is you have to know everything quite precisely to make a prediction.  For a given antenna "size," a 70cm antenna can have substantially more gain than a 2m antenna, and if its increase in gain is as much as the difference in path loss, then the signals will be the same strength for a given LOS path.

There isn't any "ground wave" at VHF or UHF.

LOS paths are "direct wave."

Most of what we work on VHF and UHF isn't anything even close to LOS (line of sight), but way beyond it, and the propagation method is primarily tropospheric forward scatter.  Tropo scatter can be more effective at higher frequencies than lower ones, which is why signals on 70cm for a given over-the-horizon path are frequently stronger than they are on 6 meters; because tropospheric propagation (other than LOS) is variable, path loss predictions are difficult.

Using a 10-band station that works 50 MHz through 10.3 GHz inclusive, I've often found signals stronger on 10 GHz than on any lower frequency for a given tropo path.  It all depends on conditions which are constantly varying.

The "calculators" are only valid for real LOS paths.

Dale Hunt:
Why should "line of sight" be significantly different on one VHF band than on another?

Hams have worked moonbounce from 10m to above 10GHz.  That's probably the longest line of sight
path we have available (though I remember some calculations of what it would take to work
"Venus-bounce".)

Scott R. Archer:
I think i'm more confused now.. hihi  I guess for Line of sight (direct wave) and due to forward scatter, 440mhz typically offers more range than 2 meters or 6 meters..  I always thought the opposite was true (lower frequencies more efficiently reached further) for some reason..  I also thought it was why VHF TV stations generally have longer range than UHF ones..

Steve Katz:
Quote from: KB9WIS on July 27, 2010, 04:09:44 PM

I think i'm more confused now.. hihi  I guess for Line of sight (direct wave) and due to forward scatter, 440mhz typically offers more range than 2 meters or 6 meters..  I always thought the opposite was true (lower frequencies more efficiently reached further) for some reason..  I also thought it was why VHF TV stations generally have longer range than UHF ones..


Actually, they don't.  If it seems that way, you've probably experienced UHF TV antennas that weren't very good, or more lossy transmission lines at UHF (which of course is usually the case -- line loss is proportional to frequency).

LOS paths are always very strong.

Over the horizon paths vary depending on not just frequency, but how far over the horizon the signal needs to scatter, and wave polarization, and weather, and a lot of things.

TV-DBS (satellite) signals come to you at 10 GHz and higher.  If VHF worked better for this application, it would have been used.

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