Antenna height vs antenna gain?

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Scott R. Archer:
I believe I already know the answer to this question, but would like to get others input, with possible real life experiences in addition to practical theory (if possible).

Basically I'm looking for max VHF line of site coverage and am curious how much antenna height tradeoff would be worth it, if I could mount a higher gain antenna.  

In this example, if I can either mount up 80' with a single (low gain) dipole design (space is limited at the top) or I can mount it only halfway up (40 feet), but I'll have a lot more room to use a much larger antenna, such as a 8db high gain collinear/stacked dipole array.

In this example, lets assume it's over average ground, flat terrain with no obstacles in the way..  Generally we hear height is king, but was curious about overall distance between trading for a lower gain antenna for more height..  Which of the 2 scenarios would most likely provide the furthest range?  

:
Hello.
Simplifying a lot, if both your station and the other station antennas are both at 12 m (40'), each one has a radio horizon of ~15km, so the path between both stations is ~30km.

Elevating both antennas to 24m (80') the radio horizon extends to ~20km on each side, providing for ~40km path between the two.

Antenna gain has nothing to do with line of sight propagation provided the ERP is enough for the signal to cover the distance between the stations.


Quote

Which of the 2 scenarios would most likely provide the furthest range? 
All other conditions the same (power, line loss, etc.), height.

73

Mike Brenza:
Height.
Experience?... Midwest balloon launch outside of Chicago with a 1 watt transmitter on board.  Copied CW signals 57 here in Maryland once it got to the 80,000 ft level and above.
But you might wish a beam antenna for weak signal work too.  Think about having both!
73s.

-Mike.

Scott R. Archer:
LU2DFM, in the past, I have seen higher gain antennas increase max line of sight distance too though.  It appears to me, that high gain antennas can pull out stations out of the noise, that lower gain antennas were not able to, therefore they should increase line of sight range to at least some degree..  

In regards to max range over flat terrain, would you suggest that doubling the height is more effective than adding 6 db of antenna gain, and if so how much more so than the latter?  I've read doubling height also increases your signal by 6db as well, so I'm kind of curious how it pans out vs using an antenna with 6db more gain at half the height..

Dan:
Quote from: LU2DFM on June 16, 2010, 01:16:54 PM

Antenna gain has nothing to do with line of sight propagation provided the ERP is enough for the signal to cover the distance between the stations.

Quote from: KB9WIS

It appears to me, that high gain antennas can pull out stations out of the noise, that lower gain antennas were not able to, therefore they should increase line of sight range to at least some degree.. 


I think we're having a problem of definition and it's not line of sight that we're actually talking about.

True line of sight (where the antennas can actually "see" each other at radio frequencies) has so little path loss in the context of  our typical output powers and receiver sensitivities that antenna is barely a consideration.  The most typical ham experience of this at LONG range is the nice signal strengths from LEO satellites that are running a watt or two.  But that's not what terrestrial propagation is out at the edges of the easiest to work range.  We are actually using scatter out at the edges most of the time I think.  And gain does matter to that.  It's not really "line of sight" even including enhanced refraction.  Eventually you're overshooting the radio horizon and scattering off of irregularities high up in the troposphere and back down to earth.

I think the real question is "what should I do to maximize my typical day-to-day FM simplex range" and that has to do with putting as much power where you need it to use troposcatter and tropospheric ducting.   You can usually work a fair distance beyond the radio horizon even with simple setups.  Things get weak and a little fluttery, etc, but that's fine.   

For the long haul on troposcatter you just want as much power near the horizon as possible, and both raising your antenna and using a higher gain antenna will help that.  I think it's right that you get 6dB per doubling of height in terms of your gain at the horizon, and a few quick checks in EZNEC agree.  I think the improvement as long as the free space path to the horizon is a fairly short part of the total path should simply improve with gain, no matter whether it's increased gain due to height or due to antenna gain. 

I think that true line of sight is almost never the goal in ham radio.  All the stations within the line of sight circle are going to be super loud no matter what your antenna is.  But increasing the height increases the gain at the horizon (and if it gets you over trees and other obstacles that's going to help too).

There's some good info on troposcatter here:

http://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm

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