[VHFcontesting] A question of altitude

[k4gun at comcast.net]

I have one more follow up question to this.  I'm not sure there is a simple answer, but I'll ask anyway.  Where does an elevation measurement matter?  This doesn't have to do with terrain issues, but when discussing how elevation affects the angle of radiation, I'm wondering how my proximity to cliffs, drop-offs and bridges changes things.  

For instance, several of the places from which I transmitted in the last contest were close to drops.  Look at a bridge.  While the antenna may be 13' above the road on which the truck is parked, the truck may be 20' from a ledge that drops an additional 20'.  When modeling the antenna would you use the 13' or the 33'?  How close or far from a drop could you be to take this into account?  



-------------- Original message -------------- 
From: jcplatt1 at mmm.com 

> Hi Steve. Good question, one that we rovers ask a lot. 6m is different 
> than the higher bands because is almost HF and because most 6m rover 
> antennas are within a wavelength (or closer) to ground, this impacts their 
> take-off angle. I fired up EZNEC and using just a simple 6m dipole moved 
> it from 13' to 17' to 21' looking at the change in gain at low take-off 
> angles (three to five degrees) compared to the model at 13'. At low 
> take-off angles, moving the 6m antenna from 13' to 17', all other things 
> being equal, equates to a gain increase of about 1.5 dB. Moving the 
> dipole from 13' to 21' equates to a gain increase of about 3.7 dB. This is 
> solely due to a lowering of the take off angle of the main lobe. My 
> thought is that if you can get your 6m rover antenna moved from 13' to 21' 
> that you would see some performance improvement, but that the move from 13' 
> to 17' would not produce much help. Another thing to think about is a 
> "high" dipole, maybe one at 25', in that it may be equal to or better than 
> a simple gain antenna mounted at 13' or lower. Mechanically speaking, 
> when stopped, it might be easier getting a simple lightweight 6m dipole 
> "high" rather than getting a physically bigger gain antenna to 17'. 
> 
> As a rover, its all a compromise which makes designing rover stations so 
> much fun ! On 2m and up its more difficult to say if the slight 
> increase in height would be noticable .... higher is always better but the 
> improvement may not be as significant as it is on 6m. 
> 
> Here in MN, the challenge is to get above the July/August corn - with 
> ethanol sales way up corn production is way up which is good for farms but 
> bad for rovers ! 
> 
> 73, Jon 
> W0ZQ/R 
> _______________________________________________ 
> VHFcontesting mailing list 
> VHFcontesting at contesting.com 
> http://lists.contesting.com/mailman/listinfo/vhfcontesting