[NLRS] 2 meter antenna elevation

Tue Jul 26 09:57:53 EDT 2011

There are more notches in the 2m elevation pattern because of the 
diffraction grating effect for the same antenna height above ground. A 
2m beam may be more directive in the elevation pattern if it has more 
properly designed elements.

One experiment is to copy a satellite beacon on a near overhead pass 
from 30 degrees elevation to LOS. Last time I did that the diffraction 
grating effect was very apparent and strong. E.g. while that antenna 
beamwidth was something like 35 degrees the lowest satellite lobe was 
only 3 or 4 degrees. And for most yagis aimed near the horizon, loss of 
satellite at 60 degrees elevation would be expected. There a lot of up 
tilt would be of benefit, or switching to an antenna like an eggbeater 
with near hemispherical coverage.

The lobes of the diffraction grating are the same whether the antenna is 
an isotropic source or very directional. And those lobes modulate the 
elevation pattern of the yagi, short or long.

There may be antenna elevation computing models with capability of using 
real terrain, its certainly a practical computation, just takes lots of 
data input though that elevation data is available on a 30 meter grid.

Remember that signals at the horizon are attenuated by buildings and 
vegetation so the practical elevation patterns in the real world tend to 
not have strong signals at the horizon.

73, Jerry, K0CQ

On 7/26/2011 6:29 AM, Dave, WV9E wrote:
>
>
> I live in the same situation as John and have worked lots of satellites,  and
> my AOS is totally affected by the terrain, in fact by several degrees depending on the azimuth of my horizontal antenna and ofcourse where
> a particular sat. is at the time elevation wise.   When birds get to 60 or
> so degrees above horizon there is a definite signal loss and being very close to a bluff perhaps terrestrial signals act in a similar way.  At a certain distance from a bluff the incoming wave angle is steeper than the beamwidth of the antenna and gain falls off. .
>   Height probably is the best method to increase gain, but I think some
> experiments may be needed to verify what K0CQ is explaining.  We
> know signals will follow the curve of the earth to some extent, but I'm
> not willing to discount the possible benefits of slight elevation on 2 meter and above antennas.  Maybe I'm just a hard case ignoring the facts.
>   I just seems to follow that signals dropping down on the top of an antenna can't have the gain that signals from the forward direction can.
>   .
>   Hopefully I'll have a chance to do some experimenting at some point and
> document the results, or if there is a study we can reference please let
> us know.. The referred article is based on 6 meter studies in ideal terrain essentially...  (http://www.w9smc.com/blackhole/news%2010%2008.pdf ) and it specifically notes that the vertical beamwidth of antennas shrinks as frequency increases.  I think to answer John, we
> simply don't know.  Yet.
>
>
>
> 73,
> Dave, WV9E
>    ----- Original Message -----
>    From: Dr. Gerald N. Johnson
>    To: nlrs at mailman.qth.net
>    Sent: Sunday, July 24, 2011 1:16 PM
>    Subject: Re: [NLRS] 2 meter antenna elevation
>
>
>
>
>    Digging out my 2010 CSVHF proceedings, I find the article is by W0FY on
>    pages 101-106. It is on 6m and illustrates the fine elevation pattern
>    details caused by the diffraction grating effect. Searching I find it
>    was published on line a couple years before at:
>    http://www.w9smc.com/blackhole/news%2010%2008.pdf
>
>    He found that the lowest lobe was attenuated by tilting a 6m beam, and
>    that lowest lobe was at a high enough elevation to get over W9RPM's
>    cliffs. It would be better the compute with Manna-Gal than EZNEC and to
>    use W9RPM's actual 2m beam and height because antenna height has a big
>    effect on the diffraction grating elevation pattern. I found it very
>    interesting to see that elevation pattern by listening to a satellite
>    beacon on 2m as the orbit rose or fell near vertically on a selected
>    pass. That's a good long range antenna range test.
>
>    W9RPM's cliffs are shadowing closer station and antenna tilt isn't going
>    to help. Only much greater antenna height or moving up on top the cliffs
>    is going to help that. The longer range stations will be dropping over
>    the cliff anyway.
>
>    Fact is VHF and UHF signals from beyond the horizon do propagate at an
>    angle. The longer range signals are refracted in the atmosphere by the
>    normal change in air density with height and that refracts rising
>    signals back down. Temperature inversions increase the refraction.
>
>    I know on some fairly long microwave links that two antennas are
>    sometimes used at each end, one for the normal refraction and another
>    pair with greater elevation for the temperature inversion events to get
>    better reliability on a path. But those antennas probably have closer to
>    1 degree beamwidths while the practical 2m yagi is closer to 30 degrees
>    beamwidth. (And my next ones will have even greater azimuth beamwidth
>    for easier aiming.)
>
>    73, Jerry, K0CQ
>
>    On 7/24/2011 12:29 PM, Bruce Richardson wrote:
>    >
>    >
>    >  I was just discussing this with John on the phone and encouraged him
>    >  to come to the list.
>    >
>    >  We know that a guy on the ridge does better than a guy in the valley.
>    >  In John's case, he really is quite close to the blocking ridge and
>    >  blasting right into the side of it.  Isn't it true that our 2m signals
>    >  get scattered by reflection and dispersion off of trees, rocks,
>    >  buildings, and ridges?  Doesn't this end up being forward scattered by
>    >  dust particles and water molecules?
>    >
>    >  Two questions: by blasting into the side of the ridge, might he be
>    >  getting back-scatter that is phase cancelling some of his signal?
>    >  Secondly, I know curvature of the earth plays into this a bit, but I'm
>    >  wondering if by elevating, he might lift that main lobe above the
>    >  ridge and strictly work on dust particle scatter to the eastward
>    >  (Milwaukee and beyond).
>    >
>    >  Dr Jerry has already replied and is kinda indicating how sharp the
>    >  lobe would have to be to get the intended benenfit.  Anyways,
>    >  interesting topic and I look forward to follow-on discussion. I'm
>    >  working on some graphics from RadioMobile to show the dilemma. I'll
>    >  post links to them when the graphics are ready. Might be tomorrow.
>    >
>    >  73
>    >  Bruce W9FZ
>    >
>    >
>    ______________________________________________________________
>