Topband: Antenna Signal/Nosie

Bill Hohnstein K0HA
Fri, 24 Sep 1999 23:11:20 -0700

K0HA wrote:
> I'm wondering if there's any equation or computer software which
> compares the [receive] signal-to-noise ratio of two antennas...

I thank those that responded.  Consensus was that comparing directivity
(gain with no antenna losses) would tell me what I was looking for.
But I was uncomfortable with comparing the directivity of my Beverages
with my transmit antenna (4 element parasitic vertical array).
I decided to try my original thought:

> Perhaps it would be something like comparing the root-sum-square
> values that do NOT include the main lobe along with the gain
> numbers(?).

Rethinking made me conclude that the main lobe needed to be included
since main lobe noise would obviously also have to be considered.
I decided to make the reference be a noise source coming from all
azimuths/elevations with the same field strength (in mV/m) that the
main lobe had (at one mile).  I did a lot of number crunching with
this.  First tests were done at just a single elevation (at the
peak angle).  Calculations were made on antenna pattern designs
which I've used on 160M.  Without prejudicing you with my reactions,
below are the numbers that resulted:
Any omni ant   0 dB
My xmit ant   -5.28
580' Bev      -6.31   Haven't helped me much over in-line w/xmit ant
881' Bev      -7.31
2 x 880' Bev  -7.86   6' spacing, stagger fed 1/4 wavelength
3 x 880' Bev  -8.08   Above, plus third spaced 0.4 wavelength (planned
                        for this season)

(The above numbers were made looking at 36 azimuth values.  I did one
comparison using 72 azimuths and saw less than 0.01 dB change.  All
of the pattern figures used were made with the 44 mS/m ground
conductivity here)
After seeing those numbers I thought, "I wonder how the figures
would change if I included multiple elevations?"  But other thoughts
occurred (eg "Is noise at 75 degrees elevation really meaningful?").
So, I tried many both weighted and unweighted calculations.
Multi-elevation conclusions:
  Unweighted numbers closely approximated the above numbers at a
    single elevation.
  All of my weighting attempts resulted in unreliable figures.
Overall conclusions (based on the many mentioned calculations, plus
many more that weren't discussed):
  The relative rankings are accurate.
  There are many more factors involved in specific cases which
    invalidate these numbers for those specific examples (noise is
    almost never from all directions, the desired signal is rarely
    precisely on the antenna peak azimuth/elevation, etc).
  The absolute dB numbers are questionable quantitatively (I doubt
    that the 1 dB difference between 580 & 881' Beverages will tell
    us anything about the numeric difference between the two on a
    real situation).

I'm not sure if all of this work has told me anything that I didn't
already know.  But, the process was interesting!  So much fun that
I think that I'll go do what I did in 1971:  plot a four tower phased
array pattern using a slide rule to make my calculations...
73,  Bill     K0HA

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