Topband: receiving antennas

[Bill Hohnstein]

Earl, K6SE wrote:

> S/N ratio of the receive antenna depends on its pattern...

> K0HA posted an interesting list of S/N ratios for his antennas using his
> own empirical method of calculation.

Comparing the root-sum-square radiation figures is a common procedure
for comparing radiation/reception from multiple directions.

> He used an "omni antenna" as the  0.0 dB S/N reference.  Knowing
> that a 1/4-wave vertical receives equally in all  horizontal directions is
> one approach we can use if we compare its azimuth plot with the
> azimuth plot of a low-noise receive antenna.

My figure used the lobe maximum azimuth/elevation as the basis of
comparison (since an omni antenna has equal radiation in all azimuths [at
one elevation] the same calculation as for a directional antenna produces
a figure of 0.0 dB).  If those angles don't match those of the desired signal
then my relative noise reduction figure is meaningless.  It would be better to
compare against a desired reception elevation, but that figure wouldn't
stay the same for path, distance, day, time, etc changes.  Vertical arrays
of either .05 and .625 wavelength tall elements would produce the
same noise reduction number, but would be much different receive
antennas.  It would probably be better to call it a figure showing the
noise reduction compared to a comparable non-directional antenna.

> This ignores the elevation plots, but should be of some use.

I made a large number of trial noise figure calculations last year.  Some
of the generalities that I found were:
  #1  For peaking a specific antenna, elevation plots can be ignored.
I made noise figure calculations at the elevation of the peak of the
main lobe alone, and also at 5º elevation steps.  The noise figure of
both were very similar and always improved identically.
  #2  I made noise figure calculations using radiation figures with
azimuth spacings of 2, 5 and 10º.  2º appeared to be overkill and even
10º figures weren't much different from them.  Most of my calculations
were made with 5º spacing when it was important, 10º when a slight
inaccuracy didn't matter.

> The width of the main lobe and other lobes off to the side and rear are
> all parts of the S/N calculation.  The total area of the azimuth and
> elevation patterns and their shapes must be taken into consideration

I chose not including the radiation figures +/- 35º from the main lobe
azimuth center because of the width of the main lobes that I was most
interested in--those resulting from my expansion from 2 to 3 parallel
(stagger fed) Beverages towards Europe.  That amount was chosen so
that the effects of narrowing the main lobes width would be shown in
the resulting figures.  In comparing wider lobe antennas the exclusion
area obvously needs to be increased.  My first thought is that the
azimuths starting at -6 or 10 dB off of the main lobe should be included
in calculating the noise figure (thus "standardizing" the number).

> It seems to me that to arrive at a precise number that represents the S/N
> improvement of a receive antenna compared to a reference antenna
> (vertical) would be to calculate the area enclosed by the receive antenna
> compared to the area enclosed by the reference antenna, with both antenna
> patterns in the same scale, regardless of antenna gain.

This would be the root-sum-square of all azimuths (and elevations) of
radiation, including the main lobe (the rms value of the antenna).  I tried
those computations and found that  they weren't representative of the
noise reduction effects of my receive antennas as compared to my
figures which leaves the main lobe out of the calculation.

I emailed W7EL regarding this matter since EZNEC2 & 3 is what
I've used to obtain my pattern figures.  I could configure an equation/
program to calculate such a noise figure but it would still require a lot
of manual entry without incorporating it directly into a NEC program.
That part is out of my league...

73,  Bill     K0HA


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