Hi Don,
Thanks for your email, I'll try to answer your questions as best as I can.
In W8JI's very rural area minimum main lobe beamwidth is more valuable to him
than minimum sidelobes. Tom's choice of 330 foot broadside spacing in a very
rural area with little or no local RFI makes lots of sense.
End fire spacing has essentially no effect on beamwidth and sidelobe levels, so
Tom's choice of 70 foot end-fire spacing makes little if any measurable
difference compared to 130 foot end-fire spacing. Larger end-fire spacing (up
to 1/4 wavelength) is somewhat more forgiving of phasing errors and mismatched
signal levels and slightly more efficient.
I live in a built up area and many RFI sources have appeared especially over
the last ten years with the explosion of RFI generating imported consumer
electronic devices and ineffective RFI regulations. Its impractical for me to
traipse over hundreds of acres of private property to identify all of the RFI
sources within thousands of feet, and much more difficult to convince dozens of
homeowners with RFI generating consumer electronics 1/2 mile away or more to do
anything about it. For me, minimum sidelobes are far more valuable than minimum
beamwidth. I have ample space for 130 foot end-fire spacing, so there's no
incentive for me to reduce end-fire spacing to 70 feet or less.
As you noticed when you computed the geometry of an 8-circle, its impossible to
optimize both broadside and end fire spacing in an 8-circle array. Broadside
spacing has a significant affect on sidelobe levels and beamwidth while end
fire spacing has essentially no effect as long as the end-fire spacing is less
than 1/4 wavelength. The W5ZN/N4HY 8-circle geometry is an optimum choice for
a minimum sidelobe BSEF receiving array.
Putting this all in perspective, one degree of phase on Topband is 18 inches,
so differences of a few feet have no practical affect on antenna performance.
In fact, variances in the environment in the general vicinity of each vertical
have significantly more effect than spacing variations of a few feet. This is
especially true when antennas and towers are within hundreds of feet (or more)
of the BSEF array if they have resonances in the rough vicinity of 160 meters.
In your case, maximizing your spacing to nearby towers and antennas is much
more important than optimizing the broadside spacing, especially if the nearby
tower or antenna is roughly resonant on Topband.
When I mentioned "45 degree beamwidth" it was in the context of rough
on-the-air comparisons of the relative performance of my BSEF receiving array,
my 900 foot Beverage and my transmitting 4-square. There's no perceptible
difference in the beamwidth of my Beverage and BSEF, but signals arriving
through the sidelobes of the BSEF are significantly weaker than on my 900 foot
Beverage. Very high angle signals arriving through the BSEF sidelobes are
often 10 dB (or more) weaker on the BSEF than on the Beverage. The actual
beamwidth of my BSEF receiving array is surely closer to 60 degrees than 45
degrees.
Good luck with your BSEF receiving array, its worth the effort and you'll enjoy
using it. When you model your BSEF array you should also model your nearby
transmitting antenna as part of your model. In my case, my BSEF array is more
than 600 feet from my nearest tower. Thats close enough to have a measurable
effect on sidelobe performance, but I haven't concerned myself with it.
73
Frank
W3LPL
---- Original message ----
>Date: Sat, 02 Feb 2013 17:41:12 -0500
>From: Don Johnson <n4dj@me.com>
>Subject: BSFE array
>To: Frank Donovant <donovanf@starpower.net>
>
>Frank,
>I was looking at all the array data. I have a couple of questions on spacing.
>The 8 circle is based on the circle being a 320 foot diameter so the broadside
>spacing is 296 feet. The geometry makes the end fire spacing about 121.6 feet
>if I did the math right. The 8 circle beamwidth looks to be 56.8 degrees at
>the 3 dB points from the plots.
>
>W8JI used 330 broadside and 70 end fire. W8JI gives patterns and the beamwidth
>looks like 49.4 degrees. He said that a broadside endfire array gave the best
>directivity of all the 4 element arrays. He did not say that 330 by 70 was the
>most directive configuration but that was what he used. That sort of implied
>it gave him the best directivity. He did not say the best beamwidth off the
>front.
>
>You noted your beamwidth of about 45 degrees. Are there any EZNEC plots of
>your array around? I was wondering why you went to
>300 by 130 feet. Was it because that gave you the best beamwidth as I think
>that was what you were looking for.
>
>I plan to model some of them in EZNEC myself. I had been thinking about doing
>something something like that myself. I have space for one about 300 by 130
>that would be broadside to Europe. It might end up being too close to my
>transmitting antenna, not sure about that yet. 330 by 70 would get me a few
>feet farther away from my other antennas. My new beverages are simple and
>working pretty good, but it is always good to improve!
>
>73,
>Don
>N4DJ
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Topband Reflector
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