Topband: cone of silence COS
btippett at alum.mit.edu
Wed May 11 15:08:44 EDT 2005
At 12:12 PM 5/11/05, KV4FZ wrote:
>Can I get the proper in phase angle by using two equal lengths of RG-6U to
>each Beverage and a T connector followed by any reasonable length of
>RG-8X to the shack? I know there will be a slight mismatch but my pre-amp
>should be able to tolerate that. I think most of us without special
>equipment to build phasers, unless there is some advantage to having BIP
>and BOP selection, would like to keep it as simple as possible. What do
Herb is referring to phasing beverages
in parallel. I was speaking of phasing them end-fire.
Parallel doesn't help much with F/R, but it will narrow the
forward beamwidth. The type of phasing, parallel or end-
fire, depends on the noise problem you are trying to solve.
If your noise is predominantly from the rear, as mine often
is here with thunderstorms from TX to FL while trying to
listen NE toward Europe, you would choose end-fire. If
your noise is predominantly in front of you, and you want
to "slice through" it with as narrow a forward beamwidth
as possible, then you want to use parallel beverages. If
you want BOTH a narrow forward beamwidth AND good
F/R, you can do both...4 total Beverages made up of 2
sets of staggered (end-fire) beverages, separated by 1/2
to 3/4 wavelengths and fed in parallel.
At 01:27 PM 5/11/05, K3BZ wrote:
>Bill... how do you phase and feed your endfire Beverages? Can you point me
>at a good info source?
There are two good sources. ON4UN's new book
from Pages 7-74 to 7-84 covering Arrays of Beverages.
Also W8JI's receiving antenna webpage at:
Assuming you build both Beverages
exactly the same, you can probably get away with a
simple T-connector approach. For parallel (in-phase)
Beverages, I would match both to 75 ohm cable, run
equal lengths to a T halfway between both, and then
simply feed the T with 50 ohm cable (as Herb suggested
above). Two 70 ohm lines in parallel are 37.5 ohms, which
results in ~1.3 to 1 mismatch to 50 ohm line. If your
Beverages are mismatched however, all bets are off.
Similarly for end-fire Beverages, match both to
75 ohms but make the line to the forward Beverage longer
by the proper phase difference (nominally 90 degrees).
Then Tee the two 70 ohm lines together as above. This
simple method only works for one band, although mine
seemed to work "OK" on 80 even though it was designed
If you want to be a purist (in case your Beverages
are not perfectly matched) you can use a Magic-T combiner
instead of a straight T-connection as above. These can be
built with only 2 binocular cores and are described here:
"The magic "T" combiner is a very useful device. It can provide
equal voltages, equal current, or equal power to matched or
unmatched loads. It is not a magic bullet."
A couple of additional points:
1. ON4UN coined a new term DMF (Directivity Merit
Figure) in his new book to help characterize an
antenna's S/N performance contribution from the
rear 180 degrees (page 7-10). Looking at Table 7-42,
you can see that the antennas with best DMF's are
all end-fire Beverage pairs of 1 wavelength or longer.
2. In John's previous book there was a formula for
determining optimum end-fire spacing based on the
main lobe's takeoff angle. Although not in his new
book (wrong?) it was (90 deg) * (cos TOA). For a
Beverage with peak response at 25 deg TOA, the
optimum stagger distance would be 90 (cos 25) =
81.5 deg (or about 122'). When I built my end-fire
pair, I used this as a start and varied both stagger
distance and phase shift to optimize the pattern.
In my particular case I found about 122' spacing
and 116 degrees phase shift gave me the best
pattern. Eznec is great for this sort of thing.
73, Bill W4ZV
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