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R: Info needed for MBM88 Antena



The Multibeam MBM88/70 cm was produced by  J BEAM in 1975 and virtually
comprises four 22 element yagi antennas that have been stacked and bayed to
form a single,compact array.
Gain of MBM88/70 is 19 dBd ,i.e over a dipole or  21,14  dBi ,i.e over
isotropic
The antenna is linearly polarized and each of the 22 rods is approximately
one wavelength long.
Each rod can be seen as two 1/2 wavelenght  elements connected togheter with
the center of this rod fastened to the boom with a  plastic insulator.
This particular assembling is not  convenient because the maximum voltage
point is exacly where the element is fastened in to the plastic insulator
wich represent a source of  losses particularly with high humidity,rain and
ice.
The slot-feed driving element is a balance one and the antenna has a baloon
made with coax cable rolled up and contained in to a small plastic box.
All J BEAM antennas supplied for Italy at that time were for 50 ohm
transmission line.
In order to connect two MBM88/70 in Right Hand Circular Polarization  first
of all it is
necessary to look at the baloon box from the reflector side and ascribe to
each of the two screws a simmetrical polarty for both antennas and mark them
 (+) (-)         ( +) (-)
Than put  both antennas on a boom and  mount the left antenna orizontal in
order to see the screws like this   (+) (-)
Than rotate vertical  the right antenna by 90° clockwise in order to see
the screws like this   (+)
                                     (- )

Now consider that the (+) screws are to be connected to the inner conductor
of the coax cable and the (-) screws to the braid of the coax cable.

In order to get RHCP it is necessary to connect the vertical antenna with a
transmission line wich is electrically 1/4 waveleght longer than the line
suppliyng the orizontal antenna.
This line is well know as a delay line or a 90° delay line and its lenght
must be calculated taking in to account the velocity factor of the coax
cable used for it.
Since the 50 ohm coax to the orizontal and vertical antennas are connected
in parallel  and forms 25 ohm in their junction,it is now necessary to use
one
of the many existing sistems to transform 25 ohm to the 50 ohm impedance
of the transmission line.

73 de i8CVS Domenico



----- Original Message -----
From: Neill Taylor <neill.taylor@ukaea.org.uk>
To: <amsat-bb@AMSAT.Org>
Cc: George Liepins <gliepins@nc.rr.com>
Sent: Friday, June 22, 2001 11:02 AM
Subject: Re: [amsat-bb] Info needed for MBM88 Antena


> "George Liepins" <gliepins@nc.rr.com> wrote:
> >Some years ago I bought two MBM88 70cm yagies made by Jaybeam Limited in
UK.
> >I was going to phase them, but never got to it.  Instead I kept the
HyGain
> >70 cm for satellite work.
> >
> >I have a friend who is interested in my antena, and wonders if it is
> >"circularly polarized."  As far as I can tell, it can be set up for
vertical
> >or horizontal polarization, but circular?  The technical specs don't
mention
> >polarization specifics, except mentioning beamwidth E and H plane ( 28
and
> >23 deg's respectively).
> >
> >Can someone , please, enlighten me concerning this antena, and how it
could
> >work as uplink for AO-40.
>
> You are right, the Jaybeam "Multibeam" MBM yagis are linearly polarized.
The design is effectively a double-yagi with a slot-feed driven element.
Since you have two of them, of course you could put one in each plane and
phase them for circular polarisation like any other pair of linear polarized
antennas.
>
> Be careful about the matching of the phasing harness, though.  My
recollection is that these yagis are 75 ohm impedance.
>
> Neill
> G4HLX
>
> ----
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