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Re: Info needed for MBM88 Antena



Hi Domenico, I think you have wrong specifications on the JBEAM MBM88
Here are the factory specifications;

Frequency  430-440 mhz
GAIN (dbd)   16.3 dbd
FRONT TO BACK RATIO 22db
3db BAND WIDTH   H 28
                                   E 23 ? (not clear on my page)
DESIGN IMPEDANCE    50 OHMS
POWER RATING (Peak)      1 KW
BOON LENGTH    3.98 Meters
LONGEST ELEMENT 42 cm
TURING RADIUS  2 METERS
WINLOADING AT 80 MPH    21.4 Kg
WEIGHT     4.7 KG


;
   73s Shlomo


AO-16,UO=22,KO-23,KO-25  Amsast U.K member #3406. QTH LOCATOR KM71JV.
Internet address  4x1as@amsat.org , 4x1as@netvision.net.il
4X1AS. Shlomo Menuhin, 30 Uriel Menuhin,Rehovot,76655, Israel, KM71JV
----- Original Message -----
From: "i8cvs" <domenico.i8cvs@tin.it>
To: "AMSAT-BB" <amsat-bb@AMSAT.Org>; "Neill Taylor"
<neill.taylor@ukaea.org.uk>
Cc: "George Liepins" <gliepins@nc.rr.com>
Sent: Saturday, June 23, 2001 00:47
Subject: R: [amsat-bb] 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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> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>
>
>
>
>
>
>
>
>
> ----
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>

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