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R: HELP please AO40 RHC Crossed Yagi connections







----- Original Message -----
From: Darren Rainer <darren.rainer@btinternet.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, December 24, 2001 4:48 PM
Subject: [amsat-bb] HELP please AO40 RHC Crossed Yagi connections


> Hi all,I am currently setting up for operation on AO40 but am a bit
> unsure how to connect up a F9FT Tonna 435MHz 2 X 19ele crossed Yagi for
> RHC polarisation.
> Now both yagis on the boom are 50 ohms. The horizontal Yagi is
> positioned 20cm in front of the vertical Yagi on the boom.
>
> What I think it needs to bring the impedance of the paralled antennas
> (25ohms) back to 50 ohms is a 1/4 wavelength of 75ohm coax coming from
> each Yagi to a T-piece (maybe use BNC connectors and BNC T 75ohm). Then
> have a BNC plug to N socket adaptor so I can connect my 50ohm RG213/
> Westflex.
>
>   Now do I need an extra length of 50 ohm or 75ohm coax on the Yagi that
> is spaced 20cm in front, to get the 90 deg phase shift or not ? If this
> is the case Im going to have a lot of loss in connectors.
>
> I have searched the net and books but cannot find any good examples on
> phasing RHC yagis.
>
> After sorting the above out what is a good 75 ohm coax for 435MHz TX
> use. I have found a length of RG62AU which some websites and books claim
> it low loss 75 ohm and other sites say it Ethernet style 93ohm. I guess
> BNC connections would be the best choice as I can use the correct
> impedance connectors to match the coax. Are T-pieces generally 75 ohm or
> 50 ohm or should I not worry.
>
> Sorry for all the questions but trying to get the setup right.
> My downlink will be FT847 WITH DB6NT MKU24 downconverter to 144 with
> G3RUH 60cm dish and patch feed. Hopefully I will get it sorted by the
> time AO40 is fully running.
> 73s Darren G4VTQ
>

Hi Darren ,G4VTQ

First of all put the horizontal dipole closest to the satellite in the boom
and the vertical dipole  200 mm on the back of  it.

The mechanical distance between dipoles is 200 mm wich is greater than 1/4
wavelength in the free space at 435 MHz because it is exactly 172,4 mm

In order to get circular polarizazion we need between the dipoles a phase
shift of  90 electrical degrees

In order to compensate for the more difference between 200 mm and 172,4 mm
it is necessary that the RF energy to the horizontal dipole arrives to it
with a supplementary delay given by the difference 200-172,4= 27,6 mm in
the free space

If we use this trick the EM field radiated in the forward direction by the
vertical dipole will arrive to the horizontal dipole with just a phase 
difference of exactly 90 degrees

To do this using RG 213 coax cable with velocity factor Vf= 0,66 che feed
line supplyng the horizontal dipole must be LONGER than that supplyng the
vertical dipole by 27,6 x 0,66 = 18,21 mm

If you call H the line of  RG 213 supplyng the horizontal dipole and V that
supplyng the vertical dipole you can use any length of coax for both lines
provided that the line H is longer than the line V by 18 mm

Since the impedance of each dipole is Z= 51,94-j1,49 on the Smith chart
at 435 MHz we must consider that it is purely resistive at that frequency.
and so if you put in parallel the open ends of the H line and V line you
get a resultant impedance of 25 ohm

In order to match 25 ohm to your 50 ohm line without to go using many
all lossy N connectors,barrels connectors and 70 ohm matching
1/4 wavelenght lines,you should use only a dual port power splitter
or power divider wich characteristic impedance is:
Zo=SQR ( Zi x Zu) = SQR ( 25 x  50) = 35 ohm

In order to get RHCP for AO40 it is mandatory to properly connect
the inner conductor and the braid  of each line  respectively to the
H and V dipole.

If you look at the orizontal dipole H from the back of reflector  you must
connect the inner conductor to your left side and the braid to your
right side of the folded dipole.

For the vertical V dipole as seen from the back of reflector you must
connect the inner conductor to the top and the braid to the bottom
of the folded dipole.

In order to prevent interactions between the antenna elements and feed
lines it is necessary to run both lines on the boom going from dipoles to
reflectors instead to go from dipoles to directors.

In this situation you will get a big bend of coax in the rear of your
antenna wich is far away from the EM field radiated in the forward
direction.

In addition the power divider should be mounted on the antenna mast.

Use all N connectors.

I hope this help.

Marry Christmas and  happy new year

73 de i8CVS Domenico

>
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