Ken,
There are a number of ways to go. A 1/2 wavelegth dipole can be built on
2-layer FR-4 PCB incorporating a 90-degree 60 ohm balanced line for
matching. Suitable ferrites are then slipped over the approximately 5 cm
line to perform the balun function.
Another approach is to build the dipole to have a 50 ohm radiation
resistance. One way to do this is to bend the ends at 90 degrees to the main
dipole axis. Dimensions for 1900 MHz:
4.56 cm along the axis and bend the ends up 1.58 cm in the same direction.
The element diameter is 0.25 cm. Z = 52 +j0 ohms.
And there is always the Gamma match to both transform the feedpoint
impedance to 50 ohms and perform the balanced-to-unbalanced function. The
Gamma match, being unsymmmetrical, does skew the radiation pattern a bit. If
you'd like I can run a NEC sim of this to see how bad it is. NEC can also be
used to determine the balun common mode impedance needed for a specific
cross polarization and pattern skew.
Then there is the one wavelength quad loop. It does not need a balun - just
impedance transformation. It is 140 ohms and 90 degrees of 75 ohm coax will
transform this to 40 ohms. You can make 84 ohm twisted pair using enamaled
wire to the matching line to get a 50 ohm feedpoint resistance. And the quad
loop can be Gamma matched.
And there must be 100 other ways to do it. Being lazy and liking foolproof
designs I would go with the bent dipole and some ferrite beads.
Dave Cuthbert
Linear Technology Corporation
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From: [email protected] [mailto:[email protected]] On Behalf Of Ken Javor
Sent: Monday, November 13, 2006 5:20 PM
To: [email protected]
Subject: balun info
Experts,
I need to build/buy a 1.9 GHz PCS band dipole receive antenna. Haven't
found anything to buy. What is a suitable material and balun design to go
>from 72 Ohm balanced to 50 Ohm coax? Could I use an old T3 balun (400 -
1000 MHz tunable dipole) and just modify the elements? I will need to
build two so I can calibrate an antenna factor. I will transmit 0 dBm or
less.
Thanks in advance!
Ken Javor
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