Baluns to feed a dipole

[Higgler]

I need to feed a printed dipole antenna at 600 Mhz from a coax. cable. I plan to connect the input of the balun to the center conductor of the cable and the other input arm of the balun to the cable outer conductor, then the two balun outputs (180 degree phase delta) go to the two printed dipole arms. This prevents the cable from being part of the antenna, I think.

I need a short balun, (I've made a slotted 1/4 wave coax. line balun- that's too long for this), hence I plan to purchase one from MaCom (TP-103 part number) or Minicircuits (

http://www.minicircuits.com/dg03-240.pdf).

I see alot of different transformer types (shown below from MiniCircuits website) and wonder if going from coaxial cable to the dipole arms must be done with item A only, or can other options be used?

Has anyone had experience doing this, and what the pitfalls may be?

Thanks, (sorry, long question)
kch


A -- Unbalanced to Balanced Center Tap
10 kHz to 1400 MHz
B -- Balanced to Balanced Center Tap at Primary/Secondary
4 kHz to 800 MHz
C -- Balanced to Balanced
4 kHz to 800 MHz
D -- Unbalanced to Unbalanced
5 kHz to 2500 MHz
E -- Balanced to Single-ended
5 kHz to 2500 MHz
F -- Single-ended to Balanced with Impedance Ratio Selection
5 kHz to 2500 MHz
G -- Balanced Transmission Line
5 kHz to 2500 MHz
H -- Balanced Transmission Line with Secondary Center Tap
5 kHz to 2500 MHz

 

[BrianR]

Are you sure you need a balun. Although theoretically you do, in practice as long as the coax has no significant effect on radiation pattern and is not carrying RF on its shield out to where it's not wanted, then does it matter. One or more ferrite beads over the coax near the feedpoint will attenuate shield only currents and have no effect on antenna radiation.

 

[Higgler]

It's low frequency and we need to keep the currents off the outer conductors, I'm assuming a balun will do that? will it? Ferrite may help though if currents get on the outer conductor of the coax.

kch

 

[BrianR]

Keeping currents off the outer conductor is also a function of the matching (VSWR) and a balun does not cure a mismatch.

It decouples the shield from the antenna element fed by it and prevents it from changing the electrical length of that element. If the antenna is resonant then the voltage node falls at the feedpoint and the coax will not form part of the antenna.

I can't be categoric, but I think it's worth a try to put a few beads over the coax close to the feed point. Much less lossy too.

 

[Higgler]

We are starting on the program in about a week, so I'll let you know how it goes after we make some progress.

thanks for the input

kch