View Single Post
Old 25-Jul-2014, 1:18 AM   #4
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 172
Balun: 4:1 and 1:1

Quote:
Originally Posted by pips View Post
...my intent is to build 4 sbgh with no reflector and stack vertical down the mast on top of a 50 foot tower.i will use pieces of pvc pipe to shield the twin line where possible.
You probably know that PVC only shields twin lead from the weather, it does not electrically shield it. PVC is a lossy dielectric at UHF frequencies, I would not use it continuously close to twin lead, probably OK if using it for spacers something like a standoff insulator, with no continuous exposure.

Quote:
...my question now is can i use and off the shelf baulen (4 to 1) or a (1 to 1)?
Off-the-shelf baluns are normally transformers and baluns combined. The 4 to 1 indicates a transformation ratio (i.e. 300 to 75 Ohms). If you use a parallel combination that yields 75 Ohms then you do not need a transformer, only a balun, a 1 to 1 balun.

Any commercial 4 to 1 balun that I have measured (many of them) will have substantial loss at UHF frequencies, ~1 dB or usually more. A ferrite choke balun can be made to have losses below a few tenths of a dB.

I do not know of any commercial 1:1 baluns. They can be made to look like a transformer, or they can be the 'ferrite choke' version.

If you should find a combination of antennas that yield 300 Ohms impedance, a 'half-wave loop' balun provides about the lowest loss.

If your transformation ratio is anything other than 4 to 1 or 1 to 1, you will need to do some more research. Odd ratio baluns or balun/transformers can be made, but most would not attempt. In such a case, probably the best approach would be to transform the non-standard impedance to either 75 or 300 Ohms, then use an appropriate balun. The transformation could be done with custom-made transmission lines and lengths.

----------------------------
Balun: A device that can convert (efficiently mate) a balanced transmission line (twin lead) to an unbalanced transmission line (coaxial cable).

.

Last edited by tripelo; 25-Jul-2014 at 1:42 AM. Reason: clarify
tripelo is offline   Reply With Quote