1. What is a Balun? The electrical energy connecting out of an antenna is most often presented as equal + and - voltages balanced to ground.
2. Why you need a Balun? To get balanced voltage into and out of unbalanced co-ax, you have to have a balun, which is short for balanced to unbalanced.
3. Measured loss in typical inexpensive baluns. Common baluns found over the counter are very poor, having too much loss of valuable signal (energy). I measured loss as high as 82.6% of signal at channel 6, close to the FM band. See the table below.
4. What to do? For broad band 300-75 ohm baluns, I don't know, not yet. I have asked Channel Master, who are said to have a good low loss balun, for more data, but they will not answer phone calls or e-mails. Any advice? Let me know.
5. Make Your Own. If you are interested in a narrow range of frequencies, such as one low band TV channel, FM broadcast, high VHF (Band III), a chunk of UHF only, or lets say about 20% bandwidth, you can make your own low loss balun out of a 1/2 wave length piece of co-ax. Cut a 1/2 wave piece of co-ax as shown. To make it for your frequency, simply find the wave length, and correct for the lower speed of light in co-ax cable. To match 300 ohm to 75 ohm, use 75 ohm cable (e.g RG 59) instead of 50 ohm cable as called for in this link. This is about 66% (solid) or about 78% (foam) shorter in co-ax than in free space.

	MEASURED COMMERCIAL TV-FM BALUN LOSS
       CHANNEL         FREQ    db LOSS  % lost
       2               57MHz   2.4dB	42.5%
       5               79      6.1	75.5
       6               86      7.6	82.6
       7               177     4.75	66.5
       13              213     4.0	60.2
       14              473     2.35	41.8
       28              557     1.7	17.8
       42              641     1.85	38.7
       55              719     2.85	48.1
       69              803     7.15	80.8