In my last house, I made an FM antenna out of good old-fashioned 300-ohm twin-lead antenna cable, mounted it in the attic and ran it down through a partition wall to my stereo. I’m planning to do this again in my new place, but I’m wondering if I wouldn’t be better served by replacing that 15-or-so feet of vertical run with quad-shield coaxial cable.
First question is, will this really buy me anything? Aside from the fact that it will save me having to buy an extra 15 feet or so of twin-lead (I already have plenty of quad-shield), will I see any noticeable improvement in reception by eliminating the vertical run of my “antenna”?
Second question is about the transitions: can I use those 75/300-ohm adapters I’ve been collecting with every new VCR I’ve ever bought to transition from twin lead to coax and back?
No improvement, and you might lose some, in fact. 300-ohm twinlead has a characteristic impedance of, well, 300 ohms. Coax cable is typically around 75 ohms. Your FM receiver is expecting to “see” a 300-ohm impedance, as well, so the twinlead is a perfect match for it. But, when you throw a length of 75-ohm coax into the mix, you introduce a mismatch. Probably nothing you’d notice. You could use the adapters, but then you have some insertion loss to deal with. Again, probably nothing you’d notice casually. In any case, you’ve got nothing to lose and nothing to gain by trying it.
The problem with twin-lead is that you have to keep it away from metal. Any nearby lumps of metal will create an impedance bump in the cable and increase losses. Coax can be routed anywhere. If you use coax, you should use a balun (75-ohm unbalanced to 300-ohm balanced transformer thingie) to connect the coax to the antenna.