That hard to find?
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Just used plain coax of any type - RG58, RG-59, RG-6 or even a thin RG-174. The improvement vs. the internal antenna is so much that a little loss means nothing. If you're very picky, then you can add a BALUN between the antenna and the coax.
Just one word of caution. An extrenal antenna picks up a lot of signals, which some tuners cannot handle. This has to do with the design of the front-end circuit.
Many tuners are designed for maximum sensitivity, assuming a poor antenna. As a result, they may easily overload when connected to an external antenna. You will know it very quickly if you live close to a high-power FM transmitter...
4yanx, I don't think this animal exists because you see. the idea behind twinlead has to do with the (elimination of) the capacitance between the two wires by spacing them apart a certain mimimum distance. If you were to throw a shield around it, you would literally short-circuit that (300 ohm) design.
What you need to do is attach (right at the antenna) a 300 ohm to 75 ohm inline transformer. Very common and no more than $1-4 at Radio Shack etc. You attach the twinlead pigtail coming out of the transformer to the two antenna terminals and then attach regular (shielded) coax cable to the other end and you're in business.
Shielded 300 ohm twinlead used to be sold by radio shack. I used it for many years and it did a super job of eliminating noise from ignition of nearby vehicles. This was in a fringe reception area, and I wanted to avoid any loss of signal due to a 300/75 transformer. The shield was left floating: not grounded at either end. Modern FM tuners take a 75 ohm antenna connection, so you are going to need a transformer anyway. I have recently installed a small lower down FM antenna, with a mast mounted RF amplifier and 75 ohm coax, and it works as well or better that the old antenna with the shielded 300 ohm twinlead..
I understand, N, and thanks, but I have used coax (and pigtails) on several occasions. This is a paricular instance where:
1) I am going to need a very long run of cable
2) I have only 300 Ohm connectors on the back of a tuner and do not want to use converters.
I'd like to use it because, due the the very long run, I figure signal loss using 75Ohm coax might be too great. "They" used to make shielded twin lead and it was relatively common. There was no issue with construction - two leads fed the regular anntena terminals and the shielded shroud connected to the tuner's ground. No big deal. However, in this day and age I can find no source for this cable.