This summer, we purchased two 1000-ft rolls of "real" balanced feed lines, namely Twinax. It is not lightweight and flexible like the lamp cord - actually it's only a bit thinner than RG-213, but well screened and protected from precipitation, salinity, temperature changes and rodents.
A brief test run last evening with the 50-degree, 500-meter beverage proved that Twinax is indeed the best solution. Some unidentified local noise sources were greatly reduced, otherwise the noise picture in general was the same. I also noted that signals coming perpendicular to the 50-degree beverage were reduced substantially. Indicating that the lamp cord compromised the beverage's side nulls. There was a slight improvement in signal levels alongside the beverage lobes as well, but this could be attributed to the Twinax test being done five minutes later than the lamp cord test (with a declining sun).
Summary: The lamp cord is an excellent feed line for beverages (and other antennas as well). The Twinax is a superior feed line.
|Lamp cord feed line|
|Twinax feed line. Murmansk stations on 657, 1134 and 1521 are attenuated compared to the lamp cord. Higher signal levels elsewhere.|