Sunday, August 14, 2011

Balanced Feed Line - Replacing Lamp Cord With Twinax

Observant readers will know that last year, we replaced our old coax feed lines (mostly RG-213 and RG-58) with cheap "Lamp Cord". Using balanced feed lines instead of unbalanced proved to be an immediate success in terms of noise reduction and flexibility. The cost was the need to put a 100:50 ohm transformer into the feed line before entering the 50 ohm receiver antenna input. Though if you didn't, you wouldn't lose more than a few dB.

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.


hg said...

Hi Bjarne,

Very interesting post! Similarly, I think Dallas Lankford reported he is also using "zip cord" (as it is called in US) for the feeding his active antennas with good results.

Quick question: how did you connect the shield? Did you leave it floating, connected to radio ground or is it connect to earth?


Bjarne Mjelde said...

Correct. Dallas has advocated the use of twin lead feed lines for many years. The shield or braid is left floating, at least until I know some better use for it :-) Remember the lamp cord had no braid.

Kari ja Johanna said...

Hi Bjarne

What kind of transfomer you use (if any) at the antenna end of lamp cord?

Bjarne Mjelde said...

Kari: I do not have the information at hand, but I will look it up. It's on a PC somewhere...

Bjarne Mjelde said...

Kari: You should indeed use a transformer at the antenna end since the antenna is 400-500 ohm and the lamp cord 100. At first I used a modified 450:50 transformer (different winding ratio), but for the more permanent transformers I had Wellbrook make some for me.