Proper feeding of non-resonant antennas

Coax becomes very lossy at high SWR. If you are feeding your non-resonant antenna (vertical, dipole, or any other design) with coax and utilizing a tuner at the shack end of the feedline, you’d be surprised by the amount of power you’re losing. Remember that your in-shack tuner does not tune the feedline to the antenna, and thus it remains at high SWR on most bands!

Balanced feedline, on the other hand, is very tolerant of high SWR and is practically lossless. It can be built inexpensively, and a variety of instructions exist online or in print. The cons of this cable type are well documented, but essentially boil down to users failing to maintain balance; if the feedline comes near conductive material, or is wildly blown about, it can become unbalanced and radiate. Despite the drawbacks, it is a demonstrably superior feedline for non-resonant antennas. Let’s compare common 600-Ohm balanced feedline against a few common types of coax:

600-Ohm Balanced Time Micro. LMR-400 Belden 8237 RG-8 Belden 8240 RG-58
Length (ft.)100100100100
Frequency (MHz)14141414
SWR3:13:13:13:1
Power In (W)100100100100
Total Loss (dB).12.721.031.95
Power Out (W)97.384.757963.8

The above chart assumes the SWR on your feedline is 3:1 — but it is likely to be significantly higher as with most non-resonant multiband antennas. Nonetheless, the 600-Ohm balanced feedline is clearly the winner. Let’s reexamine the results with a more realistic feedline SWR of 5:1:

600-Ohm BalancedTimes Micro. LMR-400Belden 8237 RG-8Belden 8240 RG-58
Length (ft.)100100100100
Frequency (MHz)14141414
SWR5:15:15:15:1
Power In (W)100100100100
Total Loss (dB).191.061.52.69
Power Out (W)95.8378.2770.9553.84

In this second example, the ham using RG-58 has lost almost half of his power in the feedline! With the high cost of radios and amplifiers, it seems silly to use lossy cable and sacrifice those expensive dB’s.

Differences in Balanced Feedline

Most amateurs opt to build their own 600-Ohm balanced feedline from wire and inexpensive spreader insulators. However, commercial 450-Ohm “window line” is available and very sturdy. Is it critical which one we select? Not particularly, as seen in the chart below:

600-Ohm Balanced450-Ohm Balanced
Length (ft.)100100
Frequency (MHz)2121
SWR5:15:1
Power In (W)100100
Total Loss (dB).19.27
Power Out (W)95.894

The 600-Ohm balanced feedline exhibits somewhat less loss, but the difference (even at 21 MHz) is negligible. If you must purchase a commercial 450-Ohm balanced feedline, you will be pleased with its performance.

Also see: W3LPL’s Coax loss charts

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