TRANSMISSION LINE "SHOOT- OUT"

COAX Vs. BALANCED-LINE

Many Hams enthusiastically "claim" that Balanced-Line is MUCH better than Coax, because it has much less loss than Coax.


TRUE or MYTH ?


Let's take a close look at reality.

BEFORE WE BEGIN, let's first look at the published loss specs of various types of coax :

NOTES: 

  1. The loss shown (in dB) in the chart above assumes a perfect impedance match (SWR = 1.0:1).
  2. In the meantime there are several other low-loss types of coax available (i.e., HyperFlex-10, Aircell-10, etc.  These are not yet shown on the chart above.  Of the models shown Messi & Paoloni "Airborne-10" and Times Microwave System "LMR-400" are the two best.

In real-life, we never have a perfectly-matched scenario.  We always have SWR ≠ 1:1.

The "total loss" in any transmission line consists of two components:

  • The "inherent loss" of the perfectly impedance-matched feedline. (SWR = 1:1)
  • The "additional loss" due to SWR.

In order to compare Coax with Balanced-Line, we have to look at "Total Loss", which includes the additional loss due to SWR.


In the three tables below, created using an online loss calculator, we shall compare total loss, looking at three different values of SWR:  3:1,  5:1,  10:1.


We can also see the native loss of a perfectly matched transmission line in all three charts.


Unfortunately, I was unable to find an online loss calculator that shows the newer low-loss coax, such as Airborne-10, Aircell-10 or HyperFlex-10.  Thus, for comparison purposes we shall compare LMR-400 to the various types of balanced-feedline.

ASSUME SWR = 3:1

COMMENTS:


On 80m, the total loss for LMR-400 is about 0.4 dB, the average loss for Window-Line is about 0.2dB.

However, an antenna fed with balanced-feedline requires an antenna tuner; antenna tuners have loss.  Good tuners have between 5% and 10% loss.  This represents a loss of 0.2 dB to 0.45 dB.

THUS, assuming the BEST antenna matchbox (0.2 dB loss), if our coax fed antenna has low enough SWR not to require a tuner, its loss is no more than the loss with balanced line + tuner!


On 10m, the total loss for LMR-400 is about 1.02 dB.  The average Window-Line loss is about 0.5 dB.  Including tuner loss, it is about 0.7 dB.  Thus, the average Window-Line loss is only about 0.3 dB less than using LMR-400 coax.


No QSO Partner on the planet can detect a difference in signal strength of 0.3 dB!


DON'T FORGET the disadvantages of balanced-line:


  • Antennas fed with balanced line require an antenna tuner and every band change requires re-tuning the tuner.


  • And, when using Window-Line rather than true Ladder-Line, when it rains or snows during a QSO, the impedance changes, SWR rises, and you must re-tune the tuner!


CONCLUSION:  Unless your feadline is several hundred ft. long, each operator must decide for his/herself if the disadvantages of using a tuner is worth the very low difference in performance that balanced-line brings.


ASSUME SWR = 5:1

COMMENTS:


On 80m, the total loss for LMR-400 is about 0.6 dB, the average loss for Window-Line is about 0.25 dB.

However, an antenna fed with balanced-feedline requires an antenna tuner; antenna tuners have loss.  Good tuners have between 5% and 10% loss.  This represents a loss of 0.2 dB to 0.45 dB.

However, at 5:1 SWR, the coax-fed antenna will also require a tuner.  Thus, tuner loss is a moot point.  The difference here is about 0.35 dB.


On 10m, the total loss for LMR-400 is about 1.5 dB.  The average Window-Line loss is about 0.75 dB.  Since a tuner is required with both types of feedline, the average Window-Line loss is only about 0.75 dB less than using LMR-400 coax.


It would be very rare than any QSO Partner anywhere on the planet could detect a difference in signal strength of 0.75 dB!


CONCLUSION: Although Die-Hard DXers "claim" 1 dB can make a difference when working weak DX stations, in my opinion that is very rare, and most people are not often trying to work stations that are barely audible above the noise level.


ASSUME SWR = 10:1

COMMENTS:


This chart showing 10:1 SWR is, IMO, where the argument for using balanced-feedline definitely makes sense!  Resonant balanced-line fed dipoles can have an SWR on their 2nd harmonic of 40:1 or more.  Obviously it would be pointless in using Coax on such antennas.


On 80m,  the total loss for LMR-400 is about 1.9 dB, the average loss for Window-Line is about 0.5 dB.

Since either type of feedline requires a tuner, tuner loss is irrelevant.

The difference here is about 1.4 dB.


On 10m, the total loss for LMR-400 is about 2.5 dB.  The average Window-Line loss is about 1.3 dB.  Since a tuner is required with both types of feedline, the average Window-Line loss is about 1.2 dB less than using LMR-400 coax.


The scenario changes significantly when comparing to thinner coax, where average 10m loss is 5 or 6 dB. The balanced feedline will be about 1 S-Unit stronger than feeding with these thin feedlines.

CONCLUSION:  This chart is for SWR = 10:1.  In many typical antennas fed with balanced feedline, the SWR can easily be 20:1 or even 50:1.  It would be ludicrous to feed these antennas with coax! THIS is where the loss difference is significant!

REALATIES:


  • There are antennas where the improvement in performance (reduction of loss) using balanced feedline instead of coax will make a significant improvement.


  • The general statement that balanced feedline has much lower loss than coax is WRONG.  There are many cases where switching from coax to balanced feedline will result in no tangible improvement in the signal.


  • There are antennas (i.e., OCFD) where "appears" that it would make sense to use balanced-line (i.e., 300 Ohm Twinline) but that is actually a very bad solution; coax is MUCH better.  (see next page).