A Case for Balanced Feedlines (September 17, 2009)
In September of 2009 a fellow on the Ham Radio Help Group mailing list posed the following during a discussion of multi-band antennas. My response appears below, slightly edited to provide links to another site. In my response I took the opportunity to promote the use of balanced feedlines over coax for multiband antennas. Tim, N9PUZ
> With all else being equal, transmitting on say 10 meters with an
> antenna cut for 10 meters a opposed to transmitting on 10 meters with
> an antenna cut for say 20, 40 or 80 and a tuner, what's the expected
> differences between the radiated signals?
Let's assume we're talking about a doublet type wire antenna.
First, if the dipole is not a half wave long on the frequency where you are operating the radiation pattern will not be the same as an antenna that is a half wave. That may or may not be a bad thing depending on whether or not there is a lobe going in the direction of interest. Of course, a true half wave dipole might not have had a lobe in the direction you needed either.
Second, the tuner keeps your radio from seeing a high SWR but the SWR losses still exist between the tuner and the antenna. The amount of loss your feedline presents is typically specified under a low 1:1 SWR condition by the manufacturer. Even really good coax can get very lossy under high SWR conditions.
This is why I'm a big advocate of balanced feedlines!
You can do some comparisons here for a real eye opener... Visit the SAARS web site and use their feed line loss calculator.
100 ft of good LMR-400 coax has a 0.644 dB loss at a 1:1 SWR. You lose about 14 Watts of power in the feedline when the antenna itself presents a 1:1 match. Now, if the SWR between the antenna and the tuner is 10:1 then you have a total loss of 2.463 dB and you lose around 43 Watts of power.
This points to why so many of us recommend and use balanced line to feed all band doublets. It would be great to have a properly cut antenna for each band but it is just not practical in a lot of cases.
Notice that if you make this comparison using 'generic', i.e.--cheap, 450 Ohm window line the 10:1 loss is only 0.702 dB and you're giving up only about 15 Watts worse case instead of 43 Watts. The window line only loses about 3.5 Watts (0.15dB) under the 1:1 condition.
One last point. The discussion above all relates to transmit loss and power. Loss is loss. If your system is giving up 43% of it's transmit power due to feedline loss then you're also giving up that amount of received signal too.
One last comment. Balanced line can be very difficult to run indoors. It needs to be kept away from metal objects such as house wiring, water pipes, furnace duct work, etc. The rule of thumb is it needs to be at least 3 times the width of the line away from metal objects but farther is always better. Interaction with nearby metal tends to unbalance the signal negating some of the advantages to balanced line and providing an opportunity to interfere with household electronics.
An alternative is to run good quality coax indoors and use a 1:1 current balun to transition to the balanced line where it goes outdoors. If you keep this coax line short you won't have much SWR loss and your life may be much simpler.
One of my favorite books on antennas...
Lew McCoy On Antennas
Pull Up A Chair And Learn From The Master
By Lew McCoy W1ICP (SK)
CQ Communications, Inc.