Feed line Considerations

 
ladderlock2 ladder

 
w7fgladder
Window line strain
relief for dipole
Window line

Open Wire
Ladder line

When attempting to load a typical 160 meter antenna it is often found to be non-resonant. This is usually due to physical constraints when installing the antenna. Often we do the best we can do with the installation and then use an antenna tuner to tune out any resulting reactances. For convenience sake the tuner is located in the ham shack. That means that the feed line between the antenna and the tuner may have high or very high standing waves (SWR). Here is where the choice of antenna feed line is signficant: with high standing waves, coax lines can have considerable loss, whereas the ladder line tolerates this high SWR very well. The following information is from QST, Dec. 1993.

Freq. Mhz.
8214 coax [db loss]
Ladder line [db loss]
1.9
26.9
8.62
3.8
13.7
1.37
7.15 (resonant freq)
0.19
0.07
10.14
2.85
0.07
14.27
5.30
0.15
18.14
6.96
0.31
21.40
0.78
0.12
24.90
3.94
0.13
28.50
5.69
0.18
The above data is from a 40 meter dipole used on 160 meters fed with either high quality coax or ladder line. Notice that as soon as the SWR gets high, say above 3:1, the coax loss becomes very significant. Using ladder line allows you much more frequency latitude, yet keep the losses to an acceptable level.

Ladder line comes in two basic forms. One is like TV ribbon, where there is a center web of plastic (or similar material). The center web can be cut out at regular intervals to reduce the dielectric losses contributed by the center web. See the 300 or 450 ohm examples shown above. This type is often called “window line”.

An even lower loss ladder line is shown in the picture in the upper right. Here we have parallel wires separated at intervals with wooden, plastic, ceramic, or other insulator. This is called “open feeders” or “ladder line”. Any parallel feed system is called loosely as “ladder line”.

Note that in many situations, coax is perfectly fine for feeding your 160 meter antenna. If you wish high frequency bandwidth, resulting in high SWR, then consider using the open line feed system to minimize losses. If your 160 meter antenna tends to be fairly resonant then, the expected SWR will be low enough that either feed type will be satisfactory.

Using ladder line assumes you have a balanced antenna system. Since your transmitter probably has a 50 ohm unbalanced output, you will need an external tuner to match your transmitter to the balanced line and most likely to counteract any high reactances that are found on most 160 meter antennas. See the easy to construct tuner article else where on this web site.  Click here for tuner article.

If your tuner does not have balanced output, you may be able to use a balun (balanced to unbalunced). The balun is simply an arrangement of wires, or coils (possibly with ferrite cores) that will allow you to connect your coax to a balanced line. With care, this system can be very satisfactory, in fact many antenna tuners have a balun inside to provide the balanced output capability. A problem may arise if the balun is used with high SWR (and can be much worse at high power); here the balun can create RFI due to non-linearities, high heating of the balun with accompanying power loss. The balun is placed at the output of the tuner and the lower end of the feed line.


BUILD A HEAVY DUTY AIR WOUND CHOKE BALUN FOR HF


QST articles about Feed Lines 

Feeding Your Station

Feed Lines

Reflections by Maxwell

Lets Talk Transmission Lines

 

 

 

 Posted by at 2:09 pm