Over the years coiled coax has been used to choke off RF
from the outside of coax feed lines. Sometimes
these are called 1:1 baluns, choke balun, but technically a balun is an impedance
transformer while these coils are chokes.
(NOTE: This discussion is limited
to air core chokes only.) The reason for a choke is to stop RF from coming down the coax, causing distortion of the antenna’s radiation pattern and RF problems in the shack.
Coax line chokes work well if they are used
correctly. However, there are some things one
needs to know about using this type of coax line choke.
1 1.
They are most effective on one band only below 40 meters.
2 2.
If carefully designed, they will perform on more than one band 40 through 10 meters (see chart below).
3 3.
Always “flat” wind these chokes. Scramble winding adversely impacts their performance.
The most important performance limiting factor is inner turn
capacitance, called parasitic capacitance.
Remember that any time two conductors are next to each other, even if
there is a DC electrical connection between them, there still is some capacitance
between them at radio frequencies (RF).
NOTE: Do not confuse parasitic capacitance with the
capacitance between the shield and the inner conductor of the coax. The capacitance between the shield and the inner conductor does not contribute anything of significance to this discussion.
As parasitic capacitance gets larger the
choke starts to look more and more like a capacitor to RF. Remember
that a capacitor's or an inductor's resistance to alternating current (AC) varies with frequency.
Experience
over the years has shown that 10 times the feed line resistance for a well
match antenna is a good number to use for a coaxial line choke's reactance. I used 500 Ohms for creating the chart below. In the real world up to 25%
either way is acceptable. That is 375 to
625 Ohms. A choke will remain
effective so long as both its coil and parasitic capacitance remain in the
acceptable range for the expected operating frequency.
The following chart gives a list of coaxial line chokes
for various ham bands.
Turns Diameter Length Coax type Band
Coverage
13 8 3.25
RG-174 160 meters only
RG-58
RG-8x
9 9 3 RG-174 80 meters only
RG-58
RG-8x
8 6 2 RG-174 40 meters only
RG-58
RG-8x
9 4 2.25
RG-174 40 – 30 meters
RG-58
RG-8x
8 4 2
RG-174 30 – 20 meters
RG-58
RG-8x
5 5 2
RG-174 20 - 15 meters
RG-58
RG-8x
6 4 2
RG-174 17 - 10 meters
RG-58
RG-8x
4 3 1
RG-174 6 meters only
RG-58
RG-8x
Length and Diameter are in inches.
Be sure to match the length of the coil keeping the
spacing even between turns. This may require using a spacer between turns while winding the coil. Once the coil is wound the spacer should be removed and the turns need to be glued in place. Deviating very far from either length or diameter will chance the frequency at which the coil will work.
Here are three graphs that will help you see why a coaxial line choke is limited. Note how rapidly the inductance and capacitance very with frequency. Notice that on each graph inductive reactance and capacitance reactance cross on 500 Ohms. Remember from above that this is the design criteria.
Notice the higher the band, larger the range of frequencies the choke covers.
Other coil sizes and coax types will work. However, they are too numerous to post.
Here are three graphs that will help you see why a coaxial line choke is limited. Note how rapidly the inductance and capacitance very with frequency. Notice that on each graph inductive reactance and capacitance reactance cross on 500 Ohms. Remember from above that this is the design criteria.
Other coil sizes and coax types will work. However, they are too numerous to post.
73
Dave, K7PDW
