Coaxial Cable Leakage

added coaxial cable measurements July 11, 2012

Related pages

Unwanted antenna coupling

coaxial lines and shielded wires

how a shield works (skin depth)

Station RF grounding

Bench Test Fixture July 2012

Data for common mode current ingress into cables is mostly non-existent. The data that is available is not very useful, because test method details are not readily available. A manufacturer might tell us shielding effectiveness is so many dB, but what does that really mean?

Another problem is the abundance of opinions and advice regarding shields and shielding, and generalizations about what does what inside a cable, including functionality or operational contribution of shields and foils inside a cable. Advice and information in amateur radio circles seems to be based on opinions, observations, or feelings about VHF or UHF performance, or how audio cables work.

Thinking about this problem, as it relates to amateur radio, I decided to build a fixture to directly measure coaxial cable ingress from common mode shield currents. Building a fixture for lower frequencies is not that difficult, because groundplanes and shields are very predictable. They are easy to test and verify.  The concerns for an "outside-in" measurement are:

• The detector system cannot respond to the outside shield common mode
• Shield common mode current has to be known
• Shield common mode impedance from cable end-to-end has to be very low (this makes the system stable by confining current to the cable tested)  
• Center conductor (inside current) has to be known
• The cable must be terminated by impedances similar to the real-world application
• The system has to be verified to ensure measurements are dominated by what we actually intend to measure

I constructed the following fixture:

Loads match cable impedance

RF load current is found with a HP RF level meter across the load

High power RF source is an IC751A through an ATR30 tuner

Current sample is a calibrated clamp-on RF current probe

I normally set shield current at either 300 mA or 1 ampere. Load current is measured by voltage drop across a 75-ohm resistor using a high impedance RF millivolt level meter. This is a significant amount of common mode current.

Verification of Fixture

To verify the fixture, I included a dummy port with load near the measurement connector. I can verify three ways:

• With the measurement device on the dummy port, dummy port voltage is read with normal cable connections and full cable excitation
• With the measurement device on the LOAD port and the cable on the dummy port, voltage on the LOAD port is read with full excitation
• With an open shield and normal measurements, which produces a near zero dB attenuation    

In the first two cases, my meter stays out-of-lock. Lock occurs with about 0.2 mV, while normal readings (so far) are in the 5-40 mV range. This shows an insignificant error. In the last case, the fixture shows within 0.5 dB of zero loss (the error is from calibration or tracking errors). 

Measurements

Ratio of outside shield current to center conductor current for 8-ft cable length:

Bright Wire link

Removing braid has a very noticeable effect on common mode impedance, because injected current changes. It does not necessarily change common mode coupling very much. Even on 160 meters, innermost foil dominates shielding.

The Dish Network cable was just removed after being outside since 2008. It is a foil / braid non-flooded cable. It appears to have excessive resistance and bad connections on the outer braid, because flexing changes RF current about 10%. Flexing does not change ingress very much, the inner bonded foil must be intact.

Connector Mounting

Connector mounting is critical.  For good CM rejection, cables entering boxes should mount on a common metallic wall. The wall must have significant conductor area to maintain low impedance between connector grounds, not moderately-sized circuit traces or wires. The exception is at an intentional interface to a shield isolation system.

This is a commonly seen connector wiring method. Techniques like this are often used where cables enter plastic boxes.

Here is how a connection method like this, with 3-inches of coax, impacts measurements of

Some Conclusions

Interesting conclusions can be drawn from this data, and observing system behavior.

First, just as it should behave, a closed cylinder or wall, even copper or aluminum foil, does an excellent job of shielding time-varying magnetic fields. The magnetic field does not easily pass through the aluminum, as long as the shield is several skin depths thick.

Second, as long as the frequency is high enough to have at least several skin depths thickness in the shield or wall, the foil shield nearest the center does all or most of the shielding work. As long as the foil is intact, common mode isolation stays about the same.

Third, the outer shield affects common mode impedance of the cable outside much more than it actually affects common mode ingress. When the braid was removed, excitation power had to be significantly increased to maintain the same common mode current. This shows a higher common mode impedance of the shield. Cable ingress did not track this increased impedance. It increased at a much reduced rate (because of time restraints, I only tested this on 160 meters).

Fourth, connector mounting is critical, especially on higher frequencies, for best CM (common mode) ingress immunity. Connectors must mount directly to  low resistance and impedance groundplane paths common to other connectors, unless the device is intentionally planned for CM shield isolation. On lower bands, it isn't so much the total metallic enclosure sealing, but rather the groundplane impedance between different ground connection areas.

Real-world Measurements (leakage into a new 3000-foot long dual-shield coaxial cable) in summer 2006

A separate receiving antenna and receiver can be used to find new multipliers while making contacts on the main radio. A second radio also can be used for two-transmitter interleaved operation on one band, provided one transmitter is allowed active at any instant of time.

The purpose of this test was to see if cable shield leakage of the transmitted signal would exceed signal level picked up by Beverages or other antennas. The goal of this was to test transmitter signal ingress into the receiver's feed line and shack wiring, comparing the level from unwanted cable and wiring ingress to levels from the same transmitter into distant receive antennas. This data should also be useful in other applications, or just to illustrate the amount of ingress through this type of common, inexpensive, coaxial cable.

This line is CommScope dual-shield F11 (roughly RG-11 size) cable. It consists of a single 100% foil shield with a single 60% coverage aluminum braid overlay. This is a flooded cable for direct burial, with snap-and-seal connectors.

Building Entrance (old pictures)

Building Entrance Panel

Test Conditions

200 watts transmitter power. Cable terminated in 75 ohms. Signal levels are measured at receiver. Antennas used were my most distant beverage antenna group approximately 2500 feet (one half mile) from my transmitting antennas. "dBm" values below for RX antenna dBm are the values with actual transmission line losses included.

160 Meters

80 Meters

There are only two cases where signal ingress can exceed signal level from the beverages that are loacted one half mile away. Both cases are where a low dipole is mounted right above and parallel to the feed line. In these cases, the feed lines from the low dipole test antennas also paralleled the F11 receiving antenna feed line for at least 200 feet in the same bundle.

160 Meters

80 Meters

Conclusion

Even with Beverages 1/2 mile from the transmitter, there are very few cases where signal ingress into the coax exceeds the signal level from the Beverages. The only cases of failure were when low dipoles parallel to the feed line were excited and compared to a Beverage with a deep null towards the transmitting antenna. This data assumes good coaxial connections. Common mode chokes were not used in this test.

Certainly for any beverage installation closer than 1/2 mile to transmitting antennas feed line ingress will not be a problem. All of the signal will be from the receiving antennas and not leakage through the coax shield. 

since Jan 12, 2007