Measuring Coax Cable Electrical Length Using a Vector Network Analyzer (VNA)

Our new 2M EME station will have Adaptive Polarity capability via MAP65. MAP65 requires that received signals from the Horizontal and Vertical planes of our antennas arrive at the receivers in our shack precisely in phase with each other.

We decided to use a pair of LMR-600 coax cables for the receive side of our feedlines. We made these cables from an unterminated length of LMR-600 coax measured to cover the distance from the top of our planned 26 ft EME tower to the ground block at the entry to our shack. The cables are approximately 82 ft long and they must be cut to be equal in length to within 1/16″!

The easiest way to measure the length of an unterminated coax cable is to determine the minimum frequency of resonance of the cable when the opposite end is an open circuit. One can then use the speed of light and the velocity factor of the cable to compute its exact length:

Length = (Speed of Light X Velocity Factor) / (Resonant Freq. X 4)

Doing these measurements with an open circuit at the far end of the cables enables trimming the length of the two cables to be matched in small increments until our two cables are exactly the same length.

Vector Network Analyzer (VNA) Measurement of Open Coax Cable Resonance

We used an Array Solutions VNA 2180 connected to a Windows PC to precisely measure the minimum Resonant Frequency of our LMR-600 coax cables as we trimmed them. Once they were equal in length to within 1/16″, we installed an N-Female connector on the unterminated end and re-verified each cable’s length. A frequency-accurate antenna analyzer can also be used to make these measurements.

We will need to repeat these steps of the receiver-end and antenna preamp box jumper cables which will make up the rest of the receive side feedlines for our EME antenna system once these components are installed. We also plan to make a final end-to-end measurement of the receive-side feedline assemblies to fine-tune the phasing of the completed feedline runs.

With this step complete, we are ready to put up our new tower and attach the feedlines. Here are some links to other articles in our series about our EME Station 2.0 project:

• EME Station 2.0 Part 1 – Goals and Station Design
• EME Station 2.0 Part 2 – Excavation, Footings, and Conduits for New Tower
• EME Station 2.0 Part 4 – New EME Tower Is Up!
• EME Station 2.0 Part 5 – Control Cables and Rotator Controller
• EME Station 2.0 Part 6 – Tower Grounding System
• EME Station 2.0 Part 7 – Building Antennas
• EME Station 2.0 Part 8 – Elevation Rotator Assembly and Sub-System Test
• EME Station 2.0 Part 9 – H-Frame Assembly
• EME Station 2.0 Part 10 – Antennas On The Tower
• EME Station 2.0 Part 11 – Station Hardware in the Shack
• EME Station 2.0 Part 12 – Station Software
• EME Station 2.0 Part 13 – H-Frame Enhancements

If you’d like to learn more about How To Get Started in EME, check out the Nashua Area Radio Society Teach Night on this topic. You can find the EME Tech Night here.

Fred, AB1OC