AEA Technology 20/20 TDR Application Note

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Abstract:

This paper discusses tips for using the AEA Technology network analyzer

at antenna sites. Adding an AEA 20/20 TDR20 TDR provides a powerful

combination for testing the feedline and antenna.

Introduction:

Measuring the impedance of antennas located at the top of a tower

presents one big problem: climbing the tower. Without climbing the tower, the

best measurement you can make combines the feedline effects with the antenna

impedance, which can make the readings confusing. Interfering signals from

other towers can create a second problem. We discuss using a pseudo feedline

(PFL) for improving impedance readings without climbing the tower. We also

show how to use filters to remove interference.

Measure the Feedline:

Start with the 20/20 TDR to check the feedline. Towers typically have very

high quality coaxial feedlines, so the TDR plot should look nearly perfect.

Sometimes you will find water in the feedline caused by a bullet hole or other

problem. Draining out the water and plugging holes usually requires a climb, but

the TDR lets you know that the effort spent climbing the tower will result in a

repaired feedline. Once the feedline has been verified, save the final reading,

this reading helps create a "pseudo feedline" discussed in more detail below.

Also take note to read about "Adapters" below. See the Appendix for a list of

App Notes showing cable issues and their plots.

A good feedline will show a flat plot (usually 50 ohms) for the entire length

of the line. Once the TDR signal reaches the antenna, the reading goes to an

open or a short depending upon the antenna design. If the connector to the

antenna has failed, you may not be able to distinguish the bad connector from a

good antenna. The network analyzer takes care of this issue.

1489 Poinsettia Ave. Suite 134 Vista, CA 92081 ◦ Phone (760) 798-9687 ◦ Fax (760) 798-9689 www.aeatechnology.com

AN114

Tower Site Tips

Written by Bill Ashley for

AEA Technology Inc.

www.aeatechnology.com

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Page 1 Measure the Feedline: Start with the 20/20 TDR to check the feedline. Towers typically have very high quality coaxial feedlines, so the TDR plot should look nearly perfect. Sometimes you will find water in the feedline caused by a bullet hole or other problem.
Page 2 2. Install a coaxial connector (same type as on the network analyzer and TDR) to one end of the coax. 3. Connect the coax to the 20/20 TDR. 4. Turn on the TDR and recall the plot made from the real feedline.
Page 3 TDR adaptors, the cable null will have some error. The easiest way to prevent this, use a 20/20 TDR model with the same connector as the network analyzer (usually N). Plan B: use an adapter that converts the TDR to the same connector as the network analyzer.
Page 4 You do not want to run out of batteries when on the tower. Conclusion: Using a BPF, TDR, and PFL with the AEA Technology network analyzer improves the ability to diagnose faults and eliminate problems incurred at antenna test sites.