Custom Products from Masters Communications Phone: (814) 313-4036

What is it, and what does it do?
A return loss bridge can turn your service monitor or signal generator / signal receiver into an accurate antenna VSWR analyzer. It will provide the same accuracy as antenna analyzers costing thousands more. The RLB1500 is a commercial duty 50 ohm, 0.5 - 1500 MHz return loss bridge (RLB) for antenna/feed line, cavity, and duplexer testing and alignment. Our bridges are individually built and tested and include an actual proof of performance graph. This unit has a 0.5 - 1500 MHz range and minimum 35 dB directivity (40 dB typical). The unit has high quality type N connectors for all ports and is configured with the input and output ports conveniently on the same side. The design will withstand 4 Watts of accidental power input to the DUT port. Use your existing rf generator and receiver/power meter/spectrum analyzer to make accurate measurements. The unit is designed for use in 50 ohm systems.

The basics of a Return Loss Bridge:
In simple terms, the return loss bridge is a device used to measure RF power reflected from a load or device under test (DUT) when power is sourced to the device through the RLB. The return loss bridge has 3 ports. An RF signal generator is applied to the input port. The output port provides an RF signal to go to a measuring device such as a spectrum analyzer, power meter or other device used to measure this signal. The DUT port connects to the device that you are testing (antenna, coax, etc.). A signal is generated into the input port. The RLB applies this signal to the DUT. The DUT will reflect a portion of the signal that it receives (due to impedance mismatch). This RF signal appears at the output port. The closer your DUT impedance is to 50 ohms, the lesser the signal will be at the output port. A perfect 50-ohm DUT will have no reflected power so a perfect RLB will have no output signal at its output port (infinite return loss). If a non-50-ohm load is at the DUT port, there will be an RF signal at the output port. The extreme cases are for the DUT to be a short or an open connection. This will cause maximum reflected power and, therefore, maximum signal at the RLB output port. Return loss is measured from the worse case (short or open) with 100% power reflected. It's the difference amount in dB of the reflected signal of your DUT compared to the worst case reflected signal. The further you go from "worse case" the better. The higher the return loss number, the closer the DUT is to 50 ohms. This is somewhat the opposite when compared to SWR (high SWR means high reflected power or poor match) For example: with an open or short connected to the DUT port and an RF signal source generating into the RLB input port, let's say we measure the RF power at the RLB output port to be -22 dBm. If the open or short were replaced with your load at the DUT port, the amount of power at the RLB's output port would then change. If the output signal is now measured to be -42 dBm, this would be a return loss of 20 dB. The amount of reflected power is 20 dB less than the worst case. This is equal to an SWR of 1.22:1. If this were an antenna, it would be considered an excellent match with about 1% of the power to the antenna reflected. A return loss of 10 dB would correspond to an SWR of 1.91:1 with 10% of your power reflected. This is generally considered a poor match. Most commercial systems want to see a return loss of 18 dB or better.

Directivity:
Directivity is the measure of how well the RLB isolates two opposite traveling (forward and reflected) signals. As the reflected signal becomes smaller and approaches the specified value of the RLB directivity, the measurement uncertainty becomes larger. To get accurate return loss (SWR) measurements, it's important to have a bridge with directivity of 10 to 20 dB higher than what you want to measure. The higher the directivity, the greater the accuracy the RLB can measure small signal levels of reflected power. A perfect 50-ohm load should have infinite return loss, which translates to no output signal from an RLB. If we could build a perfect RLB, the output port would show no signal with a perfect 50-ohm load. In the real world, the highest quality RLB would show a return loss of up to 50 dB. 50 dB would be pushing the physical limits because the N connector itself has a tiny amount of return loss. A higher quality bridge will have higher directivity than needed to provide better accuracy at all levels. The RLB1500 has guaranteed directivity of 35 dB with typical directivity being 40 dB or better over the 0.5 - 1500 MHz range. A 20 dB directivity means that you'll see as little as 1% of reflected power. A directivity of 30 dB is 0.1% and 40 dB is 0.01% ( 1/10000 of worst case reflected power level). The RLB1500 return loss bridge is individually tested and includes a performance evaluation graph.

RLB1500 Return Loss Bridge
Shown Actual Size
Color is dark gray - Rubberized coating for on bench or in-field durability.

Here is a typical return loss graph printed from an HP E6380A Service Monitor. The left graph shows the typical response when sweeping the Narda 50 ohm precision load. Sweeping to 1500 MHz shows this return loss bridge is working properly with a better than 35 dB directivity over this range. The graph on the right is a sweep of a 10-meter yagi antenna. At the top, the maximum return loss was 15 dB at 28.6 MHz. Generally, you want to sweep just 1-2 MHz to get the resolution needed to see the best impedance match, as well as the usable frequency range. The RLB1500 is a valuable tool for duplexer tuning, as the frequency which exhibits the lowest insertion loss and the frequency which exhibits the best impedance match may not coincide.

See this off-site link: Duplexer Tuning with a similar return loss bridge for details.

The RLB1500 was compared to several higher priced units and provided similar results for return losses below 40 dB.

This web page, this web site, the information presented in and on its pages is © Copyrighted 1995 and (date of last update) by Kevin Custer and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.

Updated as of date below - Kevin Custer