## Return Loss Bridge

**Return loss** (RL) may be a new concept to many radio amateurs. It may surprise you, but in this case, more loss is better. Return loss is the decibel portion of the input power that is taken by the load; **greater return loss** means **less reflected power** and lesser **SWR** (Standing Wave Ratio). In fact, when the SWR is 1:1 the return loss is infinite!

On the **picture** (click to enlarge) on the right you will see a simple build of my **Return Loss Bridge** (RLB), with 3 BNC connectors. The **transformer** is a commercial piece DLF4000, the black square in the background.

For some applications, **return loss** is easier and more useful than **SWR** measurements. Let’s look at a couple of **examples**.

Suppose you know your coax loss and SWR at your transmitter, but want to know the SWR at your **antenna feed point**. Factoring transmission-line loss into the SWR equations is difficult, but the problem becomes a piece of cake when you use return-loss measurements. Here’s the recipe:

- RLAnt = RLTX – 2 LCoax (Eq 1)

Where RLAnt and RLTX are the return losses at the antenna and transmitter, respectively, and LCoax is the coax loss. All three quantities must be in decibels. You may wonder why we double the coax loss in this calculation. The reason is that the power returned by the mismatch travels through the feed line twice—once as forward power, and then again as reflected power.

Let’s now consider some real numbers. Suppose the SWR measured at the transmitter is 2:1 and the coax loss is 1.75 dB. The first step of course requires us to convert SWR to return loss. We can translate between several measures of load match by reading a table (see Table 1) or applying math equations. [1] Table 1 shows that a 2:1 SWR equals a return loss of 9.5 dB. Plugging in the numbers, we get:

- 9.5 dB – (2 ×1.75 ) = 6 dB

The wobbulator **graph** on the right side shows with the **0 dB curve** the **short** from DDS-Generator to the logarithmic amplifier.

The **-12 dB curve** the **zero point** (RL = open, SWR = ∞) of the RLB in the **frequency range** from 1 to 31 MHZ (3 MHz per division). The **vertical scaling** is 10 dB per division.

The **center curve** shows the **directivity** (- zero curve = 30 - 35 dB, RL = 50 Ohm, SWR = 1.0) of the bridge.

The **lower curve** (-70 dBm) shows the logarithmic amplifier with **open input**.

In order to check the RLB, connect a **100 Ohm** (1%) resistor to the **OUT/Load** connector, and read a **Return Loss** value of **9.5 dB** or a **SWR = 2**.

**Table 1—ρ, ρ2, Return Loss and SWR**

ρ (V) Pr/Pf (ρ2) RL (dB) SWR 0.00 0.00 ∞ 1.00 0.05 0.00 26.4 1.10 0.09 0.01 20.8 1.20 0.10 0.01 20.0 1.22 0.13 0.02 17.7 1.30 0.15 0.02 16.5 1.35 0.17 0.03 15.6 1.40 0.20 0.04 14.0 1.50 0.23 0.05 12.7 1.60 0.25 0.06 12.0 1.67 0.26 0.07 11.7 1.70 0.29 0.08 10.9 1.80 0.30 0.09 10.5 1.86 0.31 0.10 10.2 1.90 0.33 0.11 9.5 2.00 0.35 0.12 9.1 2.08 0.40 0.16 8.0 2.33 0.45 0.20 6.9 2.64 0.50 0.25 6.0 3.00 0.55 0.30 5.2 3.44 0.60 0.36 4.4 4.00 0.65 0.42 3.7 4.71 0.70 0.49 3.1 5.67 0.75 0.56 2.5 7.00 0.80 0.64 1.9 9.00 0.85 0.72 1.4 12.33 0.90 0.81 0.9 19.00 0.95 0.90 0.4 39.00 1.00 1.00 0.0 ∞

The 6 dB return loss converts to a 3:1 SWR at the antenna feed point.

As you might guess, we can reverse this procedure. Suppose you want to know the SWR at your transmitter if your antenna SWR is 1.5 and coax loss is 1 dB. From Table 1, we see that a 1.5:1 SWR corresponds to a return loss of 14 dB. This time, the calculation looks like this:

- 14 dB + (2 × 1) = 16 dB

The SWR at the transmitter is about 1.35:1.

- Text excerpt from Jim Ford, N6JF, full article see at the Links.

### Schematic of the RLB

This basic schematic is from Wes Hayward’s (W7ZOI). The connection is made with BNC plugs.

The **resistors** are made from two 100 Ohm resistors in parallel with a tolerance of 1%.

The transformer is a commercial unit DLF4000, which gives a mean **directivity** in the **frequency range** 3 to 30 MHz of **>30 dB**, which is excellent.

As you can see in the table above, for practical reason a **directivity range of 27 dB** is sufficient.

The DLF4000 has 4 coils on the toroid. When using 2 coils for the transformer, the -1 dB lower frequency limit was -3 dB at 2 MHz, the directivity >30 dB.

A **toroid** transformer Amidon T37-6 with two times 10 turns 0.4 mm diameter wire achieved 29 dB directivity only.

### Measuring with the RLB

Together with the RaspberryPiWobbulator you will get a powerful **measurement equipment**, to measure the effective **Bandwidth** and the **Quality** (matching) of an impedance in the short wave range 1 - 30 MHz, e.g. your **antenna**.

One **practical example** is to measure the **center frequency** of a LC parallel circuit, 100 pF parallel to 1uH (adjustable core). The LC circuit must be connected on the **Output** (Load) parallel with a **50 Ohm resistor**.

In the **picture** on the right, the **center frequency** of the **parallel LC circuit** is about 11/15/21 MHz (core in, half in, out). It is interesting so see, that the core affects the **quality of the coil** at the resonance frequency. The **upper trace** is the zero curve. The **lower curve** shows the **directivity**.

Next step was to measure some **real amateur radio antennas**.

Below are three **diagrams** from three different **antennas** (click on **diagram** to enlarge). For the **cable loss** 2 x 1 dB is added to the measured value. The SWR values measured with a **real SWR meter** are behind in parenthesis.

The

**upper trace**is the**0 dB curve**, if the output connection of the Return-Loss-Bridge is open.The

**center trace**is the**return loss**curve of the antenna.The

**lower trace**is the**maximum return loss**at the output connected to 50 Ohm.The

**left diagram**is from a 80 m (3.5 MHz)**Dipol antenna**.The

**best return loss**at 3.5 MHz is about 13+2 dB, which is equal to a**SWR**(Standing Wave Ratio) of 1:1.45 (1:1.8).

The

**center diagram**is from a 40 m (7 MHz)**Dipol antenna**.The

**best return loss**at 7.0 MHz is about 19+2 dB, which is equal to a**SWR**(Standing Wave Ratio) of 1:1.2 (1:1.2).

The

**right diagram**is from a**three band beam**20, 15, 10 m (14, 21, 28 MHz)**Fritzel FB23 antenna**.The

**best return loss**at 14 MHz is about 14+2 dB, which is equal to a**SWR**(Standing Wave Ratio) of 1:1.35 (1:1.1).The

**best return loss**at 21 MHz is about 15+2 dB, which is equal to a**SWR**(Standing Wave Ratio) of 1:1.35 (1:1.3).The

**best return loss**at 28 MHz is about 13+2 dB, which is equal to a**SWR**(Standing Wave Ratio) of 1:1.45 (1:1.0).

### Dipol, Windom

I got now a **Windom Dipol** (13.8 + 27.7 = 41.5 m) from a friend and measured with the **wobbulator** and the **Return Loss Bridge** the **SWR** (Standing Wave Ratio) of the antenna, and compared the values with the **MFJ-269 SWR Analyzer**.

The **height of the antenna** is at the feed point about 9 m, at the ends 6 m above ground.

The **50 Ohm feed cable** (HSR195) has a mechanical length of 17.04 m, electrical 21.3 m (about 80 m Lambda/4).

The **diagram** on the right was measured with the wobbulator, **Frequency Range** is 1 to 31 MHz in 100 KHz steps.

The upper 3 straight curves are the **SWR values**: ∞ (blue), 1:2 (red), and 1:1.2 (green). The lower straight curve is the **50 Ohm** matching, means a SWR of 1:1.

The MFJ-269 showed the **best SWR** for the following Bands:

Band |
MHz |
SWR 1: |

80 m |
3.1 |
2.0 |

40 m |
6.7 |
1.4 |

20 m |
14.6 |
1.2 |

10 m |
29.1 |
1.1 |

With a **MFJ-941B matchbox** I achieved in all bands a SWR of better than 1:1.2. One exception was the **80 m CW band** with 1:1.4. The 80 m band is a problem, because I had to route the last 5 m in an angle of 45 about degree. In order to move the resonance point a little bit higher, I shorted the antenna at both ends for 0.5 m each.

**Conclusion**: The measurement results of the**wobbulator with Return Loss Bridge**and the**MFJ-269 SWR Analyzer**does match very good, so you can get within a few seconds with the**wobbulator**a very good overview about your antenna characteristic.

The **MFJ-269 SWR Analyzer** is very useful for measuring antennas, see the **picture** on the right.

The **Windom antenna** is connected to the input via **adapter**. The instrument does have a **N-connector**, while the antenna has a **BNC connector**.

A 2-line **LCD** show the measurement results:

The upper line the **work frequency** in MHz, and the **SWR**.

The lower line shows the **complex impedance** in **resistive and reactive** part.

Below are two analog instruments showing the **SWR** and the **load impedance**.

At the bottom of the front plate are the **frequency analog adjustment** and the **band switch**.

To **check the antenna**, you tune to the desired frequency and read the **SWR**, or you tune to a minimum SWR and read the **frequency**.

### Links

**List of pages in this category:**

-- RudolfReuter 2014-03-19 10:09:29

Go back to CategoryAmateurRadio or FrontPage ; KontaktEmail (contact email)