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!

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

attachment:wobbulator_RLB_DSC05177.jpg

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:

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:

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:

The SWR at the transmitter is about 1.35:1.

Together with the RaspberryPiWobbulator you will get a powerful measurement equipment, to measure the effective Bandwidth and the Quality (matching) of your short wave antenna.

Schematic of the RLB

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


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