measuring feedline loss

last updated 28 March 2025.

Measuring the matched line loss (MLL) of a feedline is useful both for characterizing an unknown feedline, and also for checking existing feedlines for damage. While a high SWR will often signal that there is some physical damage to a feedline, if water gets into it and increases the loss, the only symptom may be that the SWR is too low, which might not be as easy to notice on the meter.

Matched line loss is defined as the loss per unit length of cable when it is operated in a matched system (SWR = 1.0 : 1). It varies with frequency for each cable type, as shown in the graphs in the adventure with small feedlines article. For these article, I’ve standardized in presenting it as the loss per 30 meters or 100 feet (happily ignoring the 1% difference, since the data are not always that accurate anyway). The matched line loss in dB scales linearly with cable length, so if your cable is only 10m long, the actual losses are ( 10 / 30 = ) 1/3 of the stated value. Similarly, if you measure the loss as 1.3 dB for a 10m long cable, the reference value would be ( 30 / 10 * 1.3 dB = ) 3.9 dB per 30m. Often, simply knowing the loss in a particular piece of feedline is sufficient for many purposes.

For a discussion, see factors that affect feedline loss.

caveat

The most important thing to remember, however, is that these numbers are estimates. Manufactured cables are generally close to their data sheet values when new (but not always), and losses can increase with age, especially when water has gotten into the cable, or for older coax cables before they switched to using a non-contaminating outer jacket.

In most cases we aren’t concerned with very accurate measurements for ham use – we just connect the antenna and make contacts, as the ionospheric variation over time is far greater than the losses in most of our cables. But especially for weak signal work such as moonbounce, where there is little margin for additional losses, more accuracy may be important.

So, while these are just estimates, they can be very useful estimates.

measurements

There are several different ways to measure the loss in a cable, depending on the equipment available and the circumstances.

method #1 : put a power meter at each end

This is a rather simple method, but pretty straightforward to determine what we want to know. Basically, use a power meter to measure the power at each end of a cable when transmitting (into a dummy load, or an antenna with a very low SWR). The difference will tell you how many watts of power were dissipated in the cable. The ratio of the values, converted to dB, give you the actual cable loss.

This is great if you have two accurately calibrated wattmeters. If not, you can use one wattmeter and move it from one end to the other. For best accuracy, might want to use a barrel connector at the other end, so it is easy to swap the meter without changing the cable configuration, as that may affect the load impedance, and hence the measured power if the SWR isn’t exactly 1.0 : 1. But often it is close enough to just take measurements with and without the test cable between the meter and the transmitter.

If you have a 2-port Vector Network Analyze (VNA), then just connect the cable between ports 1 and 2 and the S₂₁ reading should give you the cable loss.

LIMITATIONS: this approach only works when the wattmeters are calibrated for characteristic impedance of the cable being tested, and the SWR is low (as should be the case with a good dummy load or a VNA).

proposed method #2A : measure SWR with far end of the cable open or shorted

proposed method #2B : measure return loss with far end of the cable open or shorted

These two are basically the same, since SWR and return loss can be converted back and forth with no loss of information.

So the general approach is to measure the return loss of the cable with an infinite SWR. In this case, the return loss is the total loss when the RF is sent up the cable, reflected off the open or shorted end, and travels back to the measuring point. Then dividing the return loss by 2 gives the one-way loss in the cable. If SWR is measured instead, convert it to return loss (using a table, graph, or calculator) and do the same thing.

However, this method does not work reliably, even though it is often used by professional engineers.

Here is an example: I have 15m ( 50 feet ) of RG-174, and I want to know the losses at 1 MHz.

The return loss measured with the far end open was 0.5 dB. The return loss measured with the far end shorted was 3.7 dB. That’s a big difference! Both of those were with infinite SWR, and were confirmed with a reputable cable loss calculator (one that uses the actual termination impedance in its calculations, not just the SWR). Which one is correct?

Both are correct measurements. The error is in expecting them both to give the same result. The important question is, “why?“.

Here we learn an important fact about losses under SWR: they are not linear along the length of a feedline the way that matched line loss is. To understand why, we need to consider two factors:

When there are standing waves on a feedline (SWR > 1.0 : 1), then both current and voltage will vary along the line. At high impedance points the voltage is high and the current is low. At low impedance points the current is high and the voltage is low. In this case, the line is operating at a very high SWR.
The losses in most feedlines at HF (and up through the VHF range at least) is due to the conductor resistance. More current through that resistance means higher losses where the current is high, and lower losses where the current is less. (Dielectric losses are more important at high frequencies, or when poor dielectric materials are used.)

In the case of the feedline that I measured, in the open circuit case most of the line had low current, so the losses were lower. In the short-circuit case, most of the line was operating at higher currents, and losses were higher. In order for these methods to give reasonable results, either the line must be long enough (perhaps several wavelengths) that the variation in loss doesn’t matter, or the measurement must be done on a line that is a multiple of 1/4 wavelength long, so there are equal parts at high and low currents.

LIMITATIONS: This method only works reliably when the line is long enough that the difference in losses between the conditions is insignificant to the total loss (typically several wavelengths), or the line is a multiple of 1/4 wavelength long (that is, the reactance at the measurement point is zero when the line is open or shorted).s

But that brings us to:

correct method #2 : measure the return loss in both open and short-circuit conditions and take the average

This might not be perfectly accurate, but is a lot better than measuring just one condition. It is the same as measuring the SWR in both conditions and converting it to return loss (using a calculator such as this one). Then average the two values and take half of that for the loss in the cable.

In the prior example where the return losses were 0.5 dB and 3.7 dB, the average is 2.1 dB, and half of that is 1.05 dB. The data sheet value at 1 MHz is 0.95 dB for 15m (50 feet). Given the age of the cable and the expected accuracy of the measurements, that probably is about as close as we can expect. So the loss at 1 MHz in this cable is about 1 dB.

LIMITATIONS: The SWR and/or return loss measurements must be made with respect to the characteristic impedance of the cable. So for a 75 ohm cable, you need either an SWR meter calibrated for 75 ohms, or test equipment that can be set to a 75 ohm reference impedance.

method #3 : load impedance at any resonant point

This method works with cable of any characteristic impedance, but you need to know what it is.

When a open or short-circuited cable is 1/4 wavelength long (electrically), or an integer multiple, then the impedance should be real, that is, the reactance term should be zero. The standing wave on the line goes from high impedance to low impedance (or low to high) in each 1/4 wavelength segment. So when the length is a multiple of 1/4 wavelengths, the high and low current sections should average out. In that case, measuring in a single termination condition (open or short) should give a reasonable estimate of the total cable loss. (This is another case where I find it works best with my analyzer to measure at a low impedance point, but you can try it both ways and see what works best with your equipment.)

With the cable open or shorted at the far end, find a frequency where the reactance is zero. Record the R value. Calculate the SWR on the cable using the actual characteristic impedance of the cable (either measured or nominal). For this purpose, ignore any reactance component of the characteristic impedance. Convert that SWR value into return loss, and divide it by 2. That is the loss in that length of cable.

LIMITATIONS: the reactance of the open- or short-circuited cable at the measurement point must be zero, and the characteristic impedance of the line must be known.

example

In the articles on measuring velocity factor and characteristic impedance, we used a specific 1.75m ( 5.75 foot ) piece of AC power cable. The quarter wave resonant frequency was 27.9 MHz, and the characteristic impedance was 130 – 135 ohms… let’s call it 133 ohms for this calculation. When I measured the impedance of the cable at 27.9 MHz with the far end open, I got 4.3 + j0 ohms. Then the SWR on the cable (calculated with respect to the 133 ohm impedance) is 133 / 4.3 = 30.9 : 1. Plugging this into a return loss calculator gives a return loss of about 0.56 dB. The actual matched line loss of the cable is half that, or 0.28 dB at 27.9 MHz (which is probably close enough for the whole 10m band).

To calculate the value in dB / 30m (or dB / 100 feet), multiple the value by 30m / 1.75m (or 100 feet / 5.75 feet) = 4.8 dB, which is very close to the value for RG-174 coax at that frequency.

method #4 : general formula for any line length and impedance

We can generalize a formula for any line length and characteristic impedance from the preceding methods if we are willing to do some math with complex numbers. Here’s the approach:

Measure the complex impedance (R +/- jX) of a length of feedline with the far end open, and again with it short-circuited.
Calculate the reflection coefficient for each case, relative the the characteristic impedance of the line.
Convert the reflection coefficient to return loss.
Average the two return loss values.
Take half of the result for the loss in that section of line at that frequency.

Steps 2 and 3 require a bit more math than we have had to deal with so far, but it isn’t too difficult. I haven’t found an online calculator for this yet, but it could be put in the form of a spreadsheet if one were going to do it regularly.

The reflection coefficient is a complex number that describes the amplitude and phase of the waveform that is reflected back from a mismatched load. Often the Greek letters rho or gamma are used as symbols, but I don’t have those on this keyboard, so I’ll just call it RC. The magnitude of RC, converted to dB, gives us the return loss.

RC = ( Z_a – Z₀ ) / ( Z_a + Z₀ )

where:

RC is the the reflection coefficient
Z₀ is the characteristic impedance of the transmission line
Z_a is the measured impedance

I have used bold uppercase letters for all of these as a reminder that they are complex numbers, not scalars, so all calculations need to be done following the rules for complex math.

But we only need the magnitude of RC to calculate the reflection coefficient. And if we ignore the reactive component of the feedline impedance in this case, then we can simplify the calculation to:

|RC| = the square root of { [ ( R_a – R₀ )²+ X_a² ] / [ ( R_a + R₀ )² + X_a² ] }

where:

|RC| is the magnitude of the reflection coefficient
R_a is the resistive component of the measured impedance
X_a is the reactance component of the measured impedance
R₀ is the resistive component of the characteristic impedance of the line

Then the return loss RL = -20 log |RC|

example

I measured the impedance of our sample 1.75m ( 5.75 foot ) piece of power cord on 40m and got the following results:

short circuit case: 2 + j55 ohms

open circuit case: 15 – j307 ohms

Let’s take these one at a time, just plugging in the numbers…

For the short circuit case, |RC| = the square root of { [ 17161 + 3025 ] / [ 18225 + 3025 ] } = 0.975

RL = -20 log ( 0.975 ) = 0.22 dB

For the open circuit case, |RC| = the square root of { [ 13925 + 94249 ] / [ 21904 + 94249 ] } = 0.965

RL = -20 log ( 0.965 ) = 0.31 dB

The average value is 0.265 dB, and half of that is 0.13 dB (close enough), the matched line loss of the cable on 40m. That gives a matched line loss of 2.26 dB / 30m ( or 100 feet ).

The nominal value for RG-174 coax cable on 40m is 2.95 dB / 30m, so it is a little better, but still double that of RG-58 at 1.12 dB / 30m. That seems reasonable, given the previous result at 10m, and the fact that the conductors are all copper rather than copper-clad steel. One just has to deal with the 130 ohm impedance.

LIMITATIONS: this method should work for any line of known (or estimated) impedance, and any length. However, the accuracy will degrade for very short lengths, depending on the precision of the equipment used. Consider it to be a useful estimate, rather than a laboratory-grade calculation.