All posts by Martin Blustine

I studied physics and went on to work in infrared optics, millimeter wave and microwave engineering until retirement. My interests lie in teaching, music, radio astronomy, infrared systems and microwave and antenna engineering. I enjoy writing technical papers about ham radio topics. When I am not operating CW, I enjoy homebrewing ham gear and restoring vintage HP and Agilent test and measurement equipment.
Education and Training, Featured, General

Interpreting S-Meter Readings

Introduction

Most communication receivers and transceivers have S-meters, either analog or digital. We also know that there is a 50-ohm coaxial connector on the back of most receivers. What do S-meter readings mean in terms of the 50-ohm receiver input?

High Frequency (HF) < 30 MHz and Very High Frequency (VHF) > 30 MHz receivers work to different input signal level conventions. In other words, and to confuse matters, an S9 for HF is not the same as S9 for VHF.

Nearly 100 years ago, it was decided that S9 should be 50 uV at the receiver input. However, no input impedance was specified. A signal level of S9 was meaningless until the voltage level was standardized to 50 ohms by the International Amateur Radio Union (IARU) some 50 years later. Different voltage levels at the receiver inputs were adopted at that time for HF and VHF.

While S-meter readings are useful for signal reporting and logging, it is important to remember that S-meter readings are not perfectly linear, and linearity differs from receiver to receiver. It may depend a great deal upon receiver settings.

HF Receivers

Suppose that an HF receiver is displaying a signal of S9. We are told that this signal level is defined as a voltage of 50 uV (50 microvolts) at the 50-ohm receiver input connector. This does not tell us what the signal power is incident on the antenna because we do not know what the antenna gain is, what mismatches there are, and what any other gains or losses might be. We only know that a 50 uV signal is present at the receiver input and that the receiver is displaying S9. If we perform a little calculation, we arrive at the power level at the receiver input connector.

To convert this signal power to milliwatts (mW), we divide by 1E-03 or 0.001 since a mW is 1/1000 of a Watt.

There is another way to do this if we know that there are 1E+03 mW in a Watt. We can use dimensional analysis to arrive at the right answer.

We may now convert this value in mW to dBm.

So, a signal of S9 is equivalent to a signal power level of -73 dBm into a 50-ohm input.

Example 1

Bearing this in mind, what would the power level of a signal be for an S-meter reading of S1 in units of dBm?

The signal level at S1 is 8 S-units lower. If each S-unit adds or subtracts 6 dB by convention, a signal of S1 would be 48 dB lower than S9. Subtracting 48 dB, the signal at S1 would be -121 dBm.

Example 2

Suppose we are told that the signal input to the receiver reads S9+10 dB (10 dB over S9). What would the signal into the receiver be in units of uV?

We know that a signal voltage level of S9 is 50 uV into the 50-ohm receiver input. We already know that the signal power level of S9 is -73 dBm. Thus, if we add 10 dB, the signal power level would be -63 dBm (less negative). All that is left is to convert this power level back to uV.

Let’s convert this -63 dBm input signal level to mW. In order to do this, we must take the antilog of the input signal level.

Next, let’s convert mW to Watts by dividing by 1000

Finally, we convert to Volts using the formula

We can convert Volts to uV by multiplying by 1E+06

Since the impedance level for the 50 uV and the 158.3 uV input signals are both 50 ohms, we can check the result to see if it is 10 dB higher than our S9 signal of 50 uV. We notice that the 50-ohm impedance cancels when we take the ratio of the two power levels in

Example 3

What is the 50 uV signal in dBuV?

Receiver specifications are frequently written this way.

VHF Receivers

VHF uses a different standard for S9, notably –93 dBm (5 uV) into a 50-ohm receiver input. A value of 6 dB still represents 1 S-unit. All of the calculations are similar to those for HF receivers.

Example 4

Prove that 5 uV is equivalent to an input signal level of -93 dBm into a 50-ohm VHF receiver input.

Again, there are several ways to proceed. Let’s begin by converting 5 uV to Volts.

We can convert this to power

Convert to mW by multiplying by 1000

We convert to dBm using

Example 5

Convert the 5 uV signal to dBuV

Conclusions

The reference levels for S9 are defined differently for HF and VHF receivers. In this article, it has been shown how one would convert between voltage and power levels at 50-ohm receiver inputs.

When discussing S-units, some receivers are more linear than others, and linearity may depend upon receiver settings. Nonetheless, S-units are useful for signal reporting and logging because everyone agrees on the same standards.

Title Photo Credit: Photo of Ten-Tec Orion S Meter, author: Martin Ewing. Public Domain, https://commons.wikimedia.org/w/index.php?curid=1575140

 

 

 

 

Featured, General, Newsletter, Station Equipment

Astron SS-30M Power Supply Meter LED Retrofit

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Introduction

The Astron SS-30M is a popular metered, switching power supply rated at 30 A. It is commonly used to power 100W radios.

In spite of a ubiquitous changeover to LED illumination, Astron persisted in using incandescent illumination in their power supply meters until a few years ago.

This article discusses the replacement of incandescent bulbs in power supply metering with LED illumination.

Caution. Before proceeding with any disassembly, verify that the line cord has been unplugged from the power supply, or from the wall outlet. There is no isolation transformer in these switching supplies, so it is particularly dangerous. The cover is fastened with Torx screws to prevent tampering.

Procedure

Upon opening an older Astron SS-30M [1] power supply, it was discovered that the meter scale backlighting bulbs could not be replaced without meter disassembly. That got me thinking about whether I could install my own incandescent bulbs or LEDs to replace what was inside the meter housings.

When I checked the Astron website, I found that retrofit circuit boards [2] to backlight these meters were being offered for $3 each. The boards come complete with integral, white, surface-mounted LEDs, integral dropping resistors, and wiring pigtails.  I immediately ordered two of them. This was a lot easier than retrofitting the old circuit boards with new LED bulbs and dropping resistors.

Meter disassembly is shown in Figure 1. The meters may be removed with their wiring harnesses and connectors intact. No desoldering is required to remove the meters from the next higher assembly. It is suggested that one of the harnesses and its mating connector be marked so that identification is easier during reassembly.

The meters are held to the power supply face with four spring clips (shown). These must be removed before the meters can be removed from the power supply face. Once the meters have been removed, the two screws that clamp each of the meters faces to the meter cases are removed (shown). Then, the meter faces may be swung upward to expose the meter movements. Care should be taken during this operation to remove any epoxy, RTV or plastic cement from the seam that holds the front face of the meter to the plastic meter housing. This may be done with a sharp blade. Once removed, each of the meter faces is set aside with its matching meter movement.

Figure 1. Astron SS-30M Power Supply Meter Disassembly. Take care when disassembling the meters so that the meter needles aren’t bent. Pair the meter scales with the correct meter movements. The current meter is to the left and the voltage meter is to the right. If an incorrect meter scale is paired with the wrong meter movement, one of the meter harnesses will not be long enough to reach the correct chassis connector.

The next step is to remove the meter scale. It is fastened to the meter movement with two small screws. I grasped both sides of the meter scale with one hand while removing these screws with a jeweler’s screwdriver to prevent the scale from moving and damaging the meter needle.  Once the meter scale has been removed, the internal meter movement and backlight bulbs are visible. The backlight bulb printed circuit board is held to the meter case with one small screw. This screw is removed with a jeweler’s screwdriver. Again, take care not to damage the meter needle.

Next, the printed circuit board is pulled up and away from the meter movement without touching the needle. Once out of the meter movement, the wires that power the board are snipped close to the old circuit board to leave the pigtails connected to the meter housing. The polarity of the lamp wiring is evident from the wire colors outside the meter case. In my supply, pink is used for the positive lead and black for the negative lead.

After stripping the insulation from the pigtails and tinning them, the new LED circuit boards were installed. A short length of very fine shrink tubing was slipped over each of the pigtails that had been soldered to the new circuit board at the factory. Next, the old pigtails were soldered to the new pigtails by paying close attention to the polarities. The pigtails on the retrofit are red and black.
Once soldered, the shrink tubing is slipped over each splice to prevent any chance of a short circuit between the meter circuit and the lighting circuit. Don’t heat gun the shrink tubing without protecting the meter movement. If the tubing is small enough, it may not be necessary to shrink the tubing at all.

Next, it was time to install the new circuit board in the rear meter housing. Care should be taken to protect the meter needle from the spliced leads when the board is installed with one screw. Once installed, any extra wire may be stuffed underneath the circuit board with a pair of long needle nose pliers.

Finally, the meter face is swung down over the rear meter housing, and the meter face is fastened to the rear meter housing with the original screws.

Once the meters have been reinstalled with the spring clips that hold the meters to the front panel, reconnect the meter wiring harnesses to the correct chassis connectors.

The next step is to fasten the top cover to the power supply with four Torx screws. Once closed up, an AC power cord may be connected and the supply may be powered up. If everything has been wired correctly and the wiring harnesses have been plugged into the correct sockets on the chassis, the meters should be illuminated with bright white light and the voltage meter should read what the supply was set to prior to disassembly. See Figure 2.

Figure 2. Astron SS-30M LED Meter Backlight Retrofit Completed. The OEM incandescent meter bulbs have been replaced with OEM LED bulbs supplied as a replacement part by Astron. It appears as though the left meter is illuminated by different color LEDs. That is not an artifact. When I checked my newer LED illuminated supply meters, the color and brightness were not a perfect match.

References:

[1] Astron Corporation, 9 Autry, Irvine, CA 92618. https://www.astroncorp.com/

[2] Ibid.  https://www.astroncorp.com/product-page/led-backlight-circuit-board

Antennas, Featured, General, Newsletter

Differential and Common Modes on Transmission Lines – Part III

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Introduction

In Part I of this three-part series, differential and common modes on RF transmission lines were defined and discussed.

In Part II of this article, the work of Gustav Guanella was chronicled, followed by Joe Reisert’s improvements to Guanella’s original design. The construction of a common mode choke was presented that included data for the coax used. Finally, some analyses were performed that predicted the performance of two common mode chokes. Graphical results were reported.

In this final Part III, the results of measurements performed on two common mode chokes are presented: one for 2 x FT240-31 stacked ferrite cores and another for 2 x FT240-43 stacked ferrite cores. Due to its higher initial permeability, it was expected that low-frequency choking performance for the 2 x FT240-31 material would be superior to that of the 2 x FT240-43 material. We found that this was not the case for the single sample of 2 x FT240-31.

Discussion

Two coaxial line chokes were constructed to suppress common mode currents on transmission lines. Common mode currents are apt to find their way back to the operating location on the coax shield. Common mode currents can create performance problems in the form of added receiver noise and operator problems in the form of RF bites. The choking impedance should be located at a voltage node in the feedline where the wave impedance of the standing wave is low.

The coaxial line chokes were constructed on stacked cores of FT240-31 and FT240-43 material, as the literature recommends both for EMI suppression. Eleven turns of RG-400/U coax were wound on each of the stacked cores with the Joe Reisert, W1JR, crossover winding located in the center. Each choke was housed in a connectorized Bud Industries PN-1323 box. The common mode rejection for each choke was measured with a spectrum analyzer over a 1.8 to 29.7 MHz bandwidth. The spectrum analyzer tracking generator output was split into two in-phase signals that fed the choke coax center conductor and the choke coax braid in true common mode. The resistive divider formed with two 25.5-ohm resistors (made with 51-ohm resistors in parallel) is shown in Figure 1. The divider was fed in the center by the tracking generator and the ends of the 25.5-ohm resistors fed the center conductor of the coaxial connector and the connector shell. There was a similar arrangement at the output so that the spectrum analyzer could measure the resulting common mode rejection. The conduction path of the test cable shield was carried from input to output on #16 AWG as shown. Figure 2 shows the device under test. Some undesired responses were due to some nearby equipment, coax and line cords. The distance between the test coax shields also presents some challenges, and an interconnection bridge is shown that consists of a rather long piece of #16 AWG wire. After moving some cables, line cords, and equipment around, some useful data was collected. Figure 1 – Feeding the Common Mode Choke in Common Mode. A resistive power divider was constructed that consisted of paralleled 51-ohm resistors to make two 25.5-ohm resistors. The center conductor of the choke coax and the choke coax shield were fed with in-phase signals from the spectrum analyzer tracking generator. Figure 2 – Recombining the Common Mode Signals. Similarly, a resistive power combiner was constructed at the choke output that consisted of paralleled 51-ohm resistors to make two 25.5-ohm resistors. The in-phase signals from the center conductor of the choke coax and the choke coax shield are recombined and fed to the spectrum analyzer input. The shield from the input coax and output coax is bridged with a piece of #16 AWG copper wire as shown.

The reference level of the spectrum analyzer was set for -10 dBm and all measurements were made relative to that level. The first screenshot is for 11 turns of RG-400/U wound on 2 x FT240-31, while the second screenshot is for 11 turns of RG-400/U wound on 2 x FT240-43 material. Since the toroids are wrapped with the same number of turns of RG-400/U, the #31 material, possessing higher initial permeability, is expected to exhibit a higher choking impedance at 1.8 MHz than the 43 material. This does not appear to be the case for this batch of #31 ferrite. The provenance of the #31 material is good. In any event, there is greater than 20 dB of common mode rejection for both ferrite types from 1.8 to 29.7 MHz. While the response appears to favor low frequencies for #31, the overall suppression is better for this batch of #43 material. No loss corrections have been made for the resistive power divider or resistive power combiner. Figure  -. Suppression of a 2 x FT240-31 Line Choke. The ferrite material favors the lower bands but the overall suppression is inferior when compared to that of the 2 x FT240-43 line choke. Deconstruction of the choke may disclose some defects in materials or construction. Only a single choke of this type was constructed. Figure 4 – Suppression of a 2 x FT240-43 Line Choke. The ferrite material favors the higher bands but the overall suppression is superior when compared to that of the 2 x FT240-31 line choke. Fortuitously, several chokes of this type were constructed.

Conclusions

A single choke was constructed with FT240-31 material while several were constructed with FT240-43 material because most of our operation is above 7 MHz. While the shape of the response appears to favor low frequencies for #31, the overall suppression is far greater for this batch of #43 material. These measurements will be repeated when another batch of FT240-31 material is obtained. Furthermore, it is possible that the deconstruction of the FT240-31 choke may disclose some construction or material defect.

 

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