Figure 1: Power/VSWR meter using ИН-13 neon bar-graph indicators. Click on the image for a larger version
In Part 1 I laid out the requirements of the ИН-13-based neon bar-graph VSWR/power meter. Admittedly, this is a "buy cool, old tech and figure out what project might use it" scenario - but having one tube always showing the forward power and the other tube showing either reverse power of calculated VSWR was the goal.
In the previous installment we talked about how to generate the high voltage (130 volts or so) for the bar-graph neons, the means to drive precise amounts of current through the tubes using precision current sink circuits, and the "Tandem" coupler to detect forward and reflected power.
Mounting the tubes
Figure 2: ИН-13 tubes in the raw. It is up to the constructor to determine how best to mount these tubes - and how to connect them to the circuit. Figure 3 shows how flexible wires were attached as the wires on the tubes themselves are very easily broken! Click on the image for a larger version.
In looking at Figure 1 you can see that the ИН-13 tubes are mounted to pieces of clear acrylic, but a quick look at Figure 2 shows that they don't really have a means of mounting, leaving the method to the imagination of the user.
In preparing the tubes for mounting I trimmed the wire leads and soldered flexible wires to them, covering them with "hot melt" (thermoset) adhesive to passivate the connection, making them relatively durable: The original wires will NOT tolerate much flexing at all and are likely to break off right at the glass "pinch" - which would make the tube useless. Figure 3 shows how the leads were encapsulated - the thermoset adhesive being tinted with a permanent marker - mainly to add a bit of color.
Laser-cut sheets and markings
Figure 3: Close-up of the "hot-glue" covered wire attachments for the ИН-13 tubes. Also visible are the black wire loops holding them in place and the laser-edged markings on the acrylic. Click on the image for a larger version.
In looking at Figure 1 and 3 you will also notice that there are scales indicating the function and showing scale graduations and the associated numerical values. I'm fortunate to have a friend (also an amateur radio operator) who has a high-power laser cutter and it was easy to lay out the precise dimensions of the acrylic sheets and also have it cut the holes for the mounting screws in the corners as well.
While it takes a bit of laser power to cut the sheets, a far lower power setting will ablate the surface, yielding a result not unlike surface engraving and when lit from the edges, these ablations will light up with the rest of the sheet remaining pretty dark: A total of four sheets were cut and "engraved" in this way: The front sheet for "VSWR" and its markings, the middle sheet for "Reverse Power" and the rear acrylic sheet for "Forward Power". It was possible to arrange the lettering so that only "VSWR" and "Reverse Power" were atop each other but in subdued light - and with a bit of darkened plastic in front of the display - the markings on the un-lit sheet are practically invisible. The fourth sheet mentioned was left blank, being the protective cover.
Edge lighting
Edge-lit displays go back decades - and the idea likely goes back centuries where it was observed that imperfections in glass (later, plastic) would be visible if the substrate was illuminated from the edge. Since the early-mid 20th century, one could find a number of edge-lit indicators - usually in some sort of test equipment of industrial displays - but they occasionally showed up in the consumer market - usually acrylic or similar with the markings engraved with a rotary tool or - as may be done nowadays, a laser.
While incandescent lamps would have been used in the past, LEDs are the obvious choice these days and for this I selected some "high brightness" LEDs to light the edges of the engraved acrylic sheets. For the "Forward Power" sheet - which would be that which was always illuminated in use - I chose white while using Green for VSWR and Blue for Reverse Power. I'd considered Yellow and Red, but discarded the former as it might appear too much light the white under some conditions and past experience has reminded me that - particularly in a dark room - the human eye can't see or focus on fine detail on red objects very easily.
Figure 4: Six LEDs are epoxied to the edge to evenly light the laser- etched markings in the acrylic sheet. The faces of the LEDs were filed flat to facilitate bonding and improve efficiency. Click on the image for a larger version.
Figure 4 shows some details as to how the edge lighting is accomplished. Six equally-spaced LEDs were epoxied to the bottom edge of the display, arranged to be nearly the width of the engraved text. In writing this entry I observed that photographing edge-lit displays such as this is nearly impossible owing to the variations in illumination (e.g. it's difficult to take pictures of very bright objects in the dark!) but the effect is very even as viewed by the human eye.
The six LEDs were connected as two series strings of three LEDs: As each LED requires about three volts - and I have only a 12 volt power source - doing so requires only a bit more than nine volts to power the LED arrays. As the green and white LEDs are also silicon nitride based as well, they take similar voltages.
Not readily apparent from Figure 4 is the fact that the LEDs were modified slightly. As we are trying to interface a standard T1-3/4 LED to the flat edge of a plastic sheet, it's apparent that the rounded, focused lens makes this physically difficult. To mitigate this, the top of the LED was flattened with a file and the clear epoxy was removed to just above the light emitting die. The result of this is that a flat surface is mated to another flat surface for a physically stronger bond and a more efficient coupling of light and a bit of the LED's original directivity in the form of the "lens" is removed from the equation.
Just prior to mounting the acrylic sheets in the "stack up" some black electrical tape was applied. This tape was put on both sides of the sheet, extending just above the bottom edge, to reduce the glare from the LEDs and to minimize the possibility of this light coupling into the adjacent sheet.
Mounting the tubes and sheets
As can be seen from Figure 3, the tubes are held in place with loop of solid-core insulated wire - the holes mounting them also "drilled" with the laser. The "stack-up" of acrylic sheets and the tubes - both of which were mounted on "VSWR" acrylic layer - is held together using 6-32 brass machine screws and spacers with a piece of 1/4" (5.2mm) plywood covered with black felt for the back to provide contrast.
The box and base
As can be seen from figure 1, the entire unit is in a wooden base: The same friend with the laser cutter also had some scraps of red oak and a simple base was made, decorated with an ogee cut around the perimeter with the router while atop it a simple box with mitered corners - facing at a slight upward angle - in which the display and electronics reside. On the base itself are two buttons: One switches between VSWR and Reverse Power and the other between peak and average readings. These switches have other functions as well, which will be discussed in the third installment when the final circuit and internal workings of the software is discussed.
Figure 1: Power/VSWR meter using ИН-13 (a.k.a. "IN-13") neon bar-graph indicators. Click on the image for a larger version.
Several years ago I bought some Soviet-era neon bar-graph displays - mainly because I thought that they looked cool, but I didn't have any
ideas for a specific project.
After mulling over possible uses for
these things for a year or so - trying to think of something other than
the usual audio VU meter or thermometer - I decided to construct a
visual watt/VSWR indicator for amateur radio HF use.
* * *
I actually bought two different types of these bar-graph tubes:
The ИН-9 (a.k.a. "IN-9"). This tube is 5.5" (140mm) long and 0.39" (10mm) diameter. It has two leads and the segments light up sequentially - starting from the end with the wires - as the current increases.
The ИН-13 (a.k.a "IN-13"). This neon bar-graph tube is about 6.3" (160mm) long and 0.39" (10mm) diameter. Like the ИН-9 its segments light up sequentially with increasing current but it has a third lead - the "auxiliary cathode" - that is tied to the negative supply lead via a 220k resistor that provides a "sustain" current to make it work more reliably at lower currents.
Note: It would be improper to refer to these as "Nixies" as that term refers to a specific type of numeric display - which these are not. Despite this, the term is often applied - likely for "marketing" purposes to get more hits on search engines.
Figure 2: A pair of ИН-13 neon indicator tubes. These tubes are slightly longer than than the ИН-9 tubes and have three leads Click on the image for a larger version.
For a device that is intended to indicate specific measurements, it's important that it is consistent, and for these neon indicators, that means that we want the bar graph to "deflect" the same amount anytime the same amount of current is applied to it. In perusing the specifications of both the ИН-9 and ИН-13 it appeared that the ИН-13 would be more suitable for our purposes.
This project would require two tubes:
Forward power indicator. This would always indicate the forward RF power as that was that's something that is useful to know at any time during transmitting.
Reverse power/VSWR.
This second tube would switchable between reverse power, using the same
scale as the forward power display, and VSWR - a measurement of the
ratio between forward and reverse power and a useful indicator of the
state of the match to the antenna/feedline.
Driving the tubes
"Because physics", gas discharge tubes require quite a bit of voltage to "strike" (e.g. light up) and these particular tubes need for their operation about 140 volts - a "modestly high" voltage at low current - only a few milliamps (less than 5) per tube, peak.
Figure 3:
Test circuit to determine the suitability of various inductors and transistors
and to determine reasonable drive frequencies. Diode "D" is a high-speed,
high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and
"Q" is a power FET with suitably high voltage ratings (>=200 Volts)
and a gate turn-on threshold in the 2-3 volt range so that it is suitable
to be driven by 5 volt logic. V+ is from a DC power supply that is
variable from at least 5 volts to 10 volts. The square wave drive, from a
function generator, was set to output a 0-5 volt waveform to
make certain that the chosen FET could be properly driven by a 5 volt
logic-level signal from the PIC as evidenced by it not getting perceptibly
warm during operation.
The method used for this project and the aforementioned Zenith radio is boost-type converter as depicted in Figure 3.
The switching frequency must be pretty high - typically in the 5-30 kHz range if one
wishes to
keep the inductance and physical size of that inductor reasonably
small.
As in the case of the Zenith Transoceanic project, I used the PWM output of the microcontroller - a PIC - to drive the voltage converter with a frequency in the range of 20-50 kHz. For our needs - generating about 140 volts at, say, 15 milliamps maximum, I knew (from experience) that a 220uH choke would be appropriate. Figure 4, below, shows the as-built boost circuit.
Figure 4: The voltage boost converter section showing the transistor/inductor, rectification/filtering and voltage divider circuitry.
Description:
Q301 is a high-voltage (>=200 volt) N-channel MOSFET - this one being pulled from a junked PC power supply (the particular device isn't critical) which is driven by a square wave on the "HV_PWM" line from the microcontroller: R301, the 10k resistor, keeps the transistor in the "off" state when the controller isn't actively driving it (e.g. start-up). L301, a 220uH inductor, provides the conversion: When Q301 is on, the bottom end is shorted to ground causing a magnetic field to build up and when Q301 is turned off, this field collapses, dumping the resulting voltage through D301, which is a "fast" high voltage diode designed for switching supplies - a 1N4000 series diode would not be a good choice in this application as it's quite "slow".
R304, a 33k resistor, is used to provide a minimum load of the power supply, pulling about 4.25 mA at 140 volts: This "ballast" improves the ability of the supply to be regulated as the difference between "no load" (the neon bar-graphs energized, but with no "deflection") and full load (all segments of the tubes illuminated) is less than 4:1. The resistive divider of R302 and R303 is used to provide a sample of the output voltage to the microcontroller, yielding about 2.93 volts when the output is at 140 volts. The reader will, by now, likely have realized that I could have used R304 as part of the voltage divider - but since the value of this resistor was determined duringtesting, I didn't bother removing R302/R303 when I was done: Anyway, resistors are cheap!
Setting the current:
Having the 140 volt supply is only the first part of the challenge: As these tubes use current to set the "deflection" (e.g. number of segments) we need to be able to precisely set this parameter - independent of the voltage - to indicate a value with any reasonable accuracy. For this we'll use a "current sink".
Figure 5: The precision current sinks that drive the neon tubes precisely based on PWM-derived voltage. Click on the image for a larger version.
Figure 5, above, shows the driving circuits for the two tubes using the "precision current sink". Taking the top diagram as our example, we see that the inverting input of the op-amp (U401c) is connected to the junction of the emitter of Q401 and resistor R406. As is the wont of an op amp, the output will be driven high or low as needed to try to make the voltage (from the microcontroller) at pin 10 match that of pin 9 - in this case, based on feedback from the sense resistor, R406.
What this means is that as the transistor (Q401) is turned on, current will flow from the tube, through it and into R406 meaning that the voltage across R406 is proportional to the voltage on pin 10. It should be noted that current through R406 will include the current into the base - but this can be ignored as it will be only a tiny fraction (a few percent at most)of the total current. It's worth noting that this circuit is insensitive to the voltage - at least as long as such current can be sunk - making it ideal for driving a device like the ИН-13 (or ИН-9) in which its intended operation is dependent on the current rather than the operating voltage.
At this point it's worth noting that the driving voltages from the microcontroller ("FWD_PWM" and "REV_PWM") are not plain DC voltages, but rather from the 10 bit PWM outputs of the microcontroller. The use of a 10k resistor and 100nF (0.1uF) capacitors (R405 and C406, respectively) "smooth" the square-ish wave PWM into DC.
Q401 and Q402 were, again, random transistors that I found in scrapped power supplies, but since there's at least 70 volts drop across the tube, about any NPN transistor rated to withstand at least 80 volts should suffice. It's also worth noting the presence of R407, which provides the "sustain" current on the "auxiliary" cathode.
Figure 6: An exterior view of the tandem coupler module. Visible is the top shield and the three feedthrough capacitors used to pass voltage and block RF. Click on the image for a larger version.
RF sensing
For
sensing forward and reflected power I decided to use an external
"sensing head" that was connected inline with the radio, on the "tuner"
side of the feedline.
For sensing power in both directions I chose the
so-called "Tandem" coupler which consists of a through-line sampler in
which a short length of coaxial cable carrying the transmit power (T1 in the diagram of Figure 7) passes through a toroidal core -
using some of the original cable's braid grounded at just one end as a
Faraday shield. An identical transformer (T2) is connected across the first (T1) for symmetry.
When carefully
constructed this arrangement has quite good intrinsic directivity and a
wide frequency range. Figure 6 shows the diagram of this section.
Figure 7: Schematic diagram of the "Tadem" coupler. A bidirectional coupler sends power to separate AD8307 logarithmic amplifiers - one for forward and the other for reverse. The outputs, expressed in "volts/dB" are sent to the microcontroller. Click on the image for a larger version.
The
RF sensing outputs of the second tandem coupler (T2) then goes through resistive
voltage dividers (R606/R607 for the reverse sample and R603/604 for the forward sample) to a pair of Analog Devices AD8307 logarithmic
amplifiers - one for forward power and the other for reverse - to
provide a DC voltage that is logarithmically proportional to the
detected RF power. This voltage is then coupled through series resistors (for both RF and DC protection) R605/R608 and to the outside world using feedthrough capacitors.
The use of a logarithmic amplifier precludes the
need to have range switching on power meter as RF energy from well below
a watt to well over 2000 watts can be represented with only a few volts
swing. Looking carefully at Figure 6 one can see a label that notes that the response of the AD8307 is about 25 millivolts per dB - and this applies across the entire power range of a few hundred milliwatts to 2000 watts.
All
of this circuitry is mounted in a box constructed of circuit board
material and connected to the display unit with an umbilical cable that
conveys power and ground along with the voltages that indicates forward
and reflected power.
Figure 8: An inside view of the Tandem Match (sense unit) showing the coupling lines, internal shielding and AD8307 boards. Click on the image for a larger version.
Figure 8 shows the as-built "sense unit" and the two coaxial sense lines are clearly visible. As can be seen, the "main line" coupler is physically separated and shielded from the secondary sense line, using PTFE ("Teflon") feedthrough lines to pass the signals.
The AD8307 detectors themselves can be seen at the left and right edges of the lower half of the unit, built on small pieces of perfboard. All signals - including the 12 volt power and the DC voltages of the output pass through 4000pF feedthrough capacitors to prevent both ingress and egress of RF energy which could find its way into the '8307 detectors and skew readings.
* * * * *
In a future posting (Part 2) we'll talk about the final design and integration of this project.
Last year, I was "car camping" with a bunch of friends - all of which happened to be amateur radio operators. Being in the middle of nowhere where mobile phone coverage was not even available, we couldn't resist putting together a "portable" 100 watt HF station. While the usual antenna tuner+VSWR meter would work fine, I decided to build a different piece of equipment that would facilitate matching the antenna and protecting the radio - but more on this in a moment.
A bit about the Wheatstone bridge:
The Wheatsone bridge is one of the oldest-known types of electrical circuits, first having been originated around 1833 - but popularized about a decade later by Mr. Wheatstone itself. Used for detecting electrical balance between the halves of the circuit, it is useful for indirectly measuring all three components represented by Ohm's law - resistance, current and voltage.
Figure 2: Wheatstone bridge (Wikipedia)
It makes sense, then, that an adaptation of this circuit - its use popularized by Dan Tayloe (N7VE) - can be used for detecting when an antenna is matched to its load. To be fair, this circuit has been used many decades for RF measurement in instrumentation - and variations of it are represented in telephony - but some of its properties that are not directly related to its use for measurement that make it doubly useful - more on that shortly.
Figure 2 shows the classic implementation of a Wheatstone bridge. In this circuit, balance of the two legs (R1/R2 and R3/Rx) results in zero voltage across the center, represented by "Vg" which can only occur when the ratio between R1 and R2 is the same as the ratio between R3 and Rx. For operation, that actual values of these resistors is not particularly important as long as the ratios are preserved.
If you think of this is a pair of voltage dividers (R1/R2 and R3/Rx) its operation makes sense - particularly if you consider the simplest case where all four values are equal. In this case, the voltage between the negative lead (point "C") and point "D" and points "C" and "B" will be half that of the battery voltage - which means the voltage between points "D" and "B" will be zero since they must be at the same voltage.
Putting it in an RF circuit:
Useful at DC, there's no reason why it couldn't be used at AC - or RF - as well. What, for example, would happen if we made R1, R2, and R3 the same value (let's say, 50 ohms), instead of using a battery, substituted a transmitter - and for the "unknown" value (Rx) connected our antenna?
Figure 3: The bridge, used in an antenna circuit.
This describes a typical RF bridge - known when placed between the transmitter and antenna as the "Tayloe" bridge, the simplified diagram of which being represented in Figure 3.
Clearly, if we used, as a stand-in for our antenna, a 50 ohm load, the RF Sensor will detect nothing at all as the bridge would be balanced, so it would make sense that a perfectly-matched 50 ohm antenna would be indistinguishable from a 50 ohm load. If the "antenna" were open or shorted, voltage would appear across the RF sensor and be detected - so you would be correct in presuming that this circuit could be used to tell when the antenna itself is matched. Further extending this idea, if your "Unknown antenna" were to include an antenna tuner, looking for the output of the RF sensor to go to zero would indicate that the antenna itself was properly matched.
At this point it's worth noting that this simple circuit cannot directly indicate the magnitude of mismatch (e.g. VSWR - but it can tell you when the antenna is matched: It is possible to do this with additional circuitry (as is done with many antenna analyzers) but for this simplest case, all we really care about is finding when our antenna is matched. (A somewhat similar circuit to that depicted in Figure 3 has been at the heart of many antenna analyzers for decades.)
Antenna match indication and radio protection:
An examination of the circuit of Figure 3 also reveals another interesting property of this circuit used in this manner: The transmitter itself can never see an infinite VSWR. For example, if the antenna is very low resistance, we will present about 33 ohms to the transmitter (e.g. the two 50 ohm resistors on the left side will be in parallel with the 50 ohm resistor on the right side) - which represents a VSWR of about 1.5:1. If you were to forget to connect an antenna at all, we end up with only the two resistors on the left being in series (100 ohms) so our worst-case VSWR would, in theory, be 2:1.
In context, any modern, well-designed transmitter will be able to tolerate even a 2.5:1 VSWR (probably higher) so this means that no matter what happens on the "antenna" side, the rig will never see a really high VSWR.
If modern rigs are supposed to have built-in VSWR protection, why does this matter?
One of the first places that the implementation of the "Tayloe" bridge was popularized was in the QRP (low power) community where transmitters have traditionally been very simple and lightweight - but that also means that they may lack any sophisticated protection circuit. Building a simple circuit like this into a small antenna tuner handily solves three problems: Tuning the antenna, being able to tell when the antenna is matched, and protecting the transmitter from high VSWR during the tuning process.
Even in a more modern radio with SWR protection there is good reason to do this. While one is supposed to turn down the transmitter's power when tuning an antenna, if you have an external, wide-range tuner and are quickly setting things up in the field, it would be easy to forget to do so. The way that most modern transmitter's SWR protection circuits work is by detecting the reflected power, and when it exceeds a certain value, it reduced the output power - but this measurement is not instantaneous: By the time you detect excess reflected power, the transmitter has already been exposed - if only for a fraction of a second - to a high VSWR, and it may be that that brief instant was enough to damage an output transistor.
In the "old" days of manual antenna tuners with variable capacitors and roller inductors, this may have not been as big a deal: In this case, the VSWR seen by the transmitter might not be able to change too quickly (assuming that the inductor and capacitors didn't have intermittent connections) but consider a modern, automatic antenna tuner full of relays: Each time the internal tuner configuration is changed to determine the match, these "hot-switched" relays will inevitably "glitch" the VSWR seen by the radio, and with modern tuners, this can occur many times a second - far faster than the internal VSWR protection can occur meaning that it can go from being low, with the transmitter at high power, to suddenly high VSWR before the power can be reduced, something that is potentially damaging to a radio's final amplifier.
While this may seem to be an unlikely situation, it's one that I have personally experienced in a moment of carelessness - and it put an abrupt end to the remote operation using that radio - but fortunately, another rig was at hand.
A high-power Tayloe bridge:
It can be argued that these days, the world is lousy with Tayloe bridges as they are seemingly found everywhere - particularly in the QRP world, but there are fewer of them that are intended to be used with a typical 100 watt mobile radio - but one such example may be seen below:
Figure 4: As-built high-power Tayloe bridge with a more sensible
bypass switch arrangement! This diagram was updated to include a
second LED to visually indicate extreme mismatches and provide another
clue as to when one is approaching a match.
Figure 4 shows a variation of the circuit in Figure 2, but it includes two other features: An RF detector, in the form of an LED (with RF rectifier) and a "bypass" switch, so that it would not need to be manually removed from the coax cable connection from the radio.
In this case, the 50 ohm resistors are thick-film, 50 watt units (about $3 each) which means that between the three of them, they are capable of handling the full power of the radio for at least a brief period. Suitable resistors may be found at the usual suppliers (Digi-Key, Mouser Electronics) and the devices that I used were Johanson P/N RHXH2Q050R0F4 (A link to the Mouser Electronics page is here) - but there is nothing special about these particular devices: Any 50-100 watt, TO-220 package, 50 ohm thick-film resistor with a tolerance of 5% or better could have been used, provided that its tab is insulated from the internal resistor itself (most are).
How it works:
Knowing the general theory behind the Wheatstone bridge, the main point of interest is the indicator, which is, in this case, an LED circuit placed across the middle of the bridge in lieu of the meter shown in Figure 1. Because RF is present across these two points - and because neither side of this indicator is ground-referenced, this circuit must "float" with respect to ground.
If we presume that there will be 25 volts across the circuit - which would be in the ballpark of 25 watts into a 2:1 VSWR - we see that the current through 2k could not exceed 25 mA - a reasonable current to light an LED. To rectify it, a 1N4148 diode - which is both cheap and suitably fast to rectify RF (a garden-variety 1N4000 series diodes is not recommended) along with a capacitor across the LED. An extra 2k LED is present to reduce the magnitude of the reverse voltage across the diode: Probably not necessary, bit I used it, anyway. QRP versions of this circuit often include a transformer to step up the low RF voltage to a level that is high enough to reliably drive the LED, but with 5-10 watts, minimum, this is simply not an issue.
Because the voltage across the bridge goes to zero when the source and load impedance are matched (or the switch is set to "bypass" mode) there is no need to switch the detector out of circuit but note that the LED and associated components are "hot" at RF when in "Measure" position which means that you should keep the leads for this circuit quite short and avoid the temptation to run long wires from one end of a large enclosure (like an antenna tuner) to the other as excess stray reactance can affect the operation of the circuit.
Note: See the end of this article for an updated/modified version with a second LED .
A more sensible bypass switch configuration:
While there are many examples of this sort of circuit - all of them with DPDT switches to bypass the circuit - every one that I saw wired the switch in such a way that if one were to be inadvertently transmitting while the switch was operated, there would be a brief instant when the transmitter was disconnected(presuming that the switch itself is a typical "break-before-make" - and almost all of them are!) that could expose the transmitter to a brief high VSWR transient. In Figure 3, this switch is wired differently:
When in "Bypass" mode, the "top" 50 ohm resistor is shorted out and the "ground" side of the circuit is lifted.
When in "Measure" mode, the switch across the "top" 50 ohm resistor is un-bridged and the bottom side of the circuit is grounded.
Figure 5: Inside the bridge, before the 2nd LED was added
Wired this way, there is no possible configuration during the operation of the switch where the transmitter will be exposed to an extraordinarily high VSWR - except, of course, if the antenna itself is has an extreme mismatch - which would happen no matter what if you were to switch to "bypass" mode.
An as-built example:
I built my circuit into a small die-cast aluminum box as shown in Figure 5. Inside the box, the 50 ohm resistors are bolted to the box itself using countersunk screws and heat-sink paste for thermal transfer. To accommodate the small size of the box, single-hole UHF connectors were used and the circuit itself was point-to-point wired within the box.
For the "bypass" switch (see Figure 6) I rescued a 120/240 volt DPDT switch from an old PC power supply, choosing it because it has a flat profile with a recessed handle with a slot: By filing a bevel around the square hole (which, itself was produced using the "drill-then-file" method) one may use a fingernail to switch the position. I chose the "flush handle" type of switch to reduce the probability of it accidentally being switched, but also to prevent the switch itself from being broken when it inevitably ends at the bottom of a box of other gear.
Figure 6: The "switch" side of the bridge.
On the other side of the box (Figure 7) the LED is nearly flush-mounted, secured initially with cyanoacrylate (e.g. "Super") glue - but later bolstered with some epoxy on the inside of the box.
It's worth noting that even though the resistors are rated for 50 watts, it's unlikely that even this much power will be output by the radio will approach that in the worst-case condition - but even if it does, the circuit is perfectly capable of handling 100 watts for a few seconds. The die-cast box itself, being quite small, has rather limited power dissipation on its own (10-15 watts continuous, at most) but it is perfectly capable of withstanding an "oops" or two if one forgets to turn down the power when tuning and dumps full power into it. It will, of course, not withstand 100 watts for very long - but you'll probably smell it before anything is too-badly damaged!
Operation:
As on might posit from the description, the operation of this bridge is as follows:
Place this device between the radio and the external tuner.
Turn the power of the radio down to 10-15 watts and select FM mode. You may also use AM as that should be limited to 20-25 watts of carrier when no audio is present.
Disable the radio's built-in tuner, if it has one.
If using a manual tuner, do an initial "rough" tuning to peak the receive noise, if possible.
Switch the unit to "Bridge" (e.g. "Measure") mode.
Key the transmitter.
If you are using an automatic tuner, start its auto-tune cycle. There should be enough power coming through the bridge for it to operate (most will work reliably down to at about 5 watts - which means that you'll need the 10-15 watts from the radio for this.)
If you are using a manual tuner, look at both its SWR meter (if it has one) and the LED brightness and adjust for minimum brightness/reflected power. A perfect match will result in the LED being completely extinguished.
After tuning is complete, switch to "Bypass" mode and commence normal operation.
* * *
Modification/enhancement
More recently (July, 2023) I made a slight modification to this bridge by adding a second
LED driven by the opposite swing of the RF waveform so that it would
not have any effect on the first - this LED designed to illuminate only under highly-mismatched conditions at higher power levels.
Figure 7: The "enhanced" version with TWO LEDs.
As seen in the Figure 7 (above) the "original" LED is now designated as being yellow (the different color allowing easy differentiation) - but the second LED - which indicates a worse condition - is
red and placed with a series 6.8 volt Zener diode (I used a 1N754A). The idea here is that if the VSWR is REALLY
bad and the power is high enough, BOTH LEDs will illuminate - but the
"new" (red) LED will go out first as you get "close-ish" to the match.
Figure 8: It has two LEDs now!
In testing with an open or short on the output and in "measure" mode the
red LED illuminated only above about 15 watts, so this second LED isn't
really too helpful for QRP unless the value of the 2k, 1 watt resistor
is reduced. Again, this isn't really to indicate the SWR, but having
this second, less-sensitive LED helps with the situation when using a
manual tuner in which the match is so bad that it's difficult to spot
subtle variations in the brightness of +the more sensitive (yellow) LED -
particularly at higher power levels.
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