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.

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 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.

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This page stolen from ka7oei.blogspot.com

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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.  

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.

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.

Generating high voltage from a low is one of the aspects that I tackled in a previous project on this blog when I built a high voltage power supply for the Zenith Transoceanic:  You can read about that here - A microcontroller-based A/B Battery replacement for the Zenith TransOceanic H-500 radio, with filament regulation - link.

Figure 4: The voltage boost converter section showing the transistor/inductor, rectification/filtering and voltage divider circuitry.

Description:

 

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 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.

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.

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In a future posting (Part 2) we'll talk about the final design and integration of this project.

This page stolen from ka7oei.blogspot.com

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 Halibut Electronics is working on a new satellite antenna kit!

This is kinda cool for two reasons, first because we've recently started attending the high altitude balloon meetups at Noisebridge. Satellite antennas came up during one of the meetings.

More tactically importanly though, the EggNogs docs inspired what be a better tuna can feedtrough for Projct TouCan's antenna! For notes, here's my original EggNogs documentation review reply:

The documentation looks great so far! I've made it to page 17/22. One thing:
 
 
For those of us with partners heavily into fountain pens, those of us who like to print out manuals on JIS B5 paper and then store them in Kokuyo Campus binders, page numbers in the table of contents would be very cool. (I know, I know, such a niche group :) )
 
Mostly though, I wanted to thank you for jogging my memory into a, (I hope), better solution for Project TouCans antenna ports. At present, they're inverted bananna plug posts. Banana plug screw terminals are tiny, and therefore somewhat problematic in outdoor environments. Here's an idea of how tiny
 

 

 
Suspending a two pound rig 6 meters up, eventually the screw threads begin to strip. We stuck with banana plugs though because the insulator around the conductor makes them perfect for mounting in a tuna fish can. Yes, I've read about how Yagis and dipoles are balanced and therefore have 0 volts across the antenna center, so theoretically you don't need insulators, but TouCans frequently hangs at angles to the ground and/or very close to the ground,  so, insulators.
 

 
Anyhow! The last figure on page 17, the one with the cool weather-proof washer, reminded me that gromets exist! With the correct sized gromet, we can put any size bolt pointing upward as an antenna connector.
 

 
And, even better still, we can source them from our local hardware store at the bottom of the hill!
 
Thanks Mark!
72 de KD0FNR Hamilton

Above — +/- 15 VDC input and ground ports on die cast chassis.

 
Above — Side view showing all the input and output ports.

Above — Schematic of 50 Ω differential bridge assembly. I employed a split DC supply to boost headroom and simplify op-amp biasing.I use the moderate power BD139/140 for the filter transistors: a sturdy part with low flicker noise --  no apologies.

Above — Input ports. Left: DC input (direct with a wire) using an SMA connector. Middle: AC coupled port with RCA jack. Built in 220 µF coupling cap allows testing of 50 ohm input Z audio amplifiers with no worries about the bridge causing a DC disturbance of the biasing or current.
Right: 50 Ω audio signal generator input with a BNC connector.

Above — The output of the instrumentation amp U1 gets buffered by the U2a follower. Low impedance output to use a 50 Ω terminated DSO as the detector.

Above — In analog output direct conversion or superhet receivers that use a diode ring product detector, we often employ a simple post product detector network that some refer to as a diplexer. It's not quite a diplexer, although, it does provide a 50 Ω termination to a narrow band of RF frequencies.
You might sweep this network at AF and RF with return loss bridges to study the input match versus frequency.

Above — My current post product detector network with part values chosen to try and match from 200 Hz to 200 MHz. This proved very difficult with such a simple network because the bandwidth is huge and really this calls for 2-3 networks to get it done. However, in simple receivers, this basic network works OK. The impedance match looks terrible from ~ 1 to 4 MHz, however, trying to fix this worsened the match elsewhere.

I performed the above AF measurements with my old audio return loss bridge built in 2010. It failed recently -- and that failure prompted me to design and build this new AF return loss bridge.

Compromise is a key term in simpler RF design. The network components shown gave me the best overall input Z match from 200 Hz to 200 MHz. This network also provided decent low-pass filtration of the RF lurking in the product detector's audio output. A 220 µF (or higher value) audio coupling capacitor helps keep the input noise down in the AF preamp.

Above — Testing gear used in the video: a 50 Ω Mini Circuits SMA terminator + barrel connector to 50 Ω coax -- and an RCA jack with a 2K potentiometer.