500 Watt Antenna Tuner Part 9 - Pre Build Schematics and Board Layout

 I am getting ready to order parts (again) and build the tuner.  To prepare for it, I have updated the blog (again).  But I am still in the middle of building the new EME 5m dish project at the DVRA shack.  This might take time.

I have switched from the KiCad 6.0 circuit CAD tools to KiCad 8.0.  A definite improvement.  The tuner schematic is a total of 3 pages.  They are the RF path, the directional coupler and the main page which is mostly the microprocessor circuit.  I will start with the RF path page.

The inductor and capacitor networks, from the previous discussions, should be obvious.  Relays K3, K6, K7, K9, K11, K13, K15 and K17 control taking the inductors in and out of the circuit by shorting them when necessary.  Relays K4, K5, K6, K10, K12, K14, and K16 short the capacitors to the ground by putting them in the circuit.  K2 and K18 put the capacitor network in parallel with the load or with the source depending on the location of the load in region 1 or region 2 of the Smith Chart.  Relays K1 and K19 (2 Form C) either bypass the LC network or put in in the RF path. 

Relay Driver

The relay driver is a Toshiba TBD62783A which is a "source type DMOS transistor array".  Normally, one side of the relay coil is connected to the power supply and the other side is grounded using some type of active device.  A flyback diode is incorporated to protect the device.  This device uses a P-Channel MOSFET to connect one side of the relay to +13.8 volts (I am using the general shack power supply through Anderson Power Poles to power the tuner) while the other side is permanently grounded.  It also incorporates the fly back diode inside the device.  Its input is compatible with 3 volt logic.  As you can see from the schematic, it drastically simplifies the board layout compared to the alternatives.

If you note, the routing of signals to the relay drivers and from the relay drivers to the relays is a bit random.  It is specifically chosen to simplify the board layout.  For the same reason, there are two bypass signals from the microcontroller and the signal to put the capacitors across the load or the source, even though they are inverse of each other, are separately sourced.  

Relays:

I do not plan to open or close the relays with the power applied.  So, the two parameters that matter for my design are the open contact breakdown voltage and close contact current carrying capacity.

First, I will review some items about how the LC network:

1. The inductors and inductor relays form a series circuit.  The "inductor current" is either carried by the inductor when the relay is open or by the relay when the relay is closed.
2. If the inductor relay is closed, the voltage across it is zero and if it is open the voltage across it will be the same as inductor voltage
3. The capacitors and capacitor relays form a series circuit.  The "capacitor voltage" is either the voltage across the capacitor when the relay is closed or across the relay when the relay is open.
4. If the capacitor relay is closed, the current through it is the capacitor current and if the relay is open, it is zero.

There are two factors that go into the selection of relays:

• When the relay is open, the dielectric strength between the contacts (1 Form A and 1 Form C) should be good enough not to break down at voice peaks at 800 watts peak power.
• When the relay is closed, the rated carry current should be sufficient to support the current at 200 watts average power.

The Omron G2RL dielectric strength specification is 1,000 VAC (I assume RMS) at 50/60 Hz.  K6JCA quotes a reference in his blog recommending a safety factor of 0.8 for RF applications.  This sets the limit at 800 volts.  From the Maximum Voltage and Currents table in section 4, we see this requirement is met at 800 watts of short duration peak output (voice peaks in side band transmission).

These relays also have a carry current specification of 12 Amps for single pole standard type relays.  Again, from the aforementioned table, this requirement is also met at 200 watts of average power.

To minimize the board stray inductance, the relays are located on the back side of the board.  My current estimate of the path inductance when the inductors are all shorted out is 25 nH.  We will see.

RF Path

Next, I will review the directional coupler page.  From the directional coupler discussion page, the circuits on this page should be quite familiar to the reader.  The only additional comments that I make are that during testing, I found that R24 and R27 are redundant.  That is not how I read the AD8302 data sheet.  I will have to further investigate this and add it to the blog.  Also during the prototype testing, I ended up adding a 15 tap FIR low pass digital filter when reading the magnitude and phase voltages from the AD8302.  I will have to look further into this and replace it with a biquad digital filter.

Directional Coupler

Finally, below is main page.  

Main Page

The two blocks in the upper left and right are the abstraction of the two pages that I have already discussed.  The block in the middle is the Raspberry Pi Nano.  

The two connectors J5 and J6 connect to the directional coupler as you can see in the prototype picture below.  The two inductors on the right are the voltage (white) and current (blue) sense transformers.  The two cables on the right (the blue side of the directional coupler) are the main RF signals.  The two SMA cables on the left (white) side are the forward and the reflected signal cables.  This is a prototype of magnitude and phase sensor.

As of now, I have decided not to incorporate a power supply into the package and power the tuner of the Power Pole distribution.  I am feeding the Power Pole with a 20 Amp Astron linear supply that I have owned since the early 90s.  Worst case, I will upgrade the power supply.  The Power Pole can handle up to 40 amps.  The blue area is the ground plane on the back of the board.  The 3 mm track on top of the page is a 50 ohm strip line for the bypass circuit.  There are high voltages on the board as discussed in the capacitor and inductor section.  For voltages up to 500 volt on the outside tracks that are polymer coated, the recommended clearance is 0.8 mm.  There is a gap of 1 mm between all high voltage circuits and high and low voltage circuits.

And here is the board layout as it stands now:

PWB Layout

500 Watt Antenna Tuner Part 1 - Introduction

500 Watt Antenna Tuner Part 2 - Analytical Model

500 Watt Antenna Tuner Part 3 - Simulator

500 Watt Antenna Tuner Part 4 - Capacitors and Inductors

500 Watt Antenna Tuner Part 4A Capacitors

500 Watt Antenna Tuner Part 4B Inductors

500 Watt Antenna Tuner Part 5 - Directional Coupler

500 Watt Antenna Tuner Part 6 - Matching Algorithm

500 Watt Antenna Tuner Part 7 - User Interface

500 Watt Antenna Tuner Part 8 - Software and Microcontroller

500 Watt Antenna Tuner Part 9 - Pre Build Schematics and Board Layout

 

500 Watt Antenna Tuner Part 8 Software and Microcontroller

 Controller Hardware and Programming Language Selection

This is a new iteration of this page.  I have kept the old page and included it at the end of this blog post series for anyone interested.

With the recommendation of a friend, Mike Rauch K2VPX, I started looking at the Raspberry Pi Pico.  I built a test frequency generator and the frequency measurement function for this tuner.  It worked well and I really liked what I saw.  The documentation is superb.  I also chose its native C programming enviroment for no other purpose than to keep things as simple as possible.

• 2 pins for the bypass relay (to simplify the board layout)
• 2 pin for the capacitor relay (across load - Region 1 or across source Region 2 see below)
• 8 pins for the capacitor network
• 8 pins for the inductor network
• 2 analog pins for magnitude and phase of the reflection coefficient
• 1 analog pin for power level input 
• 1 pin for frequency input
• 1 pin for gating the frequency measurement (so I am not shipping a high frequncy digital signal all over the printed wiring board)
• 1 Spare pin

Not much to spare, but it will do.  The 12 bit A/D convertors and fast floating point arithmetic are both pluses.   But the true winners are the documentation and the SDK.

Design Concepts

Most aspects of the design are fairly straight forward and I will discuss them second.  But the real time display of SWR and output power need some additional thought and research.  I spent a day and half looking for a good solution with little luck.  I finally decided to go to a library that I had used before, Plotly.  I spent a lot of time reading through API documentation, but eventually, I figured it out.  I will embellish this display, but unless I can think of something better, this is what I am going to go with.

The Stationmaster software will be interfaced with the Pico over the USB interface.  The commands from the Stationmaster to tuner are formatted as follows:

• Address (one byte)
• Command (one byte)
• Data bytes as needed (zero or more bytes)
• Terminating semicolon ";" (one byte)

Other than the address byte, this is the same format as the Yaesu radio commands.

The replies to the commands will be formatted as follows:

• Address (one byte) 
• Command (one byte) - the same as the command recieved
• Data bytes as needed (zero or more bytes) 
• Terminating semicolon ";" (one byte)

There will be two sets of commands.  One set for normal operation and another set for testing of the tuner.

Operating Commands:

• Test tuner (to see if the device at this USB port is the tuner) - returns Address, Command, ;
• Bypass - returns Address, Command, (decrease power or done - one byte), ;
• Measure power - returns Address, Command, two bytes of data,;
• Tune - returns Address, Command, (decrease power, increase power, or done), ;

Testing Commands:

• Read phase - returns Address, Command, two bytes of data, ;
• Read magnitude - returns Address, Command, two bytes of data, ;
• Read frequency - returns Address, Command, four bytes of data, ;

I add more documentation to the code and link the Github page.

500 Watt Antenna Tuner Part 1 - Introduction

500 Watt Antenna Tuner Part 2 - Analytical Model

500 Watt Antenna Tuner Part 3 - Simulator

500 Watt Antenna Tuner Part 4 - Capacitors and Inductors

500 Watt Antenna Tuner Part 4A Capacitors

500 Watt Antenna Tuner Part 4B Inductors

500 Watt Antenna Tuner Part 5 - Directional Coupler

500 Watt Antenna Tuner Part 6 - Matching Algorithm

500 Watt Antenna Tuner Part 7 - User Interface

500 Watt Antenna Tuner Part 8 - Software and Microcontroller

500 Watt Antenna Tuner Part 9 - Pre Build Schematics and Board Layout

500 Watt Antenna Tuner Part 4B Inductor Design

 One of the design challenges that Jeff, K6JCA outlines in his blog is stray inductance of the connecting wires or PCB tracks.  He used hand wiring in his construction, so I thought I might do better with a PCB layout.  I went through a design cycle with six air wound inductors and two powdered iron core inductors and did a few design iterations of the PCB with KiCad.  I did not build the board but extracted the board geometry and estimated the path inductance.  My results were the same as Jeff's.  

I decided to build all my inductors with toroid cores.  This provides me the advantage of packing them closer to each other and help keep the connecting tracks shorter and hence reduce the path inductance.  This is the approach that some of the commercial high power antenna tuners take.

Also, I noticed that some of the commercial high power antenna tuners placed the capacitor and inductor relays on the back side of the board.  This approach tends to further reduce the path inductance without increasing the PCB cost since I am using through hole relays and calling for solder mask on both sides of the board.  Preliminary estimate puts the track inductance at 25 nH.  I will explore if I can leave off the 25 nH inductor and when the tuning calls for it, just rely on the track inductance. 

For high power inductors, the recommended material to use is powdered iron cores (not ferrite), so that is what I will use.

The Mini Ring Calculator for Mac is now available, but I am familiar with the Micrometals on line tool, so, that is what I will use.  I will assume an internal enclosure temperature of 50 degrees C and maximum core temperature increase of 50 degrees C.  That meets the maximum toroid core temperature of 100 degrees C which is fine for intermittent use.  The other factor that I need is the maximum RMS current through the inductor at each frequency.

My simulator (described earlier), calculates maximum current through the inductors and maximum voltage across the inductors, but not the current at maximum voltage.  Fortunately, knowing the maximum voltage, I can calculate the current at this voltage with a spreadsheet.  

The following table is the simulator output that shows peak inductor currents and peak inductor voltages across the HF bands at average 200 watts operating power. 

	Ind Current	12.8 uH	6.4 uH	3.2 uH	1.6 uH	800.0 nH	400.0 nH	200.0 nH	100.0 nH	50.0 nH	25.0 nH
160m low	8.9	467	442	298	161	81	40	20	10	5	3
160m high	8.9	0	458	322	178	90	45	22	11	6	3
80m low	8.9	0	474	437	292	157	78	39	20	10	5
80m high	8.9	0	0	458	322	178	90	45	22	11	6
40m low	8.9	0	0	474	437	292	157	78	39	20	10
40m high	8.9	0	0	453	444	301	163	82	41	20	10
20m low	8.9	0	0	0	474	437	292	157	78	39	20
20m high	8.9	0	0	0	466	440	297	160	80	40	20
17m low	8.9	0	0	0	0	468	351	200	101	51	25
17m high	8.9	0	0	0	0	471	351	201	102	51	25
15m low	8.9	0	0	0	0	484	383	229	118	59	29
15m high	8.9	0	0	0	0	479	386	233	120	60	30
12m low	8.9	0	0	0	0	481	415	265	139	70	35
12m high	8.9	0	0	0	0	483	417	266	140	70	35
10m low	8.9	0	0	0	0	474	437	291	157	78	39
10m high	8.9	0	0	0	0	472	446	305	166	83	42
6m low	8.9	0	0	0	0	0	483	417	266	140	69
6m high	8.8	0	0	0	0	0	476	433	283	150	75
Max	8.9	467	474	474	474	484	483	433	283	150	75
Max RMS	6.3	330.2	335.2	335.2	335.2	342.2	341.5	306.2	200.1	106.1	53

Inductor Maximum Peak Currents and Voltages Table

Here is the calculated RMS current through each inductor at the maximum voltage across that inductor at a given frequency.

	3.2 uH	1.6 uH	800.0 nH	400.0 nH	200.0 nH	100.0 nH	50.0 nH	25.0 nH
160m low	5.82	6.29	6.33	6.25	6.25	6.25	6.25	7.50
160m high	5.66	6.26	6.33	6.33	6.19	6.19	6.75	6.75
80m low	4.39	5.87	6.31	6.27	6.27	6.43	6.43	6.43
80m high	4.03	5.66	6.26	6.33	6.33	6.19	6.19	6.75
40m low	2.38	4.39	5.87	6.31	6.27	6.27	6.43	6.43
40m high	2.18	4.28	5.80	6.28	6.32	6.32	6.17	6.17
20m low	-	2.38	4.39	5.87	6.31	6.27	6.27	6.43
20m high	-	2.28	4.31	5.82	6.27	6.27	6.27	6.27
17m low	-	-	3.62	5.44	6.19	6.26	6.32	6.19
17m high	-	-	3.65	5.44	6.23	6.32	6.32	6.19
15m low	-	-	3.24	5.13	6.14	6.32	6.32	6.22
15m high	-	-	3.14	5.06	6.11	6.30	6.30	6.30
12m low	-	-	2.72	4.69	5.99	6.28	6.33	6.33
12m high	-	-	2.72	4.69	5.99	6.30	6.30	6.30
10m low	-	-	2.38	4.39	5.85	6.31	6.27	6.27
10m high	-	-	2.24	4.22	5.78	6.29	6.29	6.37
6m low	-	-	-	2.72	4.69	5.99	6.30	6.21
6m high	-	-	-	2.48	4.51	5.90	6.25	6.25

                            Inductor RMS Current at Maximum Inductor Voltage

As it can be seen from this table, in many instances the inductor current at maximum inductor voltage is the same or near the maximum inductor current value of 6.3 amps.  But in some cases, especially for the higher value inductors, the current is much less than the maximum inductor current.  Here is a table of all the cases where a lower inductor current value can be used.

	3.2 uH	1.6 uH	800.0 nH	400.0 nH	200.0 nH	100.0 nH	50.0 nH	25.0 nH
160m low	5.82
160m high	5.66
80m low	4.39	5.87
80m high	4.03	5.66
40m low	2.38	4.39	5.87
40m high	2.18	4.28	5.80
20m low	-	2.38	4.39	5.87
20m high	-	2.28	4.31	5.82
17m low	-	-	3.62	5.44
17m high	-	-	3.65	5.44
15m low	-	-	3.24	5.13
15m high	-	-	3.14	5.06
12m low	-	-	2.72	4.69	5.99
12m high	-	-	2.72	4.69	5.99
10m low	-	-	2.38	4.39	5.85
10m high	-	-	2.24	4.22	5.78
6m low	-	-	-	2.72	4.69	5.99
6m high	-	-	-	2.48	4.51	5.90

Cases With Less Than 6.3 A RMS Maximum Current

I spent a bunch of time playing around with different powdered iron cores to find the smallest ones that met my design criteria.  After finding the ones that did meet my requirements, I tested the next smaller size and in all cases, they failed to meet the temperature rise criteria.  This table is the outcome of this step.  I should also note that I was using 12 AWG wire as the wire input parameter to the program.

The first inductance value is the design target.  But given the AL values (see below) and an integer number of turns, only certain inductance values can be realized.  The second inductance value is the estimated value based on the core properties (AL value).  After these inductors are built and tested, I will list their actual values.

L (nH)	Core	AL (nH/N2)	Turns	L (nH)	I (Arms)	f (MHz)	P (W)	T (deg C)
25	T94-17	2.9	3	26	6.3	30	1.7	33
50	T94-17	2.9	4	46	6.3	30	2.6	47
100	T130-17	4.0	5	100	6.3	30	4.7	49
200	T184-17	8.7	5	210	6.3	30	7.7	43
400	T184-17	8.7	7	410	6.3	7.3	3.4	22
400	T184-17	8.7	7	410	6.0	14.35	5.5	32
400	T184-17	8.7	7	410	5.5	21.45	7.0	39
400	T184-17	8.7	7	410	4.7	30	7.0	42
800	T184-17	8.7	10	850	6.3	7.3	6.1	35
800	T184-17	8.7	10	850	4.4	14.35	5.2	31
800	T184-17	8.7	10	850	3.7	18.2	4.6	28
800	T184-17	8.7	10	850	3.3	21.5	4.4	27
800	T184-17	8.7	10	850	2.8	30	4.8	29
3200	T184-17	8.7	14	1660	6.0	4	6.4	36
3200	T184-17	8.7	14	1660	4.4	7.3	5.0	31
3200	T184-17	8.7	14	1660	2.4	14.35	2.6	18
3200	T184-17	8.7	20	3310	4.4	4	2.3	20
3200	T184-17	8.7	20	3310	2.4	7.3	3.0	17

Inductor Design Using 12 AWG Wire

The parameter AL needs an explanation.  The formula for the number of turns from the RF design book is:

\begin{equation}N = 100 \times \sqrt {\frac {L}{A _{L}}}\end{equation}

Where N is the number of turns and L is inductance in micro Henries, N in turns and AL in micro Henries per hundred turns squared.  

Important to note that Amidon publishes its AL values in micro Henries per 100 turns squared while Micrometals publishes them in nano Henries per turns squared.  So, while using the Micrometals data, drop the 100 in the formula and use nano Henries for L.

It is worth mentioning that 2 and 6 material that could potentially be useful in this application have much higher AL values for their larger cores that can support the required power levels.  These higher AL values make it much harder to get close to the desired inductance values with any accuracy.

I built a few of these inductors and the inductance was not anywhere near what I expected.  To simplify the task of experimenting and finding out what is going on, I switched to 22 AWG magnet wire (working with 12 AWG wire is very yard).  The graph below is a plot of the AL value vs. the measured inductance for a T187-17 core which has a specified AL value of 8.7 nH per turns squared.

Toroid AL value vs. Measured Inductance

It is clear from the graph that for smaller inductors (less turns), the wire inductance dominates.  But as the number of turns increase since the AL value changes with the square of the number of turns, we asymptotically approach the specified AL value.  

After my experience with 12 AWG wire (which I had picked when I was going to use air core inductors), I decided to consider a thinner gauge wire since using a toroid allowed me to use less turns.  I experimented with a number of different wire gauges and 18 AWG wire seemed to meet the power loss requirements well.  I built a number of inductors to the exact number of turns predicted by the formula (and the design tool), tested them and then took off the turns needed to obtain the required inductance.  Below is a plot of designed turns vs. the experimental turns.

Measured Turns vs. Calculated Turns

Armed with this data, I was ready to run through the design process.  In the first iteration, I used the same turns as the table with the 12 AWG wire and recorded the result.  But then I reduced the turns ratio according to the above graph equation as follows:

L (nH)	Core	Designed Turns	Measured Turns
25	T94-17	3	1
50	T94-17	4	2
100	T130-17	5	3
200	T184-17	5	3
400	T184-17	7	4
800	T184-17	10	8
1600	T184-17	14	12
3200	T184-17	20	18

Specific Inductance Calculated and Measured Turns From the Graph

With these new number of turns, I calculated the power loss in the core.  What is significant in this table is the number of turns (taken from the above table) and the power dissipation (P) and temperature rise (T).  I have intentionally left off the 25 nH inductor from this list in the hope of using the PCB trace inductance in is place.

L(nH)	Core	AL (nH/N2)	Turns	L (nH)	I (Arms)	f (MHz)	P (W)	T (deg C)
50	T94-17	2.9	1	46	6.3	30	0.7	18
100	T130-17	4.0	2	100	6.3	30	1.5	36
200	T184-17	8.7	3	210	6.3	30	5.3	35
400	T184-17	8.7	5	410	6.3	7.3	3.6	25
400	T184-17	8.7	5	410	6.0	14.35	5.2	34
400	T184-17	8.7	5	410	5.5	21.45	6.0	38
400	T184-17	8.7	5	410	4.7	30	5.9	38
800	T184-17	8.7	8	850	6.3	7.3	6.6	42
800	T184-17	8.7	8	850	4.4	14.35	5.1	33
800	T184-17	8.7	8	850	3.7	18.2	4.4	29
800	T184-17	8.7	8	850	3.3	21.5	4.0	27
800	T184-17	8.7	8	850	2.8	30	4.0	28
1600	T184-17	8.7	12	1660	6	4	7.0	44
1600	T184-17	8.7	12	1660	4.4	7.3	5.4	35
1600	T184-17	8.7	12	1660	2.4	14.35	2.6	19
3200	T184-17	8.7	18	3310	4.4	4	6.6	41
3200	T184-17	8.7	18	3310	2.4	7.3	2.8	20

Power Dissipation and Temperature Rise for the Designed Inductors

What remains is the building and testing of the inductors.  Here is the test data (I have intentionally left out the 25 nH inductor in the hopes of using the PCB trace inductance in its place):

L (nH)	Turns	3 MHz	5 MHz	7 MHz	14 MHz	30 MHz
50	1	44.6			44.8	44.9
100	2	107	100	100		101
200	3	239			240	246
400	5	414			420	440
800	8	805			831	929
1600	12	1568	1576	1590	1670
3200	18	3170	3211	3275

Inductance Value Measured at Various Frequencies

It is worth noting that the apparent inductance of the toroids increasing with frequency is the effect of the stray parallel capacitance of the wires.  It is not very noticeable with fewer turns and as the number of turns increases so does the stray capacitance.

 And finally, here is a picture of the inductors:

500 Watt Antenna Tuner Part 1 - Introduction

500 Watt Antenna Tuner Part 2 - Analytical Model

500 Watt Antenna Tuner Part 3 - Simulator

500 Watt Antenna Tuner Part 4 - Capacitors and Inductors

500 Watt Antenna Tuner Part 4A Capacitors

500 Watt Antenna Tuner Part 4B Inductors

500 Watt Antenna Tuner Part 5 - Directional Coupler

500 Watt Antenna Tuner Part 6 - Matching Algorithm

500 Watt Antenna Tuner Part 7 - User Interface

500 Watt Antenna Tuner Part 8 - Software and Microcontroller

500 Watt Antenna Tuner Part 9 - Pre Build Schematics and Board Layout