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2 Meter 80W All Mode Amplifier

Published in QST magazine (May 2013) - there are several corrections and updates to the magazine article at the end of this document.

There are lots of different 2 meter low-power rigs in use out there, ranging from hand-helds for FM, older multi-mode transceivers, and even the newer all-purpose types like the FT-817 or the Elecraft 2M transverters. QRP operation can be fun, but if you're like me, you probably have the occasional need for a bit more power.

Good news...if you have a couple of afternoons to spend on a project, you can build this 80 watt multi-mode amplifier with ease. Easy because it uses one of the newer S-AV36 Toshiba modules, 50 ohms in and 50 ohms out, with gain galore. So much gain that less than 50 milliwatts will drive it to full output in any mode.

My original intent in making this was to have an amplifier capable of boosting an older 10w multi-mode radio up to 80 or 100w. I wanted to keep it low-cost and simple (no preamp or power meters), yet capable of mobile operation in any mode from 12v power.

After absorbing the specs in the data sheet, it was clear this module could be driven by almost any low-power rig; thinking about it a bit more, and keeping in mind the low cost and simplicity requirements, a few more useful features came to mind, such as:

  • A configurable input attenuator capable of matching any driver ranging from 50 milliwatts to 10 watts

  • an output low-pass filter to comply with FCC regulations for harmonic and spurious suppression

  • Low-loss antenna relay

  • RF-sensing t/r switch for remote operation, as well as a hard-key option

  • T/R sequencing to protect the S-AV36 module and prevent hot-switching the antenna relay

  • Indicator LED's and control switches

  • Reverse polarity protection

The devil is in the details for the designer, though, and it did take a little planning, but the end result was a small PC board and just a few interconnecting wires. Add a heat sink, connectors, switches, a couple of sheet metal parts for the enclosure, and that's about it.

The Photo to the left is the first prototype, using a board made with common hobby tools.

Designing the amplifier

The S-AV36 module is pretty easy to use; aside from RF in and RF out, there are two power connections; one is for bias (this turns the module on and off), and the other for main DC power, 13.5v  nominal at up to 15 amps. Since the input power required to drive it is only about 50 milliwatts, the first thing to do is design an input attenuator to match the output of the driver to the S-AV36.

The chart shown on the right lists the resistor values for 50 ohm attenuators at drive levels ranging from 1 to 10w. There are some strange values there, but these are not terribly critical, you just have to get within a few ohms to get the job done. For example, a 23db attenuator is needed for a 10w radio; the resistors chosen were those readily available from major distributors, so 58 ohms became 56, and 351 became 360 (close enough). L5 is not really necessary, it's purpose is to compensate for the stray capacitance of R7 at 2 meters (a 35w tab-mounted resistor). The input SWR was acceptable without it, but it does make the input match almost perfect.

The low pass filter

Now that the input is taken care of, let's deal with the output; the data sheet says the second harmonic will only be down about 25db, and the third about 30. Not good enough for the FCC, so we need an output filter that will put us in good graces with at least 60db total suppression. For that 25db second harmonic, we need another 35db.

The filter shown is a standard pi-type Chebyshev, 7 poles, and provides the required suppression with very little insertion loss at the operating frequency.

The last analyzer display shows the sampled harmonic content of the amplifier tested at 90 watts output. Good filter.


The Antenna Relay and switching controls

In the sprit of keeping costs low, a PCB-mount type of DPDT general-purpose relay was chosen. Less than $5 in cost, the contacts are rated at 8 amps. At 2 meters, a bit of reactance is introduced by this part, but compensated for by a small capacitor (C15) in series with it's input.

The best way to tell the amplifier to switch on is to use a control line back to the driving radio (PTT). If this is unavailable or inconvenient, the amplifier has an RF-sensing circuit that samples a bit of drive from the input connector to provide the transmit trigger.

Another little twist; switching from receive to transmit should be sequenced for two reasons; first, the S-AV36 is tough, but no self-respecting amplifier module likes seeing an open circuit while those lazy relay contacts are moving, even if it only takes 20 milliseconds to happen; it's not good for the module, and just plain rude. For this reason, the module has to be kept off while the relay contacts are settling. The other reason is to protect the relay contacts from that 80 to 100w the amplifier will generate before they finally settle; it tends to shorten the life of the relay.

Showing the full schematic now, C4, D1, and D2 sample the input, and C5, C6, R1 and R2 provide filtering and some timing, depending on the position of S1. In SSB mode, the circuit acts like a vox, providing a second or so of delay before reverting back to RCV mode. In FM mode, the switch back to receive is much quicker, as the delay is not necessary for FM operation. The circuit is sensitive, and will trigger with less than 1/2w drive.

Q1 is the switch that operates the relay. When the relay is turned on by Q1, it also turns on Q2 (the bias switch) after a short delay. This delay is provided by C9 and R4, and is about 50 milliseconds in duration, allowing those relay contacts to settle before the module becomes active.

When switching back to RCV, the bias to the module is cut off before the relay contacts open. This fast cutoff is timed by C9 and R5, and is only about 5 milliseconds in duration.

Another noteworthy component is D6, the reverse-polarity protection diode. This diode's purpose in life is to blow an in-line fuse in case you accidentally connect the power cable up backwards (come on, we've all done it).

The extra contacts on power connector J3, pins 3 and 4, provide a means to disable the RF sensing and connect PTT directly to the driver should the RF sensing be deemed unnecessary.

L1 and L4 are 4 turns #18, 4mm ID and 8mm long. L2 and L3 are 7 turns #18, 4mm ID and 10mm long.

Building the amplifier

A bill of materials (BOM) is provided for gathering the parts you don't have on hand, and is grouped by the recommended supplier. For parts supplied by Mouser, you can order all of these by ordering the project list from their web site at: http://www.mouser.com/tools/projectcartsharing.aspx. The access I.D. code for the project is c3ad150d1a . At the time of publication of the QST article, Mouser was temporarily out of stock on the 56 ohm 35w resistor (R7). A 20w resistor can be substituted there, part # PWR263S-20-56R0J. If you use the 20w part, also substitute a 270nh inductor for L5, part # imc1210err27k.

RFPARTS (www.rfparts.com) is the supplier for the Toshiba module and coax connectors; Drawings for making the chassis parts are here: front panel rear panel bottom cover. For heat sink material, a heat sink with a base thickness of .300 or better should be used. The size of the one I used was 8" long x 5.375" wide.

Here are the recommended steps, in order, for constructing the amplifier:

  • Mark the heat sink for drilling by using the PC board as a template. You can also position the Toshiba module and mark it's two mounting holes; leave a small gap of 2 to 3mm between the module body and the board for strain relief.

  • Drill and tap the module mounting holes for 6-32 screws, and the PCB holes for 4-40 screws.

  • Fill the plated-through holes surrounding (and underneath) R7 with solder; this will help to draw heat away from R7. Install all the PC board components except for the module. The relay should be installed last, and because the pins will protrude through the bottom of the board, they should be cut off flush with the board after soldering.

  • Make the two aluminum spacers as as shown in this photo.

  • Mount the board to the heat sink with four 4-40 screws. The two aluminum spacers must be positioned under the board on either end. These spacers elevate the board to a convenient height for the module, and keep the back side connections at the relay pins from shorting against the heat sink.

  • Some minor tuning of the low pass filter coils can be made at this time. Connect a dummy load to the output of the board, and a transmitter and SWR meter to the trace at the input of the filter where the module will connect. Apply 12v across the relay coil to close the relay, and spread or compress the coils for lowest SWR reading. If this is inconvenient to do, the filter can be adjusted after the amplifier is fully constructed, adjusting for max power at about 50w output. It's best to do it now, though, and you'll probably find that very little adjustment is necessary.

  • Using heat sink compound, mount the Toshiba Module with 6-32 screws. Note that the mounting bar of the module is slightly concave; this is not a defect, the manufacturer makes them this way, as do other module makers as well. Do not attempt to sand this footing flat or otherwise fill with any material except for heat sink compound, there is still plenty of contact area for heat transfer. I'm just guessing, but I believe the manufacturer makes the footing this way for strain relief in order to protect the mechanical bonds inside. Solder the module wires to the appropriate traces on the PC board (cut off the excess wire length if necessary).

  • Mount the connectors, switches and LED's, and complete the chassis wiring. The LED's have their 1K resistors soldered directly to their leads, with the wire connected to the other side of the resistor; heat shrink is used to cover the resistor and connections. Use solder lugs under the mounting screws for the connectors on the rear panel; these are for connecting coax shields, DC chassis ground, and bypass capacitors as shown in the photo.

  • Make the power cord with #14 wire; make certain to use an in-line fuse on the positive lead, and fuse it for no more than 20 amps. If you will be hard-keying your amplifier from your radio, jumper pin 5 of the connector to ground, and carry pin 4 back to your keying connection from the radio. The radio's PTT relay contacts or other switch must be capable of sinking 12v @ 50 ma to ground.

Testing the amplifier

Once everything is wired and in place, you can test the amplifier using the following procedure:

  • Connect the output to a suitable wattmeter and dummy load, and the input to your driving radio

  • Connect the power cord to a power supply capable of delivering 13.5v at up to 15 amps.

  • Place the Amplify/Bypass switch in bypass mode. Transmit, and verify that bypass mode works (most of the driver's power should pass through the amplifier to the load). Figure xx shows the bypass mode insertion loss, only about a tenth of a db.

  • Turn off the driving radio and put the amplifier in amplify mode. The "Ready" LED should illuminate. Jumper PTT to ground, and the Xmit LED should also illuminate. Adjust the IDQ trimmer (VR1) for 8 amps. Place the amplifier back in bypass mode and remove the PTT jumper.

  • Turn the radio back on, place the amplifier in amplify mode, and transmit. Performance should be similar to the data shown in the following chart:

I experimented some with various IDQ settings, and concluded that Toshiba must have designed the module to operate close to class A; setting IDQ lower tended to introduce lower overall gain and crossover distortion in SSB, and setting it higher resulted in higher gain and saturated output power. At 10 amps IDQ, for example, the amplifier could be driven to over 100w output with about half the drive required at 8 amps IDQ. This does exceed the manufacturers ratings for the device, and really doesn't make any difference on the air, so I resisted the temptation to leave it that way.

Knowing how bias levels work could be useful in the non-linear modes (FM, CW); you could set the bias level to control output power, and perhaps have a front panel control knob in place of VR1 for this purpose. For all-mode versatility, leaving IDQ set at 8 amps is best.

One last note; at 10w drive level, I noticed R7 (the 35w attenuator input resistor) ran hot; this was due to the inadequate heat transfer of the PC board I made for the original prototype, which has just a few rivets where there should have been multiple plated-through holes surrounding this part.  My solution was to use a piece of .040 copper strip soldered to the ground tab of the resistor, using it to sink heat off to the heat sink by fastening the other end to it with a #4 screw. Most of us making our own prototype boards at home don't have the ability to make plated-through holes like the commercial board houses, so if you make your own board for this project, you'll probably need to implement a similar solution. If you purchased the commercial board, you won't need the copper strip; but remember to fill the plated-through holes surrounding (and underneath) R7 with solder to help with heat sinking.
Corrections to the May 2013 QST article:

  • The value of C15 has been changed to 220pf

  • The positions of R1 and R2 were interchanged in the magazine article. R1 (511k) should be the resistor that is bypassed (or not) by S1

  • C6, C8, C9 and C11 are polarized tantalum capacitors

Clicking on the photo above will display a high resolution copy of the picture