If you have been following along with the Hermes Lite 2 content here, you know the short version: the HL2 is a capable open-source SDR transceiver, it pairs well with Thetis, and it does not cost a fortune. What it also does not do, at least out of the box, is put any meaningful power on the air. We are talking about one watt. Maybe a little more on a good day, depending on how you have things set up. That is fine for WSPR experiments and weak-signal work where a watt is actually plenty. But if you want to run SSB contacts on a busy band, chase DX, or just have a station that can compete with normal operating conditions, one watt is not going to get the job done.
So the natural next question is what to put between the HL2 and the antenna. There are a few options in this space. Some people go the kit route and build their own intermediate stage. Some drop money on something like the Hardrock-50. I went a different direction and picked up the Xiegu GPA100, which is a solid-state 100-watt linear amplifier designed specifically for QRP and low-power transceivers. The pairing with the Hermes Lite 2 is not something Xiegu markets directly, but it works, and the connection is simpler than you might think.
This article covers the amplifier itself, what the HL2-to-GPA100 connection actually looks like, what the amp is like to operate, and whether it is worth it. Short version: it is. Longer version below.
Why the HL2 Needs Help on Power
The Hermes Lite 2 was designed as a complete HF SDR transceiver at a price point that makes sense. And it succeeds at that. The receiver is genuinely good, the software integration with Thetis is solid, and the open-source nature of both the hardware and firmware means you can actually dig into what is happening rather than just hoping the black box works. But the power amplifier stage on the HL2 is minimal by design. The board is built around the idea that you are going to add external amplification if you need it. This is not a criticism, it is a design choice, and it keeps the cost and complexity of the base unit down.
One watt is actually usable in the right circumstances. On 40 meters with a decent antenna, WSPR propagation spots happen at one watt routinely. Digital modes like FT8 are efficient enough that a watt can make contacts. But voice operating is a different story. The difference between one watt and a hundred watts in a voice contact is not just loudness, it is the difference between being heard and not being heard at all. A lot of operators are used to running at least fifty watts for casual HF work, and a hundred is where most rigs top out for typical operations. If you are coming from a traditional radio background and setting up an HL2 station, you are going to want amplification.
The practical constraint is that whatever amplifier you add has to work with one watt of drive. A lot of the common external amplifiers are designed for radios putting out fifty to a hundred watts already. They need more drive than the HL2 can provide. The GPA100 is specifically built for this scenario. Its input requirement tops out at five watts, which means the one watt from the HL2 is exactly the kind of drive it is expecting.
What the GPA100 Actually Is
Before getting into the connection specifics, it is worth spending some time on the amplifier itself, because the GPA100 is a more capable unit than the basic spec sheet makes it look.
It covers 1.8 to 54 MHz, which means full HF coverage plus 6 meters. Output is 100 watts on all HF bands below 29.7 MHz, dropping to 80 watts on 6 meters. The design is solid-state throughout, so there are no tubes to worry about, no warm-up time, and no worrying about the amp surviving a rough transmit. The build quality is better than you might expect for the price. The chassis is reasonably solid metal, the front panel feels like it was thought through rather than just populated with whatever fit, and the overall unit is compact enough that it does not dominate a typical desk setup.
The LCD display is one of the features that actually impresses in regular use. It shows power output, SWR, current draw, input voltage, active band, and internal temperature all at once. That is real-time visibility into what the amp is doing that you do not always get on units at this price. If something is going sideways with your antenna, the SWR reading tells you before you damage anything. If the amp is running hot, the temperature readout makes that obvious before the protection circuits kick in.
One watt into the GPA100 comes out the other end as a hundred watts of clean signal. That changes what the Hermes Lite 2 is capable of on a real band.
Band selection is handled two ways. There is a manual mode where you use the front panel buttons to select from seven amateur bands, each with its own indicator LED so you can confirm what the amp thinks it is working with at a glance. And there is an automatic mode where the amp can track band changes from the radio if band data is available via cable. For IC-705 users there is also Bluetooth-based automatic band switching, which is a genuinely useful feature for a radio that a lot of people pair with amplifiers exactly like this one.
For the Hermes Lite 2, which does not output band data the way dedicated Xiegu transceivers do, you are going to be working in manual mode. That is not a hardship. You tune to a band in Thetis, press the corresponding button on the GPA100, and you are set until you change bands. It adds maybe two seconds to a band change. Not a problem.
Connecting the GPA100 to the Hermes Lite 2: Step by Step
The good news is that this is not a complicated integration. The HL2 is a radio. The GPA100 is an amplifier. The connection between them is RF in and RF out. Here is how to set it up.
Step 1: RF Connection
Run a short coax patch cable from the antenna port on the Hermes Lite 2 to the RF input on the GPA100. PL-259 to PL-259, a foot or two of RG-8X or similar is fine here. You are not running RF down this cable at a hundred watts, you are running one watt, so a cheap patch cable works. The GPA100 input is SO-239. The HL2 output is also SO-239. Straightforward.
Step 2: Antenna Connection
Your antenna connects to the RF output port on the GPA100. This is the coax that matters for quality since it carries the full 100-watt output. Use good coax here. LMR-400 or equivalent from the shack to the antenna feed point. Do not be the person who runs RG-58 to a hundred-watt amp.
Step 3: Power Supply
The GPA100 needs 12 to 15.5 volts DC at up to 25 amps during transmit. That is a real current draw and it needs a real supply. A thirty-amp supply has enough headroom. Astron, MFJ, or similar linear supplies are common choices. The standby draw is only about 0.3 amps, so when you are just receiving the amp barely touches the supply. But when you key up it pulls hard, so the supply needs to be able to handle it.
Step 4: PTT and Key-Up
The GPA100 handles PTT automatically via RF sensing. When it sees RF coming in from the HL2, it switches to transmit mode. When the RF stops, it switches back to receive. You do not need a separate PTT line for basic operation. This is convenient and works reliably in practice. The amp responds fast enough that you do not get clipped audio at the start of transmissions during normal SSB operating.
Step 5: Power On Sequence
Turn the GPA100 on first. Wait for the display to initialize. Then power up the Hermes Lite 2 and start Thetis. The order matters because the amp needs to be in receive mode before RF is present on the input line. Reversing this order is not going to immediately destroy anything, but it is good practice to always bring the amp up before the radio.
Step 6: Band Selection and First Transmit
In Thetis, navigate to whatever band you want to operate on. On the GPA100 front panel, select the matching band using the band buttons. Verify the correct LED is illuminated. Then do a brief test transmit at low power in Thetis, maybe twenty or thirty percent, and watch the GPA100 display. You should see power output and SWR readings. If SWR is reasonable, you are in good shape. Bring power up from there.
What to Know About Thetis Power Control
One thing worth addressing directly: how do you control power output at the radio side when using an external amplifier? The answer is through Thetis transmit settings, and it is worth spending a few minutes getting this right before you start throwing power at the amp.
In Thetis, there is a TX Drive Level control. This adjusts how much of the HL2's available output power is actually delivered to the amplifier input. For the GPA100, the input requirement is five watts maximum. The HL2 tops out at around one watt on most bands. So you are already within spec without doing anything special. But be aware that the HL2 power output varies somewhat by band, and some operators have seen slightly higher numbers on certain frequencies. Keep an eye on what the GPA100 input sees and do not overdrive it.
The cleaner your drive signal going into the GPA100, the cleaner the output coming out. The HL2 produces a reasonably clean signal when operating within its rated parameters, and the GPA100 is a linear amplifier, which means it amplifies what it receives without adding its own distortion, as long as you are not pushing it into compression. Running the amp at full output all the time is not necessary and not the kindest thing to do to the finals long term. For typical SSB operating, the amp is comfortable running at 80 to 90 watts average output with peaks at 100 during voice peaks. That is normal and fine.
For digital modes, the math is different. FT8 and similar modes transmit at a constant duty cycle, which is harder on finals than SSB. When running digital modes through the GPA100, keep output at 50 watts or below. The amp can handle more, but long-key digital transmissions at full power generate sustained heat, and thermal stress accumulates. Fifty watts is the comfortable spot for extended digital operation.
The Protection Circuits Actually Work
One of the things I was curious about before using the GPA100 seriously was whether the built-in protection was real or just a marketing checkbox. The short answer is that it is real and it does what it claims.
The amp monitors SWR continuously during transmit. If the SWR climbs above 1.5:1, it raises an alert. Above 2:1, it cuts transmit entirely. The first time this happened to me was when I had a connector that had not been fully seated on the antenna side. The GPA100 shut down before anything got damaged, and the display made it immediately clear what happened. Without that protection, that moment could have been more expensive.
The thermal protection works through a combination of active cooling and temperature monitoring. The fan is not running constantly at low temperatures, which keeps the operating noise reasonable. As the amp warms up under load, the fan speed increases. If the internal temperature reaches the limit, transmit is cut until it cools down. During a typical SSB operating session I have never triggered the thermal cutoff. The fan does its job before it gets to that point. The protection is more relevant for digital mode operators running sustained high-duty-cycle transmissions, and even there the fan management tends to stay ahead of the thermal limit.
The overcurrent protection is also present but less likely to activate under normal conditions. If something is seriously wrong on the output side, the amp will protect itself. That kind of protection is worth having and not something every amp at this price includes.
The No-Tuner Limitation: What It Means in Practice
The GPA100 does not include an internal antenna tuner. This is the most common complaint about the unit, and it is worth being honest about rather than hand-waving past it.
If your antenna is resonant on the bands you want to operate, this is not an issue at all. A dipole cut for 40 meters, a vertical with a good ground system, a Yagi: these are resonant antennas that present a reasonable impedance to the feedline, and the GPA100 handles that fine without a tuner. Most of the time a well-matched antenna system has an SWR that stays within what the amp can work with.
The problem comes if you want to use a non-resonant antenna, run an EFHW on a band it is not wound for, or work a multiband antenna that relies on a tuner to present a matched load. In those cases you need to put an external tuner between the GPA100 output and the antenna. The tuner goes after the amp, not before it, so the tuner needs to handle 100 watts. That adds cost and a piece of equipment to the chain.
For comparison, the Xiegu XPA125B includes an internal tuner and outputs 125 watts. It is a more complete solution for operators who need tuner flexibility, but it costs more. The GPA100 gives up the tuner in exchange for a lower price and a somewhat more compact footprint. Whether that trade makes sense depends entirely on your antenna situation. If you already have resonant antennas on your main operating bands, the lack of a tuner is a non-issue. If you rely on a tuner for flexibility, either budget for an external one or look at the XPA125B instead.
Real World Results on the Air
The first thing you notice when you go from one watt to a hundred watts is that the band sounds exactly the same on receive. That sounds obvious but it is worth saying because some operators expect the SDR experience to change somehow when the power goes up. The receiver in Thetis does not care what the amp is doing. You are still looking at the same waterfall, the same DSP controls, the same click-to-tune operation. Nothing about the receive side changes.
What changes is transmit. On 40 meters running SSB with the HL2 into the GPA100, contacts that were not happening at one watt start happening at a hundred. The S-meter reports on the other end go from unreadable to readable, from 51 to 57, from marginal to comfortable. This is exactly what you expect from a 20 dB power increase, but experiencing it directly on the air makes the amplifier feel worth it in a way that the spec sheet does not quite convey.
The signal reports I have gotten through this chain have been consistent with a clean transmit. Nothing in the audio has come back as sounding rough or distorted, which is what you want from a linear amplifier. The GPA100 is doing its job and doing it without adding character to the signal, which is the correct behavior. An amplifier that makes your signal sound different is adding something it should not be adding.
On 20 meters the story is similar. Conditions permitting, 100 watts through a decent antenna makes the band workable in a way that one watt simply cannot match. On a good morning with the bands open, the difference is striking. You key up and you get responses. At one watt you key up and hope.
I have also run FT8 through this chain at reduced power, around 40 to 50 watts, over extended sessions. The amp handles the sustained duty cycle without complaint. The fan runs, the temperature climbs to a comfortable plateau, and it just keeps doing its job. Nothing to worry about there as long as you are not pushing it to the full hundred watts for extended digital transmissions.
GPA100 Front Panel: What You Are Looking At
Power Output Display
The LCD shows your actual RF output power in watts. On SSB this number bounces around with your voice peaks, which is normal and expected. If you want to see a steady reading for calibration purposes, key up in AM or FM mode. The display also has a bar graph below the numeric readout that gives a quick visual sense of where output is sitting.
SWR Readout
Real-time SWR is shown alongside the power output. This is probably the most practically useful thing on the display. You can see immediately if your antenna connection is good, if something changed between sessions, or if a connector worked its way loose. The SWR reading is what triggers the protection circuit when it climbs too high, so understanding what it normally looks like helps you notice when something is off.
Voltage and Current
The display shows the DC voltage being supplied to the amp and the current draw during transmit. Voltage readout is useful for spotting a power supply that is sagging under load. If you see the voltage dipping significantly when you key up, your supply is undersized for the draw. Current readout during transmit lets you correlate power output with actual consumption, which is useful if you are running on battery for portable operations.
Temperature
Internal temperature is displayed in real time. Under normal operating conditions with the fan doing its job, the temperature stays well below the cutoff limit. During extended digital mode sessions you will see the number climb, level off, and hold. If it is climbing past where it normally plateaus, that is a signal to back off or check your cooling environment.
Band and Mode Indicators
The current band selection is shown on the display along with whether you are in AUTO or MANUAL mode. The front panel LEDs next to each band button give you at-a-glance confirmation without needing to read the display carefully. In a dark shack these are immediately readable from across the desk.
Who This Setup Makes Sense For
If you are already running a Hermes Lite 2 and want more power, the GPA100 is probably the most direct path to a functional 100-watt station. The connection is clean, the amp is capable, and the price is reasonable for what you get. You are not building something exotic or sourcing parts from four different vendors. It is a real amplifier with a real feature set that you can have on the air the same day you receive it.
If you are an IC-705 user who also wants a QRP amplifier, the GPA100 is worth a look on its own merits. The Bluetooth band switching for the IC-705 is a genuinely useful feature, and the 80 watts on 6 meters fills out a portable VHF capability that pairs naturally with the IC-705's multiband range. The amp travels reasonably well given its size and weight.
For operators who primarily run digital modes and are comfortable with 50 watts as their cap, the GPA100 works. You are not using its full output capability, but the amp is not stressed at that level and the reduced heat means longer life for the finals.
The people who might want to look elsewhere are operators who rely heavily on antenna tuners for multiband flexibility and do not want to add an external tuner to the chain. For that use case the XPA125B or a comparable unit with a built-in tuner is a better fit. The GPA100 is at its best when paired with a matched antenna system, and that is where it earns its reputation.
Frequently Asked Questions
Does the Xiegu GPA100 work with the Hermes Lite 2?
Yes. The GPA100 is designed for any QRP or low-power transceiver outputting five watts or less, and the HL2 outputs around one watt. The connection is straightforward coax in and coax out, with no special interface required. Band switching on the GPA100 is handled manually via the front panel since the HL2 does not output band data in the format the amp expects from dedicated Xiegu transceivers.
What power output does the Hermes Lite 2 put into the GPA100?
The stock HL2 produces approximately one watt of RF output, which is well within the GPA100's maximum input of five watts. You do not need to worry about overdriving the amp input with a stock HL2. The GPA100 takes that one watt and amplifies it to 100 watts on HF bands and 80 watts on 6 meters.
Does the GPA100 include an antenna tuner?
No. The GPA100 does not have an internal antenna tuner. If your antenna is resonant on the bands you want to operate, this is not a problem. If you rely on a tuner for impedance matching, you will need an external tuner capable of handling 100 watts placed between the amp output and the antenna. The XPA125B is the Xiegu option that includes a built-in tuner if that matters to your setup.
What power supply does the GPA100 need?
The GPA100 requires 12 to 15.5 volts DC at up to 25 amps during transmit. A standard 30-amp ham radio supply works well. In standby the draw drops to about 0.3 amps, so total idle consumption is minimal. Do not run this amp off an undersized supply: a sagging voltage rail during transmit causes problems and the current draw is real.
Can you run FT8 through the GPA100?
Yes, but at reduced power. For sustained digital mode transmissions keep output at 50 watts or below. FT8 and similar modes transmit at 100 percent duty cycle for the full transmission window, which is harder on finals than SSB voice peaks. Fifty watts is a comfortable level for extended digital operating. Running 100 watts through extended digital transmissions generates sustained heat that shortens the life of the finals over time.
Does the GPA100 work as a receive preamp too?
No. The GPA100 is purely a transmit amplifier. On receive, the RF signal passes through the unit with a small amount of insertion loss but no amplification. Your received signal depends entirely on the radio's receiver, which in the case of the HL2 and Thetis is where the work actually happens. The amp is transparent on receive.
Conclusion: Does the GPA100 Make the HL2 a Real Station?
The short answer is yes. Before the GPA100 was in the chain, the HL2 was a great receiver and a useful tool for WSPR, digital modes at low power, and antenna comparisons. After adding the GPA100, it became a station. A hundred watts on HF through a decent antenna is where the hobby opens up, where you can hold a frequency in a pile-up, where SSB contacts are not a prayer but an expectation.
The integration between the two is clean and simple. There is no elaborate interface, no custom cable wiring, no firmware configuration. RF in, RF out, PTT via RF sensing. You add the amp to the chain and the chain works. For the price, the GPA100 gives you real monitoring, real protection, clean linear output, and band coverage that spans everything you are likely to operate.
The lack of a built-in tuner is the only meaningful limitation, and it only matters if your antenna situation requires one. If you are running resonant antennas, that limitation does not affect you at all. For the HL2 operator who wants to get to full power without overcomplicating the station, this is a logical pairing.
You can check current pricing and ordering details for the Xiegu GPA100 at Radioddity. They carry it in stock and the link goes directly to the product page.
The Hermes Lite 2 at one watt is a receiver with a transmit capability. The Hermes Lite 2 at a hundred watts through a GPA100 is a station. That distinction matters more than any spec comparison once you are actually trying to make contacts.