One for the chasers, or folk who use their QRP transceivers with an amplifier at home.
I currently have an RM Italy HLA-150V and a HLA-300V that I can use with my IC-705 at home which has a maximum power output of 10 watts.
Are any of the mains-powered higher power amplifiers (500w+) out there designed to operate from QRP power? I note that most of the mains-powered amplifiers require a power input of 60-100w. Are there any amplifiers out there that I can use to get, say for the sake of arguments, 500w of reliable power, but drive it with an IC-705?
I am considering consolidating my equipment quite substantially for both portable and home use. I love the IC-705 interface and the ability to be able to interface it over wifi with a PC for a good workflow when at home. It is an awesome rig. There is a definite benefit to me using the rig both at home and out-and-about. I have a cherished FT-875D but I’m currently reserving this for motorbike mobile use with the ATAS which is a brilliant combination. I can also use my Heil IC headset with the IC-705.
I appreciate I could get an IC-7300 and drive an amp that way, so please indulge my question if you know of an option. Just interested if there are any mains-powered amps. Presumably valve based amps aren’t designed to work with QRP inputs because of the technology employed?
Hi Mark,
My understanding is that there is a US restriction on the number of dB gain an amplifier can have, hence the reason for the big 1 - 1.5kW ones needing up to 100 watts drive. Technically it should be possible to create an amplifier with 10w input and 1 kW output. Of course you could add a 100w amplifier to the IC705 and use that to drive the large amplifier but you would most likely then need a sequencer to make sure that everything comes on in the right order and don’t even dream of doing CW QSK with such a system.
A LunarLink LA22 will easily produce 1500W from 25W of drive. And that’s on 144MHz. The LA72 produces 1200W from 25W drive on 432MHz. A K2RIW amp on 432MHz will loaf along at 900-950W from 10W of drive. These amps were designed to be driven from quite low powers because those low powers were what was commonly available from the transceivers.
As generating 100W RF on HF was trivial using TV sweep tubes, the market for bigger valved PAs was designed around having 60-80W of drive available. When solid state took over in the late 70s, the market expected 100W HF radios and that’s what the solid state sets had to achieve. As the radios still produce 100W, HF amps tended to be designed to expect 100W drivers.
Valved PAs have had their day now as LDMOS amps are super reliable and cheap . You’ll find plenty of such amps that run 1500W with only 20W of drive needed.
Our contest group have a veritable plethora of valved PAs that are capable of “healthy” outputs using 2C39 for 23cms, 4CX250, 3CX800 on 2m/70cms. There are some monster creations crafted from 13.56MHz ISM band amps using 4CX1500s. All now spending their days in the Sunnyland Rest Home for retired Contest Amps. Too much effort to setup, tune up. And… you don’t need a blower deafening everyone as the LDMOS amps are almost silent in comparison.
We activators already have a hard enough time being heard compared to a chaser with 100 watts (and maybe a beam). If you then add 10dB more… there will soon be no more S2S with interesting stations on the summit.
Thanks for all the info. I somehow just failed to find anything other than the 600W broadband HF/6m amplifier using affordable LDMOS devices | QRPblog - which looks very interesting but not sure I can trust my electronics skills to home-brew an amplifier, but for anyone has done so I’d be interested in their experience.
I can see following a search for LDMOS that Gemini amplifiers, UK based, can be configured for 10w drive input: HF Amplifiers Archives - The DX Shop Limited and I have definitely found these previously when looking some years ago.
I thought, given the lack of info I could find, that there would be others interested in the answers.
I can see one benefit of valve-based amplifiers is that they have a better tolerance for some mismatch, is that also the case for LDMOS amplifiers, or do you need a tuner if, on some bands, the antenna isn’t resonant (I have an EFLW which is sort of resonant).
I have no experience of amplifiers other than the RM Italy 12v based ones. They are quite optomistic in their power output, to say the least…
I’d seen these, wasn’t sure about QRP drive, so that is very interesting. How do they make an amp that power that weighs only 7.5kg? That’s insane. Also, do you have the one with the built in tuner - I know that add significantly to the price, but that’s got to be a very flexible amplifier in that configuration?
I bought mine to go DXPedition with my KX3/PX3 pre-Covid…but it worked great in a COVID restricted world at TD15. It’s so light it’s almost ridiculous.
However the finals (LDMOS technology) have a reputation (think KPA1500) for being fragile if you don’t treat them with respect (I do, i.e no crazy SWR…) things can go sideways quickly.
I also used the HobbyPCB that I was loaned by the designer.
If you want to go the kit route this is 5/10w in and 500/600w out. I liked the integrated ATU and generally really liked the amp although it was definitely bulkier and heavier than the Juma. Jim Veatch, the designer, based on my experience will bend over backwards to make sure your unit gets on the air.
Hi Mark, I have the Expert PA with the tuner. It’s a very compact solution, everything in one device, without lots of cables. Also 4 antenna connections and connection for a second TRX. Once set up, everything is automatic. My 1.3K runs very well, I am very satisfied.
For a DIY project, I would perhaps also go for an MRF300 solution, as the transistors are inexpensive and easy to install. MHT1803 is the low-cost compatible variant, which can be used on HF but not on VHF. 2 transistors for 600 watts cost about 60 USD/EUR at Mouser etc.
I’m not into brute force chasing. For a long time I chased with my FT-817, considering that it was more sporting to work the activations with the same power. I’ve gone back to the main rig as my aging eyes have trouble with the postage stamp screen on the 817 but still chase with reduced power. To me it is absurd to chase a flea power rig on a summit with a high-powered amp, but each to his own.
The bulk of the weight is in the heatsink (aluminium), heat spreader (copper) and the metal case. PSU being a switched-mode will be light probably well under a kg. But it’s amazing compared to PAs of old. Denis G3UVR’s Henry Temp 2002 ( 2x 8874 tubes, 144MHz 2kW) weighed 37kg
I have two suggestions for you, both of which I have physically owned & tried:-
Acom 1200S:- This is what I currently have in my shack. I drive it with 5 watts from my Flex 6600, which gives me around 300 watts out. Up that to around 10 watts, which I’ve only tested into a large dummy load (for obvious legal reasons), I estimate around 700 watts on the output.
The cheaper option is an RM Italy BLA-350:- Not quite as much power as you said that you wanted but this was what I was using before I upgraded to the Acom, and it’sabout a third of the price if you don’t want to sink too much money into an amplifier.
It’s advertised as a 350 watt amplifier but that’s a bit optimistic. 5 watts in gave me around 120-150 watts output. I used to drive it with 10 watts, which gave me somewhere around 250-300 watts. Apparently the maximum drive is around 12-15 watts but ive heard stories of these amps throwing in the towel if you drive them too hard.
The BLA350 is also fussy about SWR. The protection on mine kicked in at around 1.6-1.7, so you really need a well matched antenna with an SWR below 1.5 across all of the bands that you plan to use it on.
The BLA-350 is obviously a budget amplifer but it’s great if you drive it properly. Most of the bad reviews I’ve read seem to be from people driving it to the max or trying to use it with antennas which are not perfectly resonant, usually resulting in them blowing it up! Drive it sensibly and it’s a brilliant little amp.
I’m not saying I’ll be using KW when chasing SOTA, it’s just that people who spend a lot more time with a home-based chaser setup with have much better knowledge about QRP driven amps than pure activators. I’ve also come to realise that I get much better quality answers to my radio related questions on this forum than anywhere else.
Also, with activators that do chase, if they do employ an amplifier then they are probably much more knowledgeable than say a contester, with regard to amps that you can drive from QRP rigs. I suspect the same would be true with dxers. Even if we are just talking a couple of 100 watts.
The reality of my current QTH is that I have a far greater problem getting my average noise level down. My shack is in the attic and not easily moved, so I’ve been researching what I can sensibly do to provide a proper RF ground. I found this article (see section Alternative 2 -The RF Suppressor Ground System that I was unaware of) - it talks about a coax based grounding solution suitable for shacks far away from the ground that I think will be my best bet.
All problems that I didn’t have when I ignored operating from home. I don’t know whether it is the current solar cycle, or a local change in noise level, but I am definitely hearing a lot more stations than I used to, but background noise is still S9 typically with my EFHW (which unfortunately I’m stuck with).
My shack is the smallest bedroom and a normal earth would involve a long lead which would cause problems as it is likely to be near resonance on some bands. The mains earth is best avoided as a source of noise. The best solution in such a case is a so-called “artificial earth”, IIRC MFJ sell one, but they are not too difficult to construct. Basically they are just a single radial tuned by a series tapped coil and variable cap to place a zero RF potential at the rig. A well-known design is by SM6AQR, which adds a tuning indicator.
I would not try to earth your station when its in an attic. Rather use mains earth, but isolate it at RF. Build one of these Clean Up Your Shack – 2019 | GM3SEK's Technical Blog and run all radio gear from it, including PC if its connected the the radio. Plus serious ferrite chokes on your feedline to the endfed. You will find your antenna much quieter.
A significant part of the noise your hearing will be coming up the mains, along the chassis of your rig, out your coax shield then round your unun and back into the receiver. Modern ferrite like the #31 used in this filter give a high impedance, but significantly have a significant restive part which absorbs the unwanted crud.
Ian’s powerpoint is excellent and explains all this in detail, link is in the page
If the antenna is OK, there is no common mode current and no RF in the shack. In my opinion, RF earthing is always the worse option, which is why I don’t attach any importance to it in my shack. But I have installed a lot of ferrite as common mode blockers. This not only means that I have no problems when transmitting, but also that reception is automatically freed from RFI. It’s better to eliminate the cause than the effect.
I’m using the MFJ artificial ground box now. What I’m concerned about is running bare counterpoise wires from the artificial ground across my desk and out of the window to form the resonant counterpoises. Is there a way of tackling this?
