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Remote Antenna Tuner  


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Joined: 2 years ago
Posts: 143
October 10, 2020 9:33 pm  

With the arrival of an FT-891 in the AB0L shack, I needed a tuner. My primary HF antenna, an OCF dipole, actually does pretty well on most HF bands but there's a few spots where SWR runs north of 2:1. My Kenwood TS-590S has an internal tuner that readily cleans up the dipole but the FT-891 has no tuner. After some thought I decided I wanted to go with a remote mounted tuner, up at the antenna feedpoint. This would eliminate SWR induced line losses on the 100' or so of coax to the shack. My napkin math told me this could easily put another 10-15 watts out the wire. 

There's surprisingly not a lot of remote mount tuner options on the market. LDG's RT-600 looked appealing but the $450 price tag sure did not. LDG also makes a lower power RT-100 for $250 and I very nearly pulled the trigger on one until I saw in the fine print that it was only rated for 30 watts on digital modes. I didn't relish the thought of blowing up a mast-mounted tuner in the dead of Winter by accidentally hitting Tx in WSJT-X with the rig set at 100 watts...

I got my Google-fu on and started exploring options. Didn't take long to stumble across David Fainitski's (N7DDC) ATU-100 tuner project. While not intended for remote use, it has an "auto tune" mode that looked promising and the specs were much better - no 30W digital limitation. Getting power up to the tuner wouldn't be a problem. I could use bias tees to inject 12V into the coax, just like the LDG tuners do. Now Dave doesn't sell actual tuners. He just came up with the design and released it to the world to do with as they choose. Turns out many did. A search for ATU-100 on eBay gets all kinds of hits from complete tuners to barebones kits. I think there's even a couple on Amazon. Some more research told me that many of these aren't very good, with corners cut on capacitors and toroids to save a few $. The Russian ones are the good ones, it turns out. Not surprising considering how fanatical Russian hams can be about the hobby. I figured $32 wasn't too bad to throw at another shack project so I bought a kit from anton_ra0sms and waited patiently for the goodies to arrive from Русская Земля.

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I'm not going to go into a lot of detail on this build as it's definitely not a beginner's project. What comes in the kit is a well-made 2-sided PCB and 100% surface mount components, 100 or so in total. There are no build instructions. You have to know how to read David's schematic and build from that. Anton even varies from the schematic a bit as true 50 ohm resistors are practically unobtanium (he provides two 100 ohm instead), and he parallels some caps so they're not an exact match to what the firmware expects. You need to know how to calculate and wind inductors. You absolutely need all the appropriate gear for SMT soldering as some of the parts are as small as 0603. And if you're like me and rush to power it up without noticing the MOSFET with a solder bridge between the gate and source and blow up a pin on the PIC microcontroller, you'll need to be able to dismount, replace and re-flash a 28 pin SSOP PIC16F1938. Ugh...

One last note on building the ATU-100 - even if you buy a pre-assembled board, changing many of the default firmware settings involves editing values in the PIC EEPROM. So, unless you've done Microchip development in the past, you'll probably need to invest in a PICkit programmer. Beware of Chinese PICkit clones (don't ask.)

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The bias tees I also got off eBay. There's not much to them; you could even make your own but at $5 each I opted for lazy. For the power injection point, I simply added a bias tee to my "Magic Box" in the shack. The Magic Box started as a rig front-end protector I built when I got started in HF. As my next door neighbor is also a ham and he regularly runs power, I figured a little protection wouldn't hurt and Phil Salas (AD5X) had a great how-to that I followed. Later on, I got annoyed with KOA randomly appearing across the HF bands so I made a simple tunable wave trap filter from an ARRL article I'd read (sorry, I don't recall the link.) Since there was plenty of room in the project box I added the filter there and declared it the Magic Box. Seemed only appropriate to do the same with this bias tee, especially since there was 12V ready to go.

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If you're now wondering why a remote mount tuner has buttons and a display, the buttons are weatherproof and allow the tuner to be put into a couple of test modes I thought could be useful, even on a ladder. I wasn't planning on a display but after I blew up the PIC I needed one for testing the replacement. Dave's software natively supports a 1602 two line display over I2C. I just so happened to have one in the junk bin. The display actually shows quite a bit of useful info. I'm already noodling how to get this back in the shack. There are I2C signal conditioner chips that claim to be able to regenerate the signals over 100' of CAT-5 so that may be a Spring upgrade.

Operation of the tuner couldn't be simpler. Pick a frequency and key up. As long as the tuner sees 5W and an SWR over 1.3:1, it looks for a new tuning solution. Even works with SSB though tuning will pause when the power drops below 5W. Usually within a few words it locks in a solution. Alternately, a quick key-up in CW, AM or FM, where there's a carrier will lock in a solution in a second or less. As long as the transceiver has a foldback circuit to protect it from high SWR, there isn't even a need to throttle power back when tuning. I did notice the FT-891 protection circuitry really cuts the power - like down to 0.3 to 0.4W when it sees an SWR over 3:1. The tuner quits tuning under 5W causing a catch 22. I'll have to see if that's a problem. The tuner can be modded for "QRP" use by reworking the windings on the tandem match core and changing some firmware settings. I'm hopeful once I get up on the roof and do my antenna maintenance I can get 10, 12, 17, 20, 40 & 80M all back well below 3:1 so this may be a non-issue.

As always, give me a shout if you have any questions. I hope something here spurs an idea or two for a Winter project in your shack!

73, AB0L.

AB0L (Formerly N0KAI)
Kenwood TM-V71A, Kenwood TS-590S, Motorola XPR4550, TYT MD-UV380, Kenwood TH-D7A, BTECH UV-5X3, µBITX V5 QRP