Several months ago, my long-time friend, a Kenwood TS-850SAT, quit. This had me looking for a good, general replacement. I’m trying to stay pragmatic, and since I no longer have beams at 100 feet, spending more than $2,000 for a rig is just not part of my current value system. I landed on the Yaesu FT-991a, which you can read about in a previous post. It offered an all-access digital-audio interface through a single USB connector, supports virtually every radio mode known today, puts out 100W, and goes from 160M to 70cm. Pretty great!
Research and Acquisition
In this search I could not avoid looking into Software Defined Radio (SDR). There are some really cool things out there. For example, the latest Kenwood and Icom, not to mention the ‘mac-daddy’ Flex Radio Systems’ offering. The FT-991a is not an SDR. It definitely has DSP augmented tuning and filtering and a slick spectral/waterfall display. But let’s face it… the screen is pretty small, and while it’s highly functional as a menu/button interface, its panoramic adapter is not very practical as a tool for hunting signals at the bottom of a solar cycle.
As a result of this contemplative process, I arrived at a “pragmatic solution” which satisfied my use-case: an SDRplay as a Pan Adapter. I was quite surprised with how well the SDRplay RSP1a worked as a 2nd receiver and quickly became addicted to the large spectral-waterfall display. I attempted to use it with an RX-antenna switch and had some success, but didn’t like the 3+ dB of front-end signal losses from sharing the antenna during receive. In addition, I didn’t care for the somewhat high (-20 dB) signal going into the front-end of the SDR during transmit.
In due time, the final outcome became clear. I needed to tap the IF of the FT-991a in a low-loss manner and bring the IF out to the RSP1a. (Maybe Yaesu will put this output on the FT-991b?) Through some good, old-fashioned Google searches, I came across a perfect little solution: a buffer-amplifier from G4HUP (sk). The PAT70 (70MHz) version is compatible with the IF of the FT-991a and other similar-IF rigs. It is available from SDR-Kits.com in the UK (I was stunned at how quickly it arrived in standard air-mail).
Modifications
Finding installation instructions specifically for the FT-991a was a challenge. The upside is: when Yaesu upgraded the 991 to the 991a, they made it easier to tap the IF and find the power supply. (The mod to add this kit goes literally on the back of the pan adapter that Yaesu added.) The best information I found was from KU0D (below) which was a huge help!
There are very few connections to make: the IF-input tap, 9 volt DC, and ground. The actual IF connector used to connect the off-board SDR is simply a coax out the back of the rig with an SMA connection.
Firstly, I located the ground and placed a lug under the standoff that holds up the Yaesu pan adapter PCBA (these mods are all done from the bottom of the rig). Once that was in place, I put the PCBA back into place for the other connection points. The next two connections were 9VDC and IF-input to the G4HUP board.
Note the Kapton tape! The FT-991a bottom cover has a conductive sponge that presses down on the can where I double-sided-taped the G4HUP buffer/amplifier. This tapes prevents the short circuits that would likely occur without it.
The final, and most scary bit, is to drill a hole in the back of the rig. I opted to use a pre-made cable from Amazon to make the final journey to the outside of the rig. Much care was exercised to insure that no metal bits ended up in the bowels of the rig.
Software Interface
I really hoped to use the SDRplay SDRuno software — which is amazing with the RSP1a, but it lacks the ability to reverse the tuning between the VFO and the LO. Depending on the mixing chain stage and which side of the mixer you’re on, the IF frequency may move opposite (down) as the VFO signal moves up. HDSDR software has settings for this as shown below:
Note the IF frequency is actually 69.450Mhz in the FT-991a. Also note that the “Mirror RF Spectrum in general” is checked. I have reached out to SDRplay and they committed to having development look into supporting this feature in the future. But until then, HDSDR is fine stuff and works well for this setup. (Both SDRuno and HDSDR work with my go-to Ham Radio Deluxe setup, which I have posted in “Ham Radio Deluxe + HDSDR + FT-991 Putting it all Together“.)
This is an inexpensive add-on that brings big value to the HF-hamming experience. The benefits of SDR, including advanced filtering if you choose to listen to your PC audio, as well as all the goodness the FT-991a brings. No RX-antenna switch needed. Nice and clean.
Hope you find this helpful,
73
