Hi there, I am a fan of QRP operations, following the “less is more” motto, and I like minimalist equipment. I normally operate QRP CW with a FT817 ND and a vertical or End Fed Half Wave portable antenna. I have also used in the past a YouKits HB-1B qrp Rig. I do sometime SOTA or other outdoor activities that normally requires some physical effort to get to the right location. Why am I boring you with this stuff? Well, what annoys me is doing some effort and not making any contact and it has happened sometime in the recent past due to poor band conditions. So what? Well, I decided to set up a portable station, being as light as possible to operate @20W on field for a typical 2 hours hike to the hills, countryside or mountains, 2 hours of operation and then back home.
I choose a Yeasu FT 891, it was a bargain, I found second hand unit for about 400€. I wanted to give a chance to the 20 Watt Xiegu X108G radio but it costed more than the Yaesu and there was no local dealer to manage any warranty issue.
I did the math to estimate average current draw in a typical scenario: 30% TX, and 70% RX in CW. Keep in mind that conversational CW duty cycle is estimated to be 40% , i.e. while transmitting, only 40% of the time you are keying down (Google around to find info on CW duty cicle). Let’s use 50% as duty cycle for our calculation to be conservative. According to some tests (again, youtube is your friend) the FT 891 draw 1 A in RX and ca. 8 A in TX @20W. Pulling everthying together it means that in RX the average current draw is 0.7 A (1A x 70%) , the average rough estimate current draw in TX@20Watt is 1.2A (8A x 30% x 50%); in the end, an average current draw of 1.9 A (0.7+1.2) is what we need to deal with. Considering I want to operate for a couple of hours I ended up looking for a 5000 mAh LiPo or LifePo battery. I found, again, a bargain on hobbyking and got a 5000 mAh Zippy LiPo 4S1P (yes I know not a LifePo) for 27€ .
LiPo batteries normally have a full charge voltage of 16.8 V (4.2 V per cell x 4) which is above the max operating voltage of the FT 891 which is 15.8 V (13.8 V + 15%). A DC-DC step down converter is necessary to drop the voltage to the right level to avoid damaging the radio.
This is what I found on amazon: a 150 W step down converter for 19 €
Lipo cells are very sensitive to discharge voltage level, each cell cannot normally be discharged below 3 Volt without compromising the performance and damaging the cell itself. A battery alarm that monitor total Lipo Battery voltage and voltage of the individual cell is mandatory to make sure to keep the battery in the right voltage operating range (3 to 4.2 V for each cell). You can programme the minimun cell voltage limit , and I set it to 3.2 Volt.
This is what I found on hobbyking for little less than 3€.
The battery came with HXT type connector, a kind of banana plug male/female combination, therefore I decided to standardize plugs and cable buying a bunch of HXT type connectors.
I bought a brand new 4 pin plug fom ebay to prepare a new cable for the radio.
Eventually , I prepared all the necessary cabling to power up the radio with the 5000 mAh battery.
Fine business!, a first test on the bench proved everything is working fine – smoke test passed! .
Now I have to test the whole system in the field (starting from my backyard :-))). STAY TUNED!