Field Day 2025 at Filtvet

	Summary These days we are preparing for NRRL Field Day 2025 at a cabin in Filtvet, Norway. We will be 6 guys with origins from LA1K using the callsign LA8G/P. Field Day is a large ham radio event all about making as many contacts as possible in 24 hours, while not being dependent on existing infrastructure. This means we need to deploy the whole antenna/radio setup in less than 24 hours and provide energy from generators and/or batteries.

What and where

We are preparing for national field day from a nice radio QTH:

A cabin close to the Oslo fjord at Sevjedammen, Filtvet, Norway.
76 MASL. Seen from south, the increase in elevation up to the cabin is very steep.
Wide and open view towards east and south

Simulation of 2 meter coverage from the cabin. Nice takeoff towards south.

We will use the call sign LA8G/P.

Who?

LA2USA Espen
LA3JPA Jon Petter
LA3DPA Knut Magnus
LA7VRA Henrik
LA8DRA Kristoffer
LA9ARA Per Øyvind

Rigging

Rigging was efficient and pain free, although the beam was slightly misaligned ;)

Antennas:

10/15/20 meter Cushcraft A3S tri-band beam rigged close to the shack
2 meter 5 element yagi on a short mast
6 meter V-antenna on a short mast
Two large masts and a tree to suspend two dipoles, one on 40m and one on 80m
Vertical multiband antenna

Rigs:

Icom FT-1000MP Mark V, used on generator for some time. Inconvenient supply voltage makes this unsuitable for battery power.
Icom IC-756 Pro III, used on generator and later on 4S Li-ion battery with diodes to drop voltage
Icom IC-7000, used on 4S Li-ion battery with diodes to drop voltage
Yaesu FT-857D on HF, used on 3S Li-ion battery

Power sources:

1000 Wh 4S Li-ion battery with a 35 A diode bridge rectifier to drop voltage ~1.2 V
915 Wh 8S Li-ion battery with 13 V buck converter
260 Wh 3S Li-ion battery
Multiple smaller 4S batteries
Briggs & Stratton P3000 Generator

Result

Preliminary result is 410+ QSOs. Score has not been calculated yet. We ran mostly SSB phone on 20/40/80m, but had QSOs on most HF bands. We ran some FT8 on 6m and 2m. We had no operators during the night and some had to leave early, so we are happy with the result considering this is mainly a social event for us.

TBD

Preparations

Fixing the A3S HF beam

The center bracket and center aluminium tube for the director and reflector was missing, and the antenna needed a full overhaul and adjustment.
First thing to do was slotting the new tubes to allow them to be compressed with a hose clamp. A Dremel cutting disc in a CNC router was perfect for the job.

Next up, new mounting brackets were made, all traps were tested and the antenna was tuned. In lack of a mast the antenna was lifted barely 2 meters off the ground to measure it with an antenna analyzer, but the antenna definitely appears to be working on all three bands.

Making another lithium battery

During field day, the rigs cannot be powered from the public power grid, so we need quite a few batteries and a generator. The FT-897D doesn't operate efficiently on very high voltages up to 14.5-15.5 volts, but is super efficient on lower voltages and provides 100 W output power all the way down to 10.5 volts. This makes it perfect for powering from a 3 S Lithium-Ion battery or similar, so I decided to make one for field day.

The cells are prismatic 25 Ah Lithium NMC cells from a VW E-up electric car, which needs to be compressed. The BMS is a generic 3S BMS with appropriate high and low voltage thresholds for the cells, short circuit protection and balancing. The battery was tested with a water heater as a dummy load, and the capacity was measured to be 23 Ah. Peak MOSFET temperature was 60 degrees celsius with a continous 14 A load.

Rig interface for DT297 headset and FT-857D

I have a Beyerdynamic DT297 V.11 headset which has a microphone amplifier built into the headset. The audio quality is great, but the mic circuit requires a 14 V or more bias voltage, and provides line level output. The mic port on the rig only provides 5 V, so a boost converter was used to increase this to 14 V, and the line level signal needed to be attenuated to mic level. The box also provides a jack for a foot pedal PTT switch. I was quite happy with how I mounted the otherwise hard to mount RJ45. It has a flange, so I made a bracket to compress it from the rear side.