3D Printing HF Portable

Build the K1FM-Loop

The latest version of the K1FM-Loop is electrically similar to the previous one, but it is designed around off-the-shelf parts or, more generically, items you can readily order online.

This antenna relies – heavily – on 3D printing. Acquiring the non-printed parts and assembling them should be fast and easy as no reworking at all is necessary. The needed tools are screwdrivers, wrench keys and a soldering iron. You also need a 3D printer, of course, or at least access to one.

Lets’s start from the parts list:

  • 3D Printed parts
  • Exciter Loop Splitter PCB (available shielded or unshielded – see below)
  • Variable Capacitor – Oren Elliot Products P/N S2-332x2P (21 blades, 2 gangs of 9.3 – 332.4 pF used in series)
  • ~40 inch selfie-stick
  • gopro bike mount
  • 15 inch photographic tripod
  • PL259 to PL259 LMR400 cable 125 inch long (for the radiator loop)
  • BNC-male straight to BNC-male straight LMR240 cable 25 inch long (for the exciter loop)
  • BNC-male straight to BNC-male straight RG316 cable ~6 ft long (for the feed line)
  • BNC-female Molex P/N 0731000105 (3 pieces for the Exciter Loop splitter)
  • SO239 panel connector (2 pieces for the capacitor enclosure)
  • MF-A05 Knob, 1/4inch shaft
  • Socket Head Cap Screw, M5-0.8mm Thread, 12mm Long, Alloy Steel, Black Oxide (8 pieces)
  • Button Head Socket Cap Screws, 6-32 x 5/16″, Black Oxide Alloy Steel (3 pieces)
  • Hex Socket Head Cap Screws Bolts, M3-0.5mm, 9mm Long, Alloy Steel , Black Oxide (8 pieces)
  • Hex Self Clinching Nuts, M3-0.5mm (8 pieces)
  • 18AWG electric cable ~5 inch
  • M3 Ring Cable Lugs Terminals (4 pieces)

The antenna is composed of three parts that I am going to describe separately. They are:

  1. Capacitor assembly
  2. Radiator / Exciter assembly
  3. Supporting structure

Let’s see them one by one:

Capacitor assembly

First of all, you need to print the necessary 3D printed parts. It might be opportune to print the Dial Bushing in a different color, in order to make more visible against the rest of the box.
I printed the box in PETG because of the enhanced mechanical and thermal characteristics. ABS would also be opportune I think, or even PLA (provided you are careful not to leave it inside of a burning car).

As the print goes, you can start working on the other parts.
All you need to do is solder a piece of cable on each gang terminal, on opposite sides. Terminate the cable extensions with a ring contact, like so:

Optionally, you could solder the same extension on the center connector of each SO239. This will allows to use the center conductor as part of the radiator (Do I think this is useful? I don’t, but I’m going to do it to avoid the complaints). Here are the connectors, ready to be used:

Now you can begin assembling the box.

3D Printing HF Portable

K1FM Magnetic Loop: 2020 edition

I really, really like magnetic loops.

After making a bit of a splash with at the Dayton Hamvention and publishing how to make your own on QST I decided to improve the design a bit more, so here I am with the latest iteration of the K1FM Magnetic Loop.

The new design improves the older one in a few key aspects:

Entirely 3D printed
Readily available, made in the USA variable capacitor
Easy to assemble

Basic geometry and main electrical characteristics remain unchanged:
– 4o to 10 meters
125 inches radiator
165pF variable capacitor (dual gang series)
QRP power (it actually can handle a lot more but, as you know, using a magnetic loop in close proximity with more than a few Watt is against FCC guidelines and potentially dangerous. Don’t do that)

Fully 3D printed, capacitor enclosure

The new enclosure requires no rework in order to be mounted. It is now quickly detachable by using the same mount type as the radiator/exciter assembly. This makes the antenna even more portable and, at the same time, opens the possibility of using other support types in place of the selfie-stick (fishing rods, PVC pipes etc.).
The capacitor uses a 3:1 planar reduction that, combined with the a fairly large knob, makes tuning quick and easy. A 3D printed indicator (blue) shows the current shaft position: just by looking at enclosure you can tell where about you are currently tuned and act accordingly when it’s time to tune again.

Exciter loop splitter

Thanks to a new splitter design, the exciter loop is now conveniently made out of a standard LMR240 BNC-male to BNC-male pigtail. The splitter also allows the possibility of using multiple radiator/exciter assemblies in order to, for example, work 6 meters.

Radiator loop mount

I also redesigned the radiator mount to follow the different bending radiuses of the radiator and the exciter loops. Both cables now snap-in with just the right amount of force, therefore zip-ties are no longer needed.

The new loop looks great and works better!
I’ve decided to call it K1FM-Loop. If you want to build your own, here are the instructions to do that

HF Portable

Portable QSO with Japan!

Last Sunday I took my Portable setup to the river, as I often do. Propagation was a bit short with Europe totally closed when suddenly – to my disbelief – JH4UYB popped up at S8.
I had heard Japan twice before with the Loop, but never as strong as that.

After beaming about 330 degrees (the same direction I normally use to work W7) I immediately started calling. Incredibly, part of my callsign was picked up right away! The rest of the QSO, however, took more than 3 minutes to complete thanks to the perseverance and the professionalism of Masaki, the Japanese operator.

HF Portable

QSO from the Pier

Once again in Italy, once again at the beach. The weather was a bit cold but propagation warmed me up good.
Heard a lot of US stations but couldn’t make the pile-up. On the other hand, I had the pleasure of a QSO with OH73ELK.


K1FM WSPR Transmitter (aka: el cheapo Antenna Testing)

I recently built a couple of identical WSPR transmitters to be used in direct comparisons between magnetic loops and other antennas, or even between various kinds of magnetic loops. They are now available on Github.

The transmitters are based upon the Si5351 clock generator and their RF section was inspired by the excellent Zachtec’s products. An onboard GPS receiver is used to automatically provide timing and location. In the absence of a GPS fix a button can be used to start transmitting. The same button can also be used to cycle between 5 predefined WSPR frequencies. There are no onboard filters therefore external filters must be used in order to maintain compliance with FCC rules.
SMA connectors provide connectivity for both the HF and the GPS antennas.

The firmware is based upon existing libraries and was created around version 1.0 of the board. Version 1.1 introduces an important extra feature: auto-calibration.

I also built another magnetic loop, identical to the one I already have. Together, they will serve as a testbed for my comparison tests.