QRP Spool Antenna - 40m EFHW

Note: This is a new design.  I had issues with tuning when insing a 3:21 UnUn.  Once I went to 2:14 - it worked great.  If you build and use this, I'd love feedback!

 

This is a very compact low power 40m End Fed Half Wave (EFHW) antenna with an integrated 49:1 UnUn.

 

• It's compact, it fits in your handle easily
• It's lightweight, weighing in at under 6 ounces (170 grams)
• It has a convenient handle for deploying and stowing the antenna
• When stowed, the handle prevents the reel from unspooling
• Simple/easy to print model, prints with default settings (it will be quieter if the sizes are printed on a smooth PEI plate, but it's not necessary) 
• It provides a connection for a counterpoise

To get the small size and light weight, it uses DX Engineering's Stealth Wire (https://www.dxengineering.com/parts/dxe-santw-150).  This is a 26 gauge wire with a 24lb break force.  So, you need to support the reel so undue stress isn't placed on the wire.

 

The 49:1 UnUn uses 2 stacked FT50-43 toroids.  AI suggests that this should reasonably handle:

• 20-25 Watts SSB
• 15-20 Watts CW
• 10-15 Watts FT8/RTTY/digital modes

K6ARK has a YouTube video on winding an FT-50-43 (single).  This is good guidance if you want to see it done. https://www.youtube.com/watch?v=8_flUymoOMo&t=737s  (I initially used his recommended 3 bifilar primary windings and 18 secondary windings (3:21) but had issues tuning it.  When I went to the traditional 2:14 it tuned with no problem.)

 

I printed with PETG and standard settings (plus supports).  I didn't want to increase the weight by adding fill or layers until I knew I needed it.

 

Parts List:

• (about 70 feet) DX Engineering Stealth Wire

   

• (2) FT50-43 Toroids
• (about 3 ft) 24 gauge magnet wire
• 100 pF ≥ 1KV capacitor

• (optional) ¾" (19.1mm) heat shrink tubing to keep cores together and to cover wrapped transformer

   

• BNC Connector

• Banana Jack connector (small length, see pictures)

   

• (8) MR85ZZ mini ball bearings
• (8) M2.5 x 12 mm for attaching sides
• (4) M2.5 x 10 mm for assembling the central hub
• (2) M3 x 12 mm for the crank hinge
• (1) M3 x 18 mm for attaching handle to crank

Because of tight tolerances, I recommend that you calibrate the flow rate and shrink rate for the filament you use to print this.

 

Boost MeI make models that I find useful.  It takes time and filament to take it from "good enough for me", to "worth sharing".

I'm encouraged by feedback and boosts.

Thanks/73