Superlight SOTA Vertical for 40-30-20m with Loading Coil by mfhepp

Last crawled date: 3 years ago

This is the enclosure and winder for a superlight, base loaded vertical antenna, mainly for SOTA operating. The antenna is ideal for use with a 6m fiber-glass or carbon-fiber mast.
It is a redesign of the approach of an antenna marketed by QRPguys and developed by Joe Everhart, N2CX: http://qrpguys.com/qrpguys-tri-band-portable-vertical-antenna.
My approach differs in the following ways from the original one:
I use one tapped coil on a singleT68-2 core instead of two coil in order to obtain the required inductances for 40 and 30 m (more details below).
Instead of two SPST slide switches, I use a single SPDT 3-pole on-off-on switch.
I added a common mode choke in order to reduce the effect of unavoidable asymmetries between the radiator and the counterpoise.
Instead of four 10 ft. ground radials, my design can also be used with
a) one elevated, resonant radial, tuned for each band by winding up the remaining wire, or
b) three elevated, resonant radials attached to each other.
Materials
1 Amidon T68-2 toroid
1 Amidon / FairRite FT50-43 toroid
1 SPDT Switch On-Off-On, e.g. MIYAMA MS-167 (Reichelt: MS 167)
Two pieces of 0.5 mm magnet wire, 100 cm and ca. 55 cm
ca. 20m of SOTA beams antenna wire (diameter ca. 0.5 mm, https://www.sotabeams.co.uk/antenna-wire-lightweight-100m/)
1 SOTAbeams Mini Wire Winder (https://www.sotabeams.co.uk/mini-wire-winders/). Instead of these, you can also use https://www.thingiverse.com/thing:1173824/ but scaled to 66% in the X and Y axis and 100% for the Z axis (3 mm high).
1 2mm plug Hirschmann 930308100 (Reichelt: MBI 1 SW)
1 gold-plated 2 mm connector (like those available for RC batteries, cf. ebay)
1 BNC connector for RG 174
1.6 m of RG 174
Various rubber cable grommets 3.4 - 5 mm inner diameter
Optional for sloped configuration from carbon-fiber masts:
1.8 m of polyester cord, e.g. SOTAbeams hi-viz antenna cord, https://www.sotabeams.co.uk/hi-viz-antenna-cord/
1 aluminium peg, like https://www.sotabeams.co.uk/aluminium-alloy-peg-price-per-one-peg/
Heat-shrink tube
Assembly
Cut the radiator to length; I recommend starting with 5.17 m plus 10 - 15 cm extra for tuning.
Prepare the common-mode choke: Twist 55 cm of 0.5. magnet wire tightly in order to obtain a 50 ohms impedance and wind 11 turns through the FT50-43 toroid.
Mount the BNC connector to the RG 174 cable and put it throuth the two 3 mm holes on the left legt of the enclosure as a strain relief (see picture).
Solder connect the other end of the RG 174 to the two ends of the common mode choke. At this point, polarity does not yet matter.
Use a continuity tester or ohmmeter to figure out which of the two open ends of the common-mode choke are connected to the ground shield of the RG 174. Mark that, e.g. with a black permanent pen.
Mount the black 2mm plug and the SPDT switch and solder the magnet wire marked in step 5 to this plug.
Put the end of the radiator wire through the 1.2 mm holes on the opposite leg of the enclosure (see picture).
Prepare the loading coil: Take the 100 cm of 0.5. magnet wire and wind 25..26 turns, then add a short tap, then another 20..21 turns. Leave at least 10 cm on either end so that you can adjust the tap by adding a turn on one side and removing one on the other.
Now tune the loading coil with a component tester or other device for measuring inductances. The total coil should have 11.5 - 12 uH, the 25..26 turns should have 3.4 - 3.6 uH. It is important to shorten the rest of the coil (i.e. the 20..21 turns) while measuring the inductance of the lower 25..26 turns.
Now connect
a) the open end of the common-mode choke with the start of the loading coil
b) the tap of the loading coil with the left (or right) connector of the SPDT switch,
c) the end of the loading coil and the radiator wire with the middle connector of the SPDT switch, and
d) the start of the loading coil with the remaining connector of the SPDT switch.
You are all set, except for the radials / counterpoise. I recommend a single, resonant counterpoise, which gives greater efficiency but is a bit more difficult to tune. Solder the 2mm plug to one end and secure it with 3.2 mm heat-shrink tube. Tie the other end to the mini-winder in order to not loose it when throwing out the counterpoise from a mountain top. Wind up all wire.
The rubber cable grommets are attached to the end of the radiator wire and allow a flexible connection of the wire to the mast; this idea is from Heinz Baertschi, HB9BCB.
Tuning
Put the SPDT switch into the position in which it shortens the entire loading coil.
Attach the antenna to a 6m mast with an initial radiator length of 5.17 m. Fold back the extra radiator wire.
The enclosure/winder should be attached to the mast with two rubber bands, which will go cross-wise from the four legs of the winder. The enclouse should be ca. 75 - 80 cm above ground level.
Now tune the antenna to resonance by winding up the counterpoise. A good starting point is ca. 5 m. Make sure that the counterpoise is sloping down from the enclosure. In particular the first meters should not touch the ground. The height above ground of the end influences the tuning. Elevating the end ca. 60 cm gives better performance, but can be difficult in the field.
Mark the tuned length of the counterpoise, ideally with something you can feel in bad light, e.g. a knot of cord or rubbert band etc.
Now tune for 30 and 40 m accordingly.
You are all set!
When using with a carbon-fiber mast, the tuning may change, and it may be unstable if the wind changes the distance between mast and radiator. In this case, I recommend a steeply sloped configuration, i.e. holding the winder/enclosure ca. 1m away from the base of the mast. For this purpose, tie the hi-viz cord to the two unused holes in the enclosure, add a loop to the other end, and fix it with a peg. The antenna may require retuning for this configuration.
Acknowledgments
The antenna is inspired by the original design by Joe Everhart, N2CX. The idea to use a single SPDT on-off-on switch and the use of rubber cable grommets is from Heinz Baertschi, HB9BCB. The tiny common-mode choke is inspired by the "pico balun" from Richard Newstead, G3CWI, available via SOTAbeams (https://www.sotabeams.co.uk/pico-balun-1-1-or-4-1-balun-kit/).