Base-loaded short Vertical Antenna for 20m

Small Vertical Antenna

Intro

DIY version of a base loaded very short vertical antenna like the Elecraft AX1/AX2/AX3. This model is highly inspired by W1RCP's version (thanks a lot - also for the video instructions for the build!). The main difference is that my telescopic antenna has an M4 thread and the lids of my tube body do not need any glue, as they snap in and hold by themselves.

A new model for a 90 degrees BNC plug allows to plug it directly into you KX2/KX3/Icom 705 etc. without the need of a BNC 90 degrees adapter!

Also provided is the parametrized Fusion360 model. So adaptations (different tube length, diameter, nut sizes, … should be easy to change!

I also used a bipod to attach the antenna to my KX3. The bipod clip and a small knob is also part of this model. An M4 screw + 2 nuts hold them together.

Antenna Theory

My telescopic antenna is 1.4m long and I wanted to get a resonance for 20m band at 14.2MHz. Calculation of the coil was done using the calculator from M9UKD with the params:

- Frequency: 14.2MHz - Length of vertical element: 1.4m - Coil position: 0m - Wire diameter: 5mm (average of 10mm on bottom and 1mm on top of the telescope antenna) The resulting coil should be 7.9µH.

Using a second online calculator from circuits.dk for single layer air cores I calculated the size of the coil (number of turns):

- Required Inductance: 7.9µH - Coil Diameter: 18.55mm (18mm tube diameter + 0.55 wire diameter) - Wire diameter: 0.55mm Results in a coil length of 12mm and 22 turns.

An antenna like this needs a counterpoise. I used 4m wire (as recommended by elecraft) and after some experiments with various number of turns and antenna length I got an SWR of 1.2 for 22 turns and a measured inductance of 7.2µH - please note that the SWR changes a lot when the counterpoise is changed, so I do not give a lot on the SWR here! The automated tuner in the KX3 manages an SWR of 1.0!

Practical Usage of the Antenna

The antenna with 1.4m length of radiating metal is by far not a DX antenna! But in some circumstances (like on a cold/windy SOTA summit where you do not have space/time for a full sized antenna) this antenna might make the difference between some QSOs and no QSOs! I managed to get contacts to Spain, Sweden, UK, Croatia and Poland from a mountain in Switzerland! ( HB/VS-125 ).

So I am very happy with the solution knowing that it does not replace a bigger/better antenna but as a quick setup for some quick QSOs!

Building

To build this antenna, the following parts are needed:

- M4 screw + M4 nut to hold the antenna - BNC plug with a hexagon nut to fix (from amazon or aliexpress) - 0.55mm diameter coil wire (or different diameter, adapt the number of windings, use the formula!) - Telescopic antenna 1.4m, M4 thread (from aliexpress) - M4 screw + 2 M4 nuts for the bipod (press the M4 nut into the hexagon hole) Building the coil itself takes around 15min. In doubt use more turns at the beginning, measure the SWR/inductance and if needed, remove turn by turn.

- Remove the coat of one end of the coil wire. - Put the end though one of the small holes on the tube body near the antenna end. - Then put the wire though the hole of the antenna. Screw the M4 screw from inside and fix the wire with the nut. - Put the large lid on the tubes opening and start to turn the wire around the tube. After the desired number of turns cut the wire (leave some centimeters for soldering). - The open end of the wire goes in one of the body holes. - Add the BNC plug and fix it with screw. - Cut the desired length of the wire and scape the coat of the wire. - Solder the end to the middle of the BNC plug. - Insert the small lid (just push it into the open space). Printing

I printed them all in PLA without any support nor brim. Orientation of the lids was done like a turtle on its shell.