Before starting the construction of this type of antenna, consider the following advantages and disadvantages.
- It’s a monoband antenna
- For any mods or relocating position you need and antenna analyzer
- Depending the parameters, the Bw can be very narrow for your purposes
- The variable capacitors need to be with 2.5-2 mm spacing between plates for 100 watts.
- The tuning point is very sharp, so a multiplier system for variable capacitors is welcomed
- It’s a small antenna, maximum 3 meters lenght for 40 m band
- No radials required
- You can use on ground radials around the mast for improving the soil parameters
- Can be made in portable pack version for mobile operations
- You need only free obstructions position, not necessary high mast (2-4 m)
- The bandwith is enough for rtty activity (more than 100 khz for SWR<1.5)
- The preliminary results shows you can work easy all Europe countries (YO4DFT – I am located in KN44HD
So, let’s find some materiales for the Hairpin antenna made with aluminium tubes. What I found:
- 2 old elements from a vertical Eco antena (diameter from 45 to 35 mm total lenght 2 m, with posibility to insert one to another to vary the total lenght)
- 4 aluminium elements for old military stock with connectors ( 1 m lenght, 30 mm diameter)
- Aluminium strip 20×1.5 mm for top connector
I decide to use the folowing parameter: 25 cm spacing and maximum 2.85 m lenght.
Building the top connector. Use metal screws (the black one) to obtain a good electrical connexions between tube and connector.
If you are using separate elements like me (not full tube, 2.85 m one piece) you have to cut a longitudinal space and use againg metal screws to obtain a good electrical connexion between elements.
I am using 2 old double sections variables with mechanical multiplier, modified (less plates) to obtain 2 mm spacing. Minimum capacity 10 pF, maximum 52 pF (each section).
Find a good enclosure and a piece of isolating material, with enough thick (15 mm in my example, I found an old textolit bar painted in blue !) to sustain the tubes. Use some Cu wires with big diametres (2.5 – 4 mm) for electrical connexions. Depending the parameters, it can be possibil to need both sections of variables to be connected (if the capacitance need to be more than 50 pF) or only one.
Find the proper mounting metal flanges. Use a common mode shocke (in my case, 20 turns of RG213 U equivalent on 12 cm diameter (on a piece of water drain pipe). Use an insulated tube for tuning the variable capacitors (the white tube in the photo). Connect the equivalent lenght of coaxial cable (as you are using for the trcvr) and start the tuning. Take care, the proximity of human body can influence the measurement. During the tuning procedure don’t touch antenna or mast. Use an insulated tube for variable capacitor (20-30 cm) to avoid your body influence.
With patience, start the tuning. After you shall obtain the resonance and R=50 ohms at desired frequency (most probably you are climbed on a ladder near the mast), step down and re-check the values at 3-4 meters away from antenna. Check the Bw for your specific needs. Re-tune if you are not satisfied.
Go into the shack and re-check the values obtained. Most probably you shall see some differences. The most important is the resonance frequency to be the same (or with minor differences).
Congratulations, you have an antenna ! But, don’t forget, is a small one, is not a miracle. You can make QSO’s 3000 km around you, more or less, depending the obstructions, depending the soil, etc.
Good luck and 73’s !
24 hours WSPR tests, 1 watt power
(14.03.2021 09:00 UTC – 15.03.2021 09:00 UTC)