Hairpn modified antenna for 40 m band – improvments and updates (h=5.5 m above the ground)

For ease of adjustment I found that it is necessary to mount a gear for variable air capacitors.

SWR graph - 7050 +/_ 50khz 

Resonant frequency = 7071 khz
Bw (swr<1.5 = 268 khz) Bw (swr<2 = 484 khz )


Hairpin modified antenna – estimated performance based on WSPR data analyse

So, we have an short antenna, easy to build and install. It was working in WSPR mode more than 3 days. It’s time to find the performances… and to see if our expectations are to big, or what can we improve.


First analyse is a simple one. Just go to VK7JJ site (http://wsprd.vk7jj.com/) and follow the instructions. You can see when you see a lot of useful informations about your antenna, the antenna location and propagation.

It looks fine… it’s only an example of useful info. But is not enough. The chart say you can work from YO stations from VK, JA, North and South America. Yes, using WSPR mode.

What about other modes ?


Let’s go to KP4MD site (https://www.qsl.net/kp4md/wsprmodes.htm#application) and download the mode-comparison excel file.

Using your WSPR data you can find if you can work a specific station in another mode (CW, RTTY, SSB, etc.).

Let start with the bad news…

After you shall download and process the data, you shall find that the probability to work VK7JJ in RTTY or SSB mode is very low to very, very low. Red colour is not good….

Let’s check other DX stations

Sorry, no K1ABK, no JA5NVN.


OK, dont panique, let see what can we work.

TF4X                        3927 km


MM7BWK              2544 km


OH3FR                     1867 km


So, if you want more, try to find if you can use a higher position or re-locate the position, or improve soil parameters, etc.

For example, my location is very unfriendly regarding the obstructions around me. As you can see in the graphs (from 1st part, VK7JJ site), the existing freeway to 310 degrees azimuth is confirmed by number of spots and the obstruction from 230 degrees (a new big house, etc.) is also confirmed by no data/spots on that direction.

Hairpin elevated antenna 40 m band – tips & tricks

 

Before starting the construction of this type of antenna, consider the following advantages and disadvantages.

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

Advantages

  • 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)