Endfed trapped dipole for (15) 20, 30 and 40m
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| Pointed out in the picture are: the feeding point (bottom right), the 20m trap (at the top of the pole), and the 30m trap (lower left) Caravelas Mt, SOTA ref CT/TM-012 activation, august 2024 |
After a lot of SOTA activations and a lot of antenna types been used, at the end I decided to design and build one which meets some points:
- Easy and quick setup,
- ATU not needed
- lift up and lowering for changing each band not being necessary
The llast condition should be being able to handle a max of 20w power output, more adecuated for the equipment used on SOTA activations.
After having tested some antenna types -long wires, random, ground plane verticals, center fed dipoles, doublets, linked dipoles, etc.- and a lot of reading around Internet, I chose a resonant endfed (EFHW) trapped dipole design. The bands should be 20, 30 & 40m (plus 15m as the 3rd armonic of 40m) as the most common bands.
Even the design of a linked dipole is quite easier than using traps, then, during the activations, you are forced to stop the operation, leave the operating position, drop the antenna down, change the links configuration and sit again, every time you want to change the band. This is specially hard when you are searching for S2S QSOs on different bands.
The traps design is more complicated, that's true, but the effort is made only once. Then, while activating, the ease of operation will be hugh. Band changes will be "automatic" -you will not need to move or stop operating- It will also diminish the damage to the pole caused for its manipulation.
Being a endfed antenna its construction will be easier, as you will only need one trap for each band (not as on the center fed dipoles, were a trap by leg is needed)
The endfed antennas need a shorter coaxial cable than the "normal" dipoles. This will make the antenna weight lighter and the coax attenuation smaller.
Amongst all the designs I have seen and studied, I found one that caught my attention: an EFHW for 20 30 and 40m, no tuner needed. I am talking about Dan's AI6XG one (https://www.ai6xg.com/post/trapped-20-30-40-meter-efhw-antenna)
After reading it carefully, I decided to use his design, but after some slight changes. Dan's design does not use the most adequated toroids neither by material nor size. I wanted my antenna to be able to manage up to 15-20w too, so I thought the toroid sizes should be larger than Dans'. So, I will use a FT82-43 for the impedance transformer on the feeding point, and T68-6 for the traps (as the material 6 is more suitable to the trap frequencies -13.9 and 10.0 MHz (see https://toroids.info).
When building the antenna, some points must be specially cared. The most critical is the resonance frequency of the traps.
There are two types of trap designs:
- You can make them resonant on a middle point between the two bands to be isolated (f.i. around 12 MHz on an antenna for 20 and 30m).
- Slightly below the LOWER frequency of the HIGHER band. That is, for an 20 & 30m antenna, the trap should be tuned at 13.9 MHz. If the antenna should be trapped at 30m -10.100 MHz, the resonance should be at 10.000 MHz.
The first design is easier, but then the lenghts of each band will be affected for the other band's lenght, making the adjustment quite harder. As we are talking about a three bands antenna, the matter will be larger!
Tuning of the traps is quite critical, and as far as the capacitor values commercially available are what they are, will we have to play with the coil sizes and turn numbers. It must be done using a well calibrated VNA or a dip-meter (if you can already find one)
Before adding or subtracting turns from the coils, you can try putting the turns closer or wider. That will change slightly the resonance of the trap.
Be aware that after ending the coil adjustments, you should keep it fixed to avoid changes of the resonance. Glue, wax, or heat shrink tube can be used but, caution, this can change a bit the frequency. Check it before cutting the wires.
Trap assembly can be done in several ways: on a fiberglass circuit, on your own 3D design, a piece of plastic, or even on the air (with short ropes for strain relief). Just take into account that then the traps will suffer while mounted on the field or when you mount & dismount the pole.
Regarding the impedance transformer, even the theory says that, for an 1:49 ratio, 21 turns should be used (with a tap at turn 3 for the secondary wiring) starting with 3 or 4 turns more is recommended, and then after having adjusted the 3 wires (1 of each band) try to add or remove until you find the best SWR ratio. The secondary will be always at turn 3.
The said lengths of the wires are only approximated. As always, when working with antennas it is better starting with a larger length and cut than finding that we are short for the wanted frequency.
Contrary to we have said of a minimum SWR adjust for the Z transformer, the different lengths of each wire must be made to be resonant at the given frequencies, no matter which SWR ratio at. This should also be made using a VNA.
The setup must start with the upper bands (20m in this case with the 13.9 trap connected at the end of the wire), once this done, we will connect and adjust the 30m wire with the 10.0 MHz trap.
Wire length adjustments must be done while the wire is set at its final position (that is, angle and height). For this, we can use a longer wind wire that later we make shorter as more segments are added.
If we made the upper bands adjustments correctly, they should not be affected when adding more segments. If it is, probably the trap design is not ok, check it again.
Regarding the position of the antenna/pole on the field, please note the following:
- The impedance transformer should be, at least, at 1.5 - 2m height from the ground. Many antenna designs connect the antenna right to the rig at ground level. This could be more comfortable, but the ground losses will be larger too.
- The far end of the wire, that is, the segment of the 40m band, should be at least 4m height. Otherwise tuning the antenna for 40m could be impossible.
- The antenna could be slightly directional, on the direction of the wire, mainly on the upper bands, as the configuration is something like a sloper design. (view the picture below)
- Regarding the counterweight, we have several options:
Use the coaxial wire itself. This should be at least 1.5-2m long for the feeding point to be at 1.5m or higher. If we make it a bit longer, say 2.5-3m, it will work better on the lower bands.
If when we use this option we have CMC (RFI at the rig, CW keyer, or mic) we can put a 1:1 RF choke of 10 turns of the coaxial itself onto a FT82-43 toroid, located just on the BNC connector of the transceiver.
We can also use a wire of 0.045 wave lengths of the lower band (40m) which should be connected at the Z transformer on the opposite end of the primary coil. It can be left on the ground below the radiating wire.
I hope this antenna to give you as many good times as to me. Do not hesitate to contact me to solve any problem or doubt. Find my e-mail address at qrz.com
73 de Mikel EA2CW | AE2CW