- Who am I?
- https://w6nbc.com/articles/2020-TBDdoubledelta.pdf
- Looks great on paper, does it live up to it?
- Which is better, Parallel or Series?
- Can you rotate it by 45deg instead of 90deg by using a single loop?
- 3D model for bracket: https://github.com/SmittyHalibut/DoubleDeltaSlotAntenna/tree/mainline/Brackets
- 22' tall loop, 2' off the ground, 24' height at top
- 4' spreader arms, 8' total spread at top
- Tuner: RT-600 (same as W6NBC used)
- Balun: 11 turns of speaker wire on FT140-43
- John's design has: Radio -> Feeline -> Balun -> Tuner -> Antenna
- Uses an unbalanced tuner in a balanced circuit.
- I built: Radio -> Feedline -> Tuner -> Balun -> Antenna
- Uses a balun designed for 50+j0 ohms in a wildly varried and complex impedance circuit.
- Neither are perfect. I'm proceeding with my analysis. I've already got the data.
- NanoVNA-F
- Learn to calibrate!
- NanoVNA Saver software
- 3MHz to 30.27MHz in 2727 steps of 10kHz (27 segments of 101 data points each.)
- Measures S11: Power output, and power and relative phase of reflection.
- Saves data in the standard "Touchstone" file format:
.s1p
files. - NanoVNA Saver does lots of visualizations and calculations on its own! Its very neat.
- I wrote a python script to analize the data in the Touchstone file.
- For 5 data points across each ham band:
- What does the antenna look like natively? It's all over the place, as expected.
- What is the theoretical perfect matching L network?
- What matching network can the RT-600 make based on its limitations?
- 0 to 1270pF in 10pF steps
- 0 to 12700nH in ~100nH steps
- What impedance is presented to the radio, given this "actual" matching network?
- Extra credit: Pick an "optimal" single matching network per band. What impedance does that present to the radio across the band?
- This analysis is of MY PARTICULAR BUILD.
- The build is very flexible on exact dimentions, because the tuner will handle it.
- If you build yours differently than I built mine, your numbers are likely to be different!
- Include snapshot of raw data, showing:
- Parallel is good or great from 60m to 10m, except 30m and 20m
- Series is good (but not great) at 60m, and 15m and above. It's GREAT on 30m, not pretty reactive on 40m, 20, and 17m.
- Single is good or great from 60m to 10m, but nearly unusably high SWR on 30m and 20m.
- Very minimal, non-scientific, practical experience matches this data.
- All the benefits that John describes in his article.
- Covers all bands from 60m to 10m to varying degrees.
- Possibly usable at 6m too, I didn't measure.
- Easy to construct, could be made "field-day" portable (but probably not SOTA portable.)
- 80m and below is a complete loss. A larger/taller loop would probably be better.
- 20m is not great in any configuration. Usable, but with very reactive impedance.
- If I cared strongly about 30m, I would consider building a complex switching network to do Series on 30m, and parallel on other bands.
- I'm willing to trade 30m to get simplicity, so I will only build mine for Parallel.
- But someone with different goals, or an antenna with different numbers, I could see wanting the complexity.
- Would it help?
- The modeled beam width is -3db at 90deg, so it wouldn't help reception sensitivity much.
- The nulls are pretty narrow, so rotating 45deg might help point nulls.
- Can it even be done?
- The Single loop numbers are similar to the Parallel numbers: Good or great from 60m to 10m, except 30m and 20m
- Should it be done?
- RT-600 remembers the matching network for a given frequency.
- Changing the antenna impedance on a given frequency will require the tuner to completely retune every time you switch from Dual to Single, losing the benefit of the memory.
- The tuner turned out to be more of a pain than I expected, so anything that makes the tuner less effective is not good, IMHO.
- If you DO want the full switching options, I've designed a switching network that will do:
- Parallel, In and Out of phase.
- Series, In and Out of phase.
- Single N/S loop, or E/W loop.
- Using 4 Relays: 3 DPDT, 1 3PDT (for safety interlock to make sure only valid states are selected.)
- I'm considering designing a board with:
- SO-239 input, a spot for a balun, relays, and screw terminals for the antenna loops.
- Microcontroller to control the relays
- Using George's serial protocol for remote control over RS-485 or ZigBee
- I won't steal his thunder. Watch for his presentation in the QSO Today Ham Expo in March 2021.
- It will make the antenna a bit cheaper to construct, eliminating the need for a remote tuner.