Just for fun, I fed my original post into ChatGPT to see if "she" had any insights, and here was "her" response.....a fun exercise..
"What you're describing is a fascinating experiment! It sounds like you're observing the effects of an asymmetrical antenna design, and there are a few key factors to consider that could explain the differences in the SWR and Smith charts when you reversed the dipole legs.
1. **Non-Balanced Antenna and Common-Mode Currents:**
Since the antenna is a form of off-center fed dipole (OCFD), it is inherently unbalanced. An off-center fed dipole typically has a significant imbalance in the current distribution between the two legs, which results in common-mode currents on the shield of the coaxial feedline. The role of the coax shield becomes crucial, as it can have a different effect on the antenna impedance depending on the configuration of the dipole legs.
Reversing the two dipole legs changes the relative phase and current distribution between the two antenna legs. As a result, this can alter the electromagnetic field around the antenna, affecting the impedance seen by the transmitter and causing different interactions between the antenna and the coaxial feedline. The coaxial cable’s shield could be acting differently in each configuration, either radiating or picking up different signals depending on how the antenna’s currents are distributed.
2. **Impact of Asymmetry:**
In an ideal balanced antenna (like a center-fed dipole), the two legs of the antenna are of equal length and the currents in each leg are balanced. However, in your case, the legs are of different lengths and are presumably attached to the transformer in a way that creates an imbalance. When you reverse the legs, the imbalance changes, which likely affects how the antenna is coupled to the environment and the transmitter.
In terms of impedance matching, reversing the legs could change the impedance seen at the antenna terminals. This could be why you observe different SWR, return loss, and Smith chart behaviors — it's reflecting a change in the impedance that the transmitter sees. This effect could be accentuated by the unbalanced nature of the antenna and the non-ideal placement of the coaxial shield.
3. **Coaxial Shielding and Proximity Effects:**
Since your coax shield is short (4 feet) and potentially has significant current on it due to the unbalanced antenna design, the proximity of the coax to the walls (and to the antenna) could influence the impedance. Even though the nanoVNA was dangling hands-free, subtle changes in the position of the shield, or how the shield is coupled to nearby structures (walls, etc.), might affect the measurements. This is especially true when dealing with an unbalanced antenna where the coax shield may not behave symmetrically in each configuration.
4. **Transformer Characteristics:**
The transformer you're using has a 6:4 turn ratio, which means the impedance transformation may also behave differently depending on how the antenna legs are connected. If the two dipole legs are different lengths, the impedance seen by the transformer secondary might change, which could lead to significant variations in the reflected impedance. This could explain the differences you’re observing in the SWR/return loss between the two configurations.
### What Could Be Happening:
- **Reversing the dipole legs likely changes the current distribution** and the balance between the two legs of the antenna. This results in different impedance characteristics and different interactions with the feedline.
- The **coaxial shield** might be more involved in one configuration than the other, leading to different behaviors in the SWR/return loss measurements.
- **The transformer’s impedance transformation** could also be affected by the dipole leg configuration, which would further explain the differences in the measured SWR.
### Suggestions for Further Investigation:
- **Try using a balun:** A simple 1:1 or 4:1 balun could help balance the antenna, reducing the impact of common-mode currents on the coax shield and potentially stabilizing the impedance.
- **Check the positioning of the coax shield:** Experiment with different orientations or placements of the coax cable and observe how that affects the measurements. Small changes in the physical setup might reveal further insights into how the shield and antenna interact.
- **Use a longer feedline:** If possible, try a longer feedline to see if the behavior changes, as the coax shield’s interaction with the antenna may be more pronounced over a longer distance.
Your experiment is a great example of how even small changes in antenna configurations and feedlines can lead to noticeable differences in impedance measurements. The combination of antenna imbalance, coaxial shield effects, and transformer characteristics all play a role in shaping the observed results."
Roger Need
