Tell me length of inverted V antenna for 7135 kHz using 2.5 sq mm insulated copper wire used for home wiring
That’s a great question for an amateur radio operator! The length of a half-wave dipole antenna (which is what an inverted V is based on) is primarily determined by the frequency and the velocity factor of the wire.
For your specific antenna design:
- Frequency (f): 7135 kHz (or 7.135 MHz)
- Wire: 2.5 sq mm insulated copper wire (likely PVC or similar insulation)
1. Velocity Factor (VF) Estimation
The insulation on the copper wire slows down the radio wave, which is accounted for by the velocity factor (VF).
- For bare copper wire, the VF is very close to 1.0 (or about 0.95-0.98).
- For PVC-insulated hook-up or house wire, the VF is typically lower, often falling in the range of 0.93 to 0.97.
I will use a standard conservative value of VF = 0.95 for insulated wire for this calculation. You may need to fine-tune this by trimming the antenna once it’s installed.
2. Full Dipole Length Calculation
The formula for the total physical length of a half-wave dipole antenna in feet is:
L (in feet) = 468/f (in MHz) * VF
And in meters:
L (in meters) = 142.5/f (in MHz) * VF
Calculation:
- Use the formula in meters: f (in MHz) = 7.135 MHz; VF ~ 0.95
- L (in meters) = 142.5/7.135 * 0.95
- L (in meters) ~ 19.97 * 0.95
- L (in meters) ~ 18.97 meters
3. Inverted V Leg Length
An inverted V antenna is a half-wave dipole bent at the center. The total length is the same as a horizontal dipole, but it’s typically cut into two equal halves (legs).
- Total Antenna Length: 18.97 meters
- Length of Each Leg: L(leg) = 18.97 meters/2 ~ 9.485 meters
Conclusion and Next Steps
The required length for a half-wave inverted V antenna at 7135 kHz, using insulated copper wire with an estimated velocity factor of 0.95, is:
- Total Wire Length: 18.97 meters
- Length of Each Leg: 9.485 meters
⚠️ Important Antenna Tuning Note
The calculated length is a great starting point, but the final, most accurate length is determined by your specific environment (height above ground, surrounding objects, type of insulation, etc.).
- Cut the wire slightly longer than the calculated length (e.g., 19.5 meters total, or 9.75 meters per leg.
- Install the inverted V.
- Use an Antenna Analyzer or SWR meter at the feed point to check the Standing Wave Ratio (SWR) on 7135 kHz.
- Gradually trim equal lengths off each end of the antenna legs until the lowest SWR is achieved near your desired frequency 7135 kHz.
I made this antenna as described and mounted it in inverted V fashion with apex at about 10m from the ground and ends about 3m from the ground. Checked the SWR in my FT-710 radio and got SWR of near 1:1 on 7135 kHz. SWR was near 1:1 down up to 7100 kHz
SWR on 7190 kHz was also near 1:1.
At lower end of the 40m band it was around 2:1. That is tunable with auto-tuner in the radio. So I did not go for any trimming.