Leg Lengths: Inverted V vs Horizontal Dipole Antennas

 

The halfwave dipole antenna element lengths are measured differently when building an inverted V dipole.

You usually make an inverted-V dipole slightly shorter than a flat horizontal half-wave dipole because the downward-sloping legs change the antenna's capacitance and feed point impedance.

A common rule of thumb is:

  • Horizontal half-wave dipole:


Each leg:


  • Inverted-V dipole:
    Start with 2–5% shorter than the horizontal calculation, then trim while checking resonance.

A practical formula is:


or


Example: 20 meters (14.2 MHz)

Horizontal dipole:

  • Total:


  • Each leg:


Inverted V:

  • Total:


  • Each leg:


That's about 3.4 inches shorter per leg.

Why?

When the legs slope downward:

  • The ends are closer to ground.
  • The antenna has greater end capacitance.
  • It becomes electrically longer than the same physical wire length.
  • Therefore, less wire is needed to resonate at the same frequency.

How much shorter?

The exact amount depends on:

  • Included angle (typically 90–120°)
  • Height above ground
  • Ground conductivity
  • Wire diameter
  • Nearby objects

Typical reductions are:

Included Angle

Length Change

120°

1–2% shorter

90°

2–4% shorter

60°

4–6% shorter

Best practice

Most amateur operators:

  1. Cut each leg about 2–3% long initially.
  2. Raise the antenna to its final height.
  3. Measure SWR or resonance with an antenna analyzer.
  4. Trim both legs equally until the desired resonant frequency is reached.

This approach accounts for your actual installation, which has a much larger effect on resonance than the theoretical correction for the inverted-V geometry alone.

Comments

Post a Comment

Popular posts from this blog

Antenna Theory - What's Happening

  Antenna Theory – A small bit, anyway It is an early Sunday evening, and you have been in the shack for over an hour. You have been tuning the HF bands on your new Icom 7300 transceiver. Linked to the 40-meter half-wave dipole that stretches from the far yard tree to the house eave, you are experiencing noiseless, 1.2 SWR transmissions and picking up DX stations in South America and the Pacific rim. Your wife calling you to dinner quickly snaps you out of your Tuesday, after work nap. It's time to get up from the Lazy-Boy lounge chair. While at the dinner table, you reflect on the dream that you just had. It was all a fantasy; you get no such performance from your ham shack setup. You know it is not good, but you don’t know why.   It's time to dig into what may be happening. Radiation occurs when electric charges accelerate. Constant-velocity current does not radiate; time-varying current does. In a half-wave dipole, the RF source forces electrons to speed up, slow dow...
Read more

A Blog Site to share Ham Stuff

I am an experimenter, a builder of things, and I have been a teacher. I am a man of Maths and of Science; when curious, I will dwell until I have an understanding. As a Ham, I am probably a poor excuse. I enjoy scanning the bands. It is interesting to me to observe how well the reception may be on a given band, at a particular time.  But I find that my interest is not in long Ham visits, rather it is in seeing what and where I can detect.  Signal reception, of course, is governed by many, some often nasty factors. However, all else being fine, how well you will receive depends in a major way on your antenna setup. A primary purpose of this blog will be to share experiences with antenna construction. There seems to be no end of the technical stuff that a newer Ham has to become acquainted with. This site will present technical stuff, too. What I hope to achieve is to present the material in a down-to-earth understandable way.  When reading a particular post, if you see an ...
Read more

Antenna Considerations

  Because you are tying a plastic insulator to the end of the antenna leads to connect to paracord, should you add extra length to the legs of your dipole?   When tuning a multiband HF antenna should you start by adjusting the length of the lowest received frequency band, or the highest received frequency band? Tune the lowest band first (longest element   What is the purpose of adding a balun at the feed point of a dipole antenna ? The main purpose of adding a balun (short for balanced-to-unbalanced transformer ) at the feed point of a dipole antenna is to match the balanced antenna to an unbalanced transmission line (like coaxial cable) and reduce unwanted currents. What are the expected input impedances of half wave dipole antennas? A center-fed half-wave dipole is usually close to 70–75 Ω , making it a good match to 50 Ω coax, especially if a 1:1 balun is used to reduce mismatch effects. What is a unun ? A unun is just an unbalanced-to-unbalanced transfor...
Read more