End Fed Half Wave antennas are again very fashionable with hams, accompanied by extraordinary claims and somewhat sparse understanding (the way of modern ham radio).
To add some light I have created a set of NEC 4.2 models of a half wave antenna on 20m to give some insight into the behaviour of a bottom fed vertical half wave over real ground.
This analysis does not consider harmonic operation, antennas are a half wave at 14.2MHz.
Four models are used:
- 20mHW-VEP – bottom fed vertical above perfect ground;
- 20mHW-VEA – bottom fed vertical above real ground;
- 20mHW-VCA – centre fed vertical above real ground (ie ground independent feed);
- 20mHW-HCA – centre fed horizontal at 5m height above ground;
NEC 4.2 model description:
- no conductor loss;
- real ground assumed to have conductivity=0.005S, εr=13, of course results are dependent on these values;
- conductors are ~10m long, 20mm diameter;
- bottom fed vertical half wave uses a 10m x 20mm vertical driven ground electrode;
- centre fed vertical is raised 200mm above ground;
- feed line and feed line common mode current are excluded;
- the centre of all antennas is ~5m above ground (real or perfect).
Above are the patterns from the models for discussion.
Lets just remind ourselves that GAIN and EFFICIENCY are related, GAIN=DIRECTIVITY*EFFICIENCY.
Half wave bottom fed vertical over perfect ground
Above, the blue curve is the vertical half wave bottom fed over perfect ground. It has a pattern (6dBi omni gain at low elevation) and radiation efficiency (100%) that hams dream of, and no doubt some achieve it in their dreams. The outcome is the result of ignoring the effects of real ground.
Half wave bottom fed vertical over real ground
The red curve is the vertical half wave bottom fed over real ground. Radiation efficiency is just 25.7%, even though, gain at low elevations is relatively good, gain near the zenith is very low.
Above is a current magnitude distribution on the conductors. Current flows down into the ground electrode, it is relatively small and diminishes quickly. That current results in conversion of RF energy to heat, though the quantum is rather difficult to assess. The following model is a similar, but avoids that loss element.
The feed point impedance (at resonance) is around 1300Ω, and some form of impedance transformation will usually be necessary. Finding an efficient and convenient solution for impedance transformation and transmission is key to extending the benefits of the antenna to an antenna system.
Half wave centre fed vertical over real ground (ground independent feed)
The green curve is the vertical half wave centre fed over real ground (elevated 0.2m). Radiation efficiency is just 24.5%, even though, gain at low elevations is relatively good, gain near the zenith is very low.
The characteristics are very similar to the previous model, and the suggestion is that a lot of energy is converted to heat in the soil, but it is mainly due to the effects of E and H fields around the antenna than more immediately around the ground electrode used in the previous model.
Half wave centre fed horizontal over real ground (ground independent feed)
The pink curve is the horizontal half wave centre fed over real ground (elevated 5m). Radiation efficiency is just 73%, even though, gain at low elevations is relatively poor, gain near the zenith is very good.
The height of 5m was chosen to be similar to that of the other configurations, but most hams would deploy such a dipole at considerably greater heights where efficiency is better, but more importantly the pattern changes greatly and low elevation gain improves significantly.
Antennas do not create energy (a bit of a radical notion for lots of hams gauging from the way the speak).
For antennas above real ground, gain averaged over the hemisphere must be less than 0dBi, which means that if gain is higher than 0dBi in some directions, it must be less in some others.
Of the two antennas presented above, a low vertical half wave and a low horizontal half wave, there is a large difference in gain distribution:
- low horizontal gives high gain at the zenith, good for NVIS paths; and
- low vertical has poorer efficiency but gives good gain at 5-45°.
Neither gives good gain at 0° elevation, that is the way of real ground. All those patterns you see in ham handbooks with maximum gain at 0° elevation are, like the blue curve above, over perfect ground… and we do not usually build stations on perfect ground.
Though one might initially take fright at the low efficiency of the bottom fed vertical half wave over real ground, it is the gain in the directions that most suit operations that really matter, but that is no reason to squander power in inefficient transmission and impedance matching solutions.
- The model results and discussion apply specifically to the scenarios described, and extension to other scenarios may not be valid. Buildings, vegetation, ground slope, and any of the variables mentioned above are some of the factors which may change behaviour.
- Bottom feed of a half wave radiator over real ground drives a small current into the ground system, but losses due to that current should be low even for only modest ground systems.
- Finding an efficient and convenient solution for impedance transformation and transmission is key to extending the benefits of the antenna to an antenna system.