Multiband: traps
What is the primary function of antenna traps?
To enable multiband operation.
A trap is a parallel resonant circuit in an element. At its resonant frequency it presents a high impedance and effectively cuts the element short there; at other frequencies it is a low impedance and the full length is in use. One physical antenna, several electrical lengths.
The cost is asked directly:
Which of the following is a disadvantage of multiband antennas?
They have poor harmonic rejection.
A single-band antenna is a filter — it presents a bad match to your second harmonic and radiates it poorly. A multiband antenna is, by design, a good match on several bands at once, including ones your harmonics land on. Spurious emissions get out more easily.
Simple variations on a dipole
What is the common name of a dipole with a single central support?
Inverted V.
One mast, two legs sloping down. Slightly lower feed impedance and a slightly broader pattern than a flat dipole, and one support instead of two — which is why so many first HF antennas are inverted Vs.
What is the feed point impedance of an end-fed half-wave antenna?
Very high.
Thousands of ohms, following from the current distribution: minimum current and maximum voltage at the end. Feeding it needs a matching transformer with a large ratio, and it is why an end-fed wire connected straight to a radio puts RF all over the station.
Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?
A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
Deliberately low, so most of the radiation goes up and comes back down within a few hundred miles. Everything the DX operator avoids, done on purpose.
Wide bandwidth: log-periodic
Which of the following describes a log-periodic antenna?
Element length and spacing vary logarithmically along the boom.
Which of the following is an advantage of a log-periodic antenna?
Wide bandwidth.
At any frequency in its range, a different subset of elements is near resonance and does the work. The result covers a decade or more of frequency with a consistent pattern — the reason television receiving antennas look like this.
The trade is size and modest gain compared with a Yagi of similar boom length.
Receiving antennas
What is the primary use of a Beverage antenna?
Directional receiving for MF and low HF bands.
A very long wire — one to several wavelengths — laid close to the ground and terminated at the far end. It is a poor radiator and an excellent receiving antenna: directional, quiet, and unbothered by the ground losses that would ruin it for transmitting.
Receive-only antennas are a real category. On 160 and 80 metres, hearing is the hard part, and an antenna optimised purely for it is worth the space.
In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern?
Broadside to the loop
A small loop responds to the magnetic field and has deep, sharp nulls in the plane of the loop — which is what makes it the classic direction-finding antenna. You locate a source by rotating for a null, not for a peak: nulls are far sharper than maxima.
Mobile and VHF specials
How does a “screwdriver” mobile antenna adjust its feed point impedance?
By varying the base loading inductance.
A motor moves a contact along a coil, changing the loading inductance and retuning the antenna from inside the car. It is the practical answer to mobile HF, where a resonant antenna is impossible and a fixed loading coil works on one band only.
In which direction is the maximum radiation from a VHF/UHF “halo” antenna?
Omnidirectional in the plane of the halo.
A dipole bent into a near-circle. It gives horizontal polarisation — matching VHF weak-signal practice — while remaining omnidirectional, which a straight horizontal dipole cannot. Standard for mobile VHF SSB work.
The selection table
| Need | Antenna |
|---|---|
| Several bands, one antenna | trapped dipole (poor harmonic rejection) |
| One support, simple | inverted V |
| 50–300 miles reliably | low horizontal dipole, 1/10 to 1/4 wavelength up |
| Very wide frequency range | log-periodic |
| Quiet directional receiving on 160/80 m | Beverage |
| Direction finding | small loop, using its nulls |
| HF from a vehicle | screwdriver |
| Horizontal and omnidirectional at VHF | halo |
Check yourself
- Why does a multiband antenna make harmonic radiation more likely to get out?
- You need to find the direction of an interfering source. What antenna, and do you look for a peak or a null?
- What is the feed point impedance of an end-fed half wave, and what follows from it?
Answers
- It presents a good match on several bands, including the ones your harmonics fall on. A single-band antenna acts as a filter against them.
- A small loop, and you rotate for the null — nulls are much sharper than peaks.
- Very high, thousands of ohms. It needs a large-ratio matching transformer, and fed without one it puts RF current through the station.