ADVANCEDTEMPLATE.MZT

ADVANCED LESSON 50

LEARNING OBJECTIVES and NOTES

Antennas 2

5c.4 Identify folded and trap dipoles and quad antennas in addition to those in earlier syllabuses.
5c.5 Recall that an antenna trap is a parallel tuned circuit and understand how it enables a single antenna to be resonant and have an acceptable feed-point impedance on more than one frequency.
Recall that this technique may be extended to multi-element antennas such as Yagis.

Revise the antenna types from Foundation Lesson 07 and Intermediate Lesson 19
The drawing shows three other types of antenna commonly used by Radio Amateurs

The folded dipole is like a dipole with a parallel half wave wire connecting the two ends. This has an impedance of 300 ohms and can be fed with 300 ohm ribbon with a balun at the TX/RX end to transform the 300 ohm impedance to50 ohm impedance. ( a 6:1 balun would be required.

The advantages of using a folded dipole are:

In a beam it is often used as the driven element where the parasitic elements bring the impedance of the 300 Ohm driven element down to about 50 Ohms
It has wider bandwidth than a dipole and so would give a low SWR over a whole band.

The trap dipole is used to enable a dipole to operate on more than one band.

A dipole is cut to the highest frequency required. For example 40m.
Two traps are then constructed that are tuned to have a null at 40m. These are parallel tuned circuits which have a high impedance at their resonant frequency.Traps can be made from a coil and high voltage capacitors. They can also be made from lengths of coax cable. It is important that they have a low Q to cover the band.
The traps are connected to the end of the dipole. Wire is then connected to the other side of the traps on each side and adjusted to resonate on the lower band - e.g. 80m.

Advantages and disadvantages of a Trap Dipole

A trap dipole is less efficient and works over a narrower bandwidth than a dipole
The radiation pattern is different to a dipole
However, it does allow more than one band to be used from the same 50 ohm feedline.

Traps are also used in three element beams where the driven element has two traps which allow it to work on, for example on 20m, 15m and 10m. The director also has two traps so that it can work on 20m, 15m and 10m and the reflector has two traps so that it can work on 20m, 15m and 10m
Quad antennas
These consist of several full size loops mounted on a set of light weight supports. There is a separate driven element loop for each band and a separate director or reflector loop for each band.

The quad antenna has a high gain and wider bandwidth than a 3 element Yagi beam.
Single band quad antennas are used on 2m and 70cm where they give good gain and take up less room than a conventional Yagi.

You will also need to remind yourself of the antennas mentioned in the Foundation and Intermediate syllabus.

At Foundation level you had to identify the half-wave dipole, λ/4 ground plane, Yagi, end-fed wire and 5/8λ antennas.

At Intermediate level recall the structure of a three-element Yagi