Simple DIY FM antennas: Building a Folded Dipole Pt.1

Overview

These simple antenna construction projects are primarily designed to be fun to encourage more interest in long distance FM reception, DAB+, digital television or the two metre band amateur radio. Once again, the additional criteria (or criterion) are that an antenna design should offer simplicity in construction & measurable performance.

Copper pieces © 2006 FreshElectrons

Copper pieces © 2006 FreshElectrons

Building a folded dipole antenna is only marginally more difficult to construct than the popular DIY circular FM loop project. Again it’s an all copper affair! Annealed copper tubing was chosen as antenna material for the earlier project.

For construction of the folded dipole, inflexible copper pipe is used. 90 degree ‘elbows’ are chosen for the joins which means no special tools are required to bend the copper.

To aid readability, this construction article is published in two parts. The focus in this article is on getting everything ready for the build. The second part is the ‘home stretch’, the fun part with a step-by-step guide to building the antenna.

Why build a folded dipole?

Unlike a multiple element yagi antenna, a folded dipole may perform satisfactorily under challenging indoor reception scenarios where multiple metal signal reflections often exist.

Copper tubing prior to construction  © 2014 FM DXing at WordPress

Copper tubing prior to construction © 2014 FM DXing at WordPress

The straight copper pipe must be cut into five pieces. The final step is to glue the antenna together so it does not fall apart. Antennas don’t perform well in pieces! Although gluing may not be considered mandatory it is recommended, especially if the antenna will be mounted vertically. Otherwise, the heavy pieces of copper pipe may fall to the floor.

The most important part of the construction is getting the dimensions reasonably accurate for the required frequency. Once that step is complete, then one proceeds with the fun ‘hands on’ part, which is cutting the copper pipes and gluing the folded dipole together!

Project cost

The author spent $45.50 to build this project. This figure might be considered an indicative minimum cost.

For the apartment dweller whose tool shed is likely to be entirely bare, the maximum cost of the entire project is $86. Perhaps that maximum cost might seem expensive? But another way of looking at it is to consider that many of the materials (such as a quality tube cutter, Wire Glue adhesive, tape measure, surplus ‘off-cuts’ of copper tubing, PVC pipe & marker pen) can be used for years to come in future projects.

Metal pipe cutter 6-35 mm © 2012 Robert & Mihaela Vico

Metal pipe cutter 6-35 mm © 2012 Robert & Mihaela Vico

Copper elbows © 2012 Robert & Mihaela Vico

Copper elbows © 2012 Robert & Mihaela Vico

Cost projections exclude discretionary items such as mounting tripods & masthead preamplifiers. Buying copper tube in 1.5 metre / 59 inch increments (the shortest lengths the author has seen at the retail level) may increase the tabulated representative price of tubing by as much as 23%. To minimize cost, it is recommended to purchase the longest piece of ¾ inch copper pipe one can reasonably transport home and then cut it into the lengths required.

Feedline is a mandatory additional cost. Its cost is likely to be insignificant. Obviously, cost will vary with the length and type of feedline required. For detailed discussion, please refer to the feedline section in part two of this publication.

ESSENTIAL MATERIALS COST PLACE OF PURCHASE (PICKUP)
¾ in diameter straight copper tube x 3 m OR $25 OR Discount hardware retailer
¾ in diameter straight copper tube x 4 m $33 Discount hardware retailer
Store brand tube cutter OR $8 OR Discount hardware retailer
Brand name tube cutter (recommended) $17.50 Discount hardware retailer
Copper elbows to suit ¾ in tube x 4 pcs $10 Discount hardware retailer
Alligator clips with 12mm jaw x 2 pcs $2.50 Electronics retailer
OPTIONAL MATERIALS COST PLACE OF PURCHASE (PICKUP)
Wire Glue $10 Electronics retailer
Permanent ink marker pen $1.50 Discount hardware retailer, supermarket or newsagent
F-type outdoor balun $5 Electronics retailer or Discount hardware retailer
Store brand tape measure $3 Discount hardware retailer
Highest pressure grading (thickness) PVC pipe x 2 m $3.80 Discount hardware retailer

Before building, one must ascertain the dimensions

According to Martin E. Meserve K7MEM, the most important measurement for the folded dipole is its total length, measured tip to tip.

The recommended dimensions for a folded dipole can be calculated using simple software or by using raw calculations. A useful rule of thumb is that a folded dipole is typically 2% longer than a single dipole.

There are at least four ways of obtaining the measurements. One recommended way is to use software calculations. This software provides precise measurements for constructing a folded dipole. The only input required is the resonant frequency of the antenna & the diameter of the tubing. The software is simple to use.

Option 1. Download VK5DJ’s Yagi Calculator software. Use the Design Yagi function.

The software will enable one to manipulate the antenna design with MANA-GAL Basic antenna modelling software to allow further analysis or optimization.

Option 2. Download Al Legary VE3SQB’s software Yagi-Uda Antennas.

Option 3. Use the on-line Folded Dipole antenna calculator. Unfortunately this calculator only allows the calculation of dimensions for tubing diameters of 10.2 mm, but its calculations may be considered a useful rule of thumb.

Software calculations  © 2014 FM DXing at WordPress

Software calculations © 2014 FM DXing at WordPress

Option 4. The lazy way! An alternative route is to copy. That’s right, simply copy the dimensions of a commercially made aluminium folded dipole from an FM yagi. As always, it is recommended not to deviate from the dimensions, including the use of an identical thickness of tubing.

A commercial design made from aluminium

The following dimensions were taken from commercial folded dipole found in a 5-element FM yagi.

ANTENNA CHARACTERISTIC SPECIFICATION
Material Aluminium tubing
Theoretical Gain 2.14 dBi
Aluminium Length – Longest piece 1.455 metres (57.3 inches)
Total Length – measured tip-to-tip 1.54 metres (60.6 inches)
Straight Tubing Diameter Approximately 13 mm (more likely 12.7 mm)
Spacing Gap between the dipole ends – ‘Feed Gap’ in centre of antenna 25 mm
Spacing Gap between the tubing – Horizontal section separation 50 mm

An example construction made from copper

Using software calculations, the author constructed the following dipole. She appears in the photographs in this series.

ANTENNA CHARACTERISTIC SPECIFICATION
Material Copper tubing
Gain (theoretical) 2.14 dBi
Maximum Gain (theoretical modeling in free space) 0.32 dBd (2.47 dBi)
Copper Length – Longest piece 1.5 metres (59 inches)
Bandwidth (theoretical modeling in free space, SWR < 1.5) 85.9 – 98.7 MHz (12.7 MHz)
Total Length – measured tip-to-tip 1.54 metres (60.6 inches)
Straight Tubing Diameter 19 mm
Bend Diameter 20 mm
Spacing Gap between the dipole ends – ‘Feed Gap’ in centre of antenna 50 mm
Spacing Gap between the tubing – Horizontal section separation 50 mm

Terminology

What was that? Did I hear you ask what the #$%^ is the ‘feed gap’? What on earth is ‘tip-to-tip length’? Relax, in his introduction, Martin K7MEM offers a thorough explanation of the terminology used. Antenna gain is often quoted with reference to the half wavelength dipole, that is dBd.

Folded dipole terminology © 2014 FM DXing at WordPress

Folded dipole terminology © 2014 FM DXing at WordPress

Modelling

For those who enjoy software modelling, Graham Daubney F5VHX cautions that the folded dipole may be difficult to model accurately in software. David Jefferies PhD suggests, ‘Any modelling process needs careful validation by measurements’.

Build a copper folded dipole antenna easily without soldering © 2015 FM DXing at WordPress

Build a copper folded dipole antenna easily without soldering © 2015 FM DXing at WordPress

Indicative specifications

For comparative purposes, the author examined the on-line specifications of eight aluminium or stainless steel folded dipoles marketed for FM broadcast applications. In these models for sale, maximum forward gain figures were 0, 1 or 2 dBd & front to back ratios were 4-7 dB. Those manufacturers purporting to offer the highest specifications typically took into account overall performance when the antenna was mounted vertically on a horizontal conductive pole. There was some standardization and simplicity to be found in the crude analysis nonetheless; a typical element diameter was 19 mm & maximum bandwidth 10% of the centre frequency!

Bandwidth is typically measured using VSWR (Voltage Standing Wave Ratio). Rescue Electronics tested the bandwidth of their FM folded dipole designed for FM reception. The SWR was measured at less than 2 (a mismatch loss of only 0.51 dB, calculated at Antenna Theory) between approximately 80-110 MHz.

Holl_Ands has provided SWR modelling plots for his folded dipole (which includes FM coverage) which are invaluable for comparisons between copper tubing diameters.

Stay tuned for part two… it is expected to be published shortly… but the author is too distracted, ‘at the trough’ trying to emulate the DX pig!!!

Further reading

Bandwidth of the Folded Dipole by Natalia K. Nikolova PhD

Bandwidth Explained at Antenna Theory.com

Copper FM J-pole Antennas by Michael Martens, KB9VBR

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