TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

A PORTABLE VHF ANTENNA: the “Baby-Yagi”

After doing some activation in the mountains carrying vertical antennas, soon we missed a more powerful

antenna to reach bigger distances comfortably.

Therefore we thought it was the time to build a well known antenna; the yagi.

Many amateurs use the yagi up in the mountains. Talking about 145 MHz, the elements of this type of

antenna are around one meter long. There are plenty of designs available in Internet. Some of them fit the

gamma match to adapt the impedance of the antenna to the required 50 ohm by the rig or handheld. Other

designs play with the distance between elements to allow a direct coaxial feed.

What I present here is based on an existing design but I have added some variation to create an antenna

that can be quickly assembled/disassembled and that has a very compact profile to be able to put it in a

rucksack.

It has the following characteristics:

Direct 50 ohm feed with a coaxial cable

Elements are built out of Aluminum rods and cut in half to help for transportation

Reduced boom size: 53 cm (20.8 inch) for a 3 element yagi

Homebrew manufacturing using wood sticks or PVC pipes

Quick fixation of the elements using screw and a bent retainer: fast and no need for tools.

Can be built with 2, 3 or 4 elements.

Base design: the cheap yagi from WA5VJB

The design I choose as the basis for my model is the so called “cheap yagi”. You can find that on Internet.

The particularity of this design is that it is a direct coax feed model, and the boom is quite reduced

compared to standard yagis. Both features seemed perfect for a portable antenna.

Its author, Kent Britain WA5VJB, build and distribute some models for microwaves, but has published data

for VHF band as well.

The innovation of his design is that uses for the driven element a partial folded dipole. Let’s see why.

A standard dipole has an impedance of 73 ohm and looks like this:

When transformed into a folded dipole the impedance raises up to 300 ohms and looks like this:

The driven element of WA5VJB is a hybrid version that is 150 ohm and looks like that:

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

How can we get the final 50 ohm for this yagi? It is achieved by adding elements around until to get the

desired impedance. He used the aid of simulation models and fine tuned the antenna with a trial and error

method on the field.

The resulting antenna is simple to build, has a good bandwidth, and can be extrapolated to add more

elements. All at the cost of 1/2 dB loss; it does worth a try!

Original yagi dimension

Shown below are dimension for a model centered in 145.000 MHz appropriate for FM section.

Driven element: although the original model uses a rod of 3.2 mm diameter, we built it with a 4mm one without functional variation:

*All dimension in centimeters

The Hamtennas version: the “Baby-yagi”

Bearing in mind the idea of building a really collapsible version so that it could be for the field, we

considered cutting the elements by half. That was the key point that gave birth our Baby-yagi.

It is a 3 element model that once disassembled fits neatly in our rucksack.

To achieve that goal all elements are cut in the middle. The Driven element is then transformed into a

single rod and a long narrow U shape. Here you see all rods prepared and the Boom aside:

Reflector Driven D1 D2

Length 102.9

Spacing 0 17.8

Length 102.9 92.7

Spacing 0 21.6 50.2

Length 102.9 94.0 82.5

Spacing 0 21.6 48.3 101.6

3 elements

4 elements

2 elements

97.8 cm

2.5 cm Coax

Braid connect

Center connect

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

Boom fixing

To ease the fixing of elements into the boom, we designed a quick fit and lock system by means of some

small wood square with one side diagonal cut and some end stop counterparts. This way the aluminum

rods are trapped in the middle space:

Materials:

- Wood rod 3x1 cm. - Aluminum rods (blank, not anodised) Ø4mm - Screws, washers and wing nuts M4 - Plywood and wood square rods - Coax RG-174 & female BNC connector - Copper foil with adhesive side 6mm. - Nylon ties

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

Connecting the elements in the Boom

Due to the fact the elements are cut in half, we need to have them together again once in the Boom to

ensure a good electrical connection between them.

To do so, we add in the Boom a copper foil with one adhesive side. Such foil is sold for electrical

maintenance works, decoration or electrick guitar repair.

Feeding the Driven element

Using the copper foil we place it in two feeding points: one for the center of the coax cable at one side of

the U shape rod and the other of the coaxial braid in the joint between both rods. A tiny hole of 1mm

dimeter is done through the Boom to pass a solid conductor for the braid. It is then bend over the copper

foil and soldered. On the opposite side another copper foil is added and the RG174 coax cable is soldered:

To finish, a female BNC connector is added to the end of the short coax. All soldered points are protected

against corrosion and stiffer with Epoxi adhesive:

Upper part Lower part

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

This is the Baby-yagi ready for the first trial:

Simulation

This graph shows radiation of this antenna at 2 meters above ground.

The antenna is fed directly at 50 ohms. Maximum gain is 8 dBi at 9,7º elevation. Compared with a vertical

dipole installed at the same height, Baby-Yagi has 5,2 dBd.

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

Mast support

Although the antenna is light enough to be hold by hand during an activation, it’s of great help to use a

fishpole to hang it and have our hands free. To make it possible a two element suport is built with wood

square rods: one will keep the Boom straight and the other will hold it perpendicular to the fishpole using

nylon ties.

SWR measurement

Our Baby-yagi built according to sketch provides a nice bandwidth. Here you see the SWR measurements,

very good performance is achieved:

Field tests

First we check directivity in a mountain. I speak with station distant 50 km away; his exchange is 59 when

beaming towards him and drops to 53 when turned 180º.

The antenna is tested and works consistenly. Using the 5 watts of a simple handheld, I can easily log

stations covering different areas beyond intermediate peaks and obstacles.

As an example, here it is my detailed log for a recent activation with this antenna:

0

0,4

0,8

1,2

1,6

2

2,4

142 142,5 143 143,5 144 144,5 145 145,5 146 146,5 147

SWR Yagi

TECHNICAL PAPER

Prepared by: EA2BD / EA2BSB. 2014

EB2FDT/1 Summit S. Cristobal 85km

EC2PM Hondarribia 73km

EA2CWM Billabona 42km

EB2AFU Bilbao 71km

EA2EBX Laguardia 42km

EA2OX Portugalete 79km

EA1HRR Castrourdiales 95km

EB2GKK/P Summit Saioa 65km

Weight

The baby-yagi has resulted extremely light and collapsible, and yet rugged enough.

Its weight including all elements plus a one meter long RG58 coax is:

- 3 ele Baby-Yagi: 305 grams

Here you see a picture of it alongside with a commercial 3 elements model from Arrow Antennas, where

you see its compactness:

All ready. Let’s enjoy amateur radio up in the mountains! 73

We thank Colin G8TMV for the aid with the spell check.