Ken Hough's Website
Amateur Radio

Antennas -- Yagis

This section includes reports on construction and use of Yagis for 2m and 70cm

The Yagi antenna, or to give the full name, the Yagi-Uda array, was invented in Japan in 1926. Yagis are now widely used for reception of VHF/UHF frequencies, including broadcast television and FM radio, and of course, amateur radio.

Yagis can provide 'gain' in a particular direction, while suppressing response in other directions.

Typically, Yagis are made up of a driven element with a reflector element and one or more director elements, as indicated on the left.

For many years, it was common for radio amateurs to build Yagi antennas according to an approximate "rule of thumb" as follows:
Driven element -- 1/2 wave dipole
Reflector -- 5% longer and spaced 0.2 λ behind the driven element
Directors -- first director 5% shorter than the driven element and spaced 0.2 λ in front of the driven element. All additional directors decreasing in length by additional 5% with all spaced at 0.2 λ

Ref. ARRL Handbook 1957

More recently, computer programs have become readily available which can be used to develop more optimal designs for Yagis. Results typically will have higher gains than "rule of thumb" designs and are likely to be better matched to working feeder impedances. Element spacing may look strange, as indicated here on the left.


Yagis for the 2m band:

I built two 5 element Yagi antennas for use on the 2m band, one using "rule of thumb", and the other with the aid of the 'Yagi Calculator' program. The "rule of thumb" design did work and had a gain that I estimated to be approx 6 dBd. The better optimised second antenna had a gain estimated to be approx 8 dBd. The design gain for the second antenna was given as 7.8dBd, so clearly the antenna was working well.

Despite having the higher gain, overall length of the computer optimised antenna was less than that of the "rule of thumb" antenna.

Gain was estimated by comparing signal strengths received via the Yagis with those received via my (standard) 2m J pole. This was done using the "S" meter of a Yeasu FT897D -- assuming that at approx S9, 1 S point corresponds to 6dB.

Practical implementation:
Both of the antennas described above were intended to be short term experimental devices and were built on wooden booms. All elements were made from 10mm diameter aluminium tubing which were fixed to the boom by means of single bolts. The driven element was a single-piece straight dipole (ie not a folded dipole). 50 Ohm feeder cable was coupled to the dipole via a gamma match.

Tubing was bought in 1 metre lengths. The reflector element was extended to the required length of slightly longer then 1 metre by inserting short pieces of 8mm diameter tube into the ends of the 10mm tube.

The second (computer designed) antenna has been in outdoor service now for over a year without any problems. Perhaps one day I shall get around to re-building it properly.
This second 2m Yagi antenna is shown here on the left standing on my garage rooftop 5 miles south of Kendal and directed towards the Furness Peninsula. Despite unfavourable geography, I am able to have regular QSOs with stations on Walney Island, approximately 35 miles away.

I really must get around to re-building this antenna, but it just keeps on working!


Closeup view of gamma match

This comprises an insulated wire that passes concentrically inside an outer aluminium tube. The length of the wire is such that it extends only partly into the tube. Adjustment of the length of the link is done by moving the clamp, and adjustment of the capacitance between wire and tube is done by moving the tube while keeping the clamp position fixed.

This is a simple, but very effective means of matching feeder and antenna impedances.



Yagi for the 70cm band:

I used 'Yagi Calculator' again to produce a design for an 8 element Yagi for the 70cm/435MHz band. This time I decided to use a folded dipole with a half wave coax matching balun.

The folded dipole was made from 10mm x 3mm aluminium strip. The reflector and director elements were made from 6mm diameter tubing. Yagi Calculator provided detailed dimensions for the folded dipole and for a half wave matching balun made from RG58 coax. A small plastic project box was used to house the the terminal ends of the dipole, the balun, and a 50 Ohm BNC connector. Then the box was made fully weather proof by applying silicone sealant. The original carbon steel lid fixing bolts were replaced by bolts made of brass.

Presently, I have no reference antenna for 70cm so cannot give a comparative assessment of performance. However, this antenna enables me to easily open the Blackpool repeater (GB3FC) while using only 1 Watt of RF.

Communication with stations further down the Lancashire coast are seriously impeded by high levels of QRM. A Yagi can provide significant forward gain, but that is of limited advantage in the face of QRM from that direction.
The 70cm yagi is shown here standing on my garage roof and pointing towards the Lancashire coast and North Wales.


A close-up view of the reflector, driven, and first director elements of the 70cm yagi. The black plastic box which carries the feeder connector and contains a half wave coax balun.


About Yagis:

Description of Yagis



Software for computer design of Yagis:

DL6WU-OD.xlt
Yagi Calculator

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05/05/13