Portable Yagi for 144MHz



Yagi



This simple 7 element Yagi is ideal for portable operation. The elements can be removed and replaced in a few minutes. The 2.42 Metre boom will fit inside my car. All of the elements are made from 6mm solid aluminium rod. The boom is made from 50x25mm (2x1 in) timber.


Yagi elements
Element lengths and spacing.



driven element


The driven element is a half wavelength dipole. Unlike the more common type of folded dipole, this dipole is folded on one side but not on the other. For want of a better name, lets call it a 'half folded dipole'. The co-ax cable braid is connected to the centre of the dipole. The co-ax centre conductor is connected to the folded dipole section (see diagram above).

The idea of using a half folded dipole was borrowed from the W7ZOI portable Yagi (1)

The boom length for my Yagi has to be around 2.4 metres. A shorter boom would give less gain, a longer boom would not fit in my little Honda motor car.



Performance


According to the table in the ARRL Handbook, the maximum gain available from a Yagi with a boom length of just over one wavelength is about 12 dBi. I used NEC-2 to create models of several different Yagi designs. The NEC-2 models show a maximum gain variation of less than 1dB between the best and worst designs. The lowest gain was for a standard 'TV aerial' type of design with 0.2 WL spacing between each element and all directors cut to the same length. The highest gain (12dBi) was for a 7 element Yagi designed with QY4 software by W7RAI.

gain plot


This design offers a good compromise between maximum gain, a clean pattern and good front to back ratio.

Frequency: 144.300 MHz
Gain: 11.62 dBi (in free space)
F/B: 23.6 dB

In free space, the driven element would have a feedpoint impedance of about 150 Ohms. The effect of the parasitic elements reduces the impedance to somewhere in the region of 50 Ohms. The measured SWR is 1.1:1 at 144.300MHz and well under 1.5:1 across the entire 2M band (144 to 146MHz in EI). The results of on-air tests with other amateurs suggest that the performance of the portable Yagi is very close to the NEC model. Most stations report a F/B ratio of about 25dB.

Many thanks to Paraic EI7IR (145KM) for helping with QRP tests and Gary EI8GQ who built an exact copy of the portable Yagi.

The NEC-2 plots and graphs were generated by Xnecview(2) software for X-Windows by PA3FWM

graph



Construction



Each element is secured to the boom by three wood screws (see photo). To remove the elements, loosen screw number 2 and slide the element out sideways.

element



feed


I used nickel plated solder tabs to connect the co-ax to the driven element. Note that the end of the aluminium rod is hammered flat to make drilling easier.

feed-2



image
/P in south Co. Cork. June 15 2003



(1) W7ZOI Yagi
(2)Xnecview homepage

EI9GQ HOME BREW RADIO PAGE