Canvey Island, Essex, England Justin@g0ksc.co.uk
Twin boom quad
144MHz LFA Yagis
144MHz LFA Yagis

Low Noise LFA Yagis designed by G0KSC free to build for personal use.

144MHz LFA Yagis
70cms LFA Yagis
70cms LFA Yagis
Twin-Boom G0KSC Quads
G0KSC Twin-Boom Quads
Twin-Boom G0KSC Quads
G0KSC Custom Dish feeds - Above installation @ HB9Q
Custom low-noise dish feeds
Custom low-noise dish feeds
G0KSC Custom Dish Feeds

Above installation @ HB9Q

G0KSC Custom Dish feeds - Above installation @ HB9Q
previous arrowprevious arrow
next arrownext arrow
 
Read Time: 2 - 4 minutes

The full EME 2M LFA Yagi article will appear in DUBUS issue 4 2009

I have been spending a lot of development time recently trying to enhance the performance of LFA arrays with multiple reflector elements. Traditionally, multi-reflectors have been used upon Yagis seeking very high banwidth such as those used in TV antennas which can cover 50MHz or more. However, my experiments were focused on increasing performance over a much narrower bandwidth (1MHz) in the hope that I could produce ground breaking performance from a wide band amateur antenna.

 

First glace at the Azimuth pattern shows a clean pattern with good rear end suppresion

 

When looking at the VE7BQH G/T table, we can see that very narrow band antennas produce the best sky temperature and G/T figures. However, these antennas are difficult to set up in the first instance and prove difficult to maintain performance with and keep in tune due to varying weather conditions. This is due to the twitchy tendancies of narrow band antennas. The LFA has already set new standards for temperature and G/T figures for a wide band, stable antenna and also out performs the narrow band equivlants at the same time but what if these margins could be improved? What if the goal posts moved further away from the traditional Yagi design? The intention of this experiment was to see if the gap in performance between traditional Yagis and LFA Yagis could be made wider with these additional reflectors and if so, by how much?

 

The Elevation plot shows impressive supression to the rear of the antenna contributing to the huge G/T advantage this antenna portrays

 

For 2M and particularly EME use, the most important attribute an antenna can offer is low noise properties rather than out and out gain. If you can not be heard, you can turn the power up or get a bigger amp. If you can't hear, your can't hear. A pre-amp will amplify the noise as much as it will anything else and therefore, you have to have a low noise antenna which offers gain in just one direction with massive supression everywhere else. From my experimentation, I have established it is the characteristcs of the rear bubble (within a Yagi pattern) on a Yagi which contributes greatly to the antenna temperature and G/T. Forward lobes if kept close to the main lobe, do not contribute to any increase in these figures. (this section elaborated on within the DUBUS article)

The above antenna uses 3 reflectors of different sizes and takes the same boom space as a typical 15 element 2M Yagi (4.89W/L) but exceeds anything near it for sky temperature, bandwidth and G/T performance. Early experiments with a 4 bay array have shown sky temperature figures of 214.5 Kelvin and -0.31G/T. The closest antenna on the list is the YU7EFs 15 element with 218.1 Kelvin (lower is better) and -0.46G/T (more positive is better). While the LFA still betters this antenna in both respects it is a very wideband and stable antenna. The YU7EF 15 element is very narrow in bandwidth with a VSWR of 3.18:1 across 1MHz while the multi-reflector LFA shows a VSWR of less than 1.1:1 over the same range. The YU7EF is the best of the rest as far as performance is concerned but may carry instability issues compared with wider band antennas. The multi-reflector LFA provides the best of all worlds including both antenna peformance and bandwidth stablility/VSWR too.

 

Equalling the impressive performance figures is the incredible SWR curve of the multi-reflector LFA

For the rest of this article and build dimensions, see DUBUS magazine issue 4 2009. Note, this multi reflector design is an EXCLUSIVE DUBUS design and will not appear anywhere else other than DUBUS magazine.

 

Also featured in DUBUS issue 4 2009

The new (Next Generation) Low Noise 19 element 2M LFA which makes its debut in DUBUS issue 4 2009

The above antenna is 6.56 W/L long. When configured as part of a 4 bay array, this antenna shows a sky temperature figure of 206 Kelvin and +0.97G/T.

This makes this antenna at least +0.22G/T better than the best of the rest at the same boom length (although the compared antenna is longer at 6.58 W/L Long). Want one? Subscribe to DUBUS magazine!