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Building an 11 element VK7HH LFA yagi

10 January, 2018 - Reading time: 5 minutes

As per my previous post I have been looking at building a LFA yagi. Essentially this is a folded dipole loop turned on its side (i.e. mounted parallel with the rest of the elements). The antenna is essentially a 12.5 ohm antenna with the full wave loop stepping up the impedance to 50 ohms. This enabled direct feeding with coax (with the appropriate balun of course).

I contacted Justin G0KSC several times to get details on a 11 element design for 144MHz. I currently have a YU7EF 11 element - EF0211 design

YU7EF has a EF0210LT design which has suppressed sidelobes to lower noise pickup. I wanted an antenna such as this to lower my noise level on 2m, but also have the "low noise" loop fed arrangement. I set about designing a yagi using MMANA. Now according to G0KSC, the loop cannot be accurately modelled in MMANA. However I decided to "have a play" anyway and build the design I came up with. 

I call it a VK7HH 11 element "hybrid". Hybrid because it's a mix of the YU7 suppressed sidelobe antenna, and the G0 loop fed system. As I type this, I have the yagi being built in my shed, so results to come.

The link to the MMANA file is here. I might post the antenna dimensions on this page, if I get time in the future. As can be seen in the below free space plot, the antenna has very suppressed sidelobes. My goal was to keep these under 30dB. The antenna has a peak gain of 12.32dBd at 144.3MHz (design freq). This is probably 0.5-0.7dB less than the "optimum gain" for this length of boom. However I think there is more to gain in the compressed pattern of the antenna. The purpose of my antenna is for use on Sporadic E openings, so gain isn't a massive necessity either. This antenna also models well in a 4 bay stack, suitable for EME work with very small spacing for stacking. Full details below. 

Gain = 12.32dBd (14.47dBi)

F/B = 31.1dB

SWR = 1:03:1 (144.3MHz)

Bandwidth = 1.6MHz <1.5 SWR 

As can be seen, the pattern is exceptional for this antenna. All sidelobes and behind the antenna are less than 30dB down on the main lobe. So how should this antenna perform in theory? 

Below is a MMANA (read not accurate apparently) comparison of my hybrid antenna (blue) the YU7EF EF0210LT (low sidelobe antenna) (red), my current YU7EF 11 element (green).

As is evident, both antennas beat my current antenna for sidelobe suppression and F/B ratio. Especially so on the elevation plot (right plot). My hybrid antenna has 0.5dB more gain than the EF0210LT for the longer boom and extra element, but 0.6dB down on my current antenna. I explained earlier in this article that isn't too much of a concern for me. 

A lot can be taken away from this - at least in theory, antennas can be modeled in MMANA or similar, and optimized for the users particular needs. Below is a 4 stack array and the associated pattern.

Horizontal = 2.3m, Vertical = 2.4m. Different stacking distance may be suitable for extra gain, etc. Antenna can also be stacked in a two bay array.