31 December, 2017 - Reading time: 8 minutes
Sometime ago I built a decently sized 6m yagi here at home - aYU7EF EF0606. This yagi has performed very well over the past few years, and I've had to do very little maintenance on it. I've worked all of mainland VK on Sporadic E, JA, ZL, Fiji and the Philippians. One thing that has eluded me though is USA on WSPR. N3IZN runs 6m beaming VK during the summer months here, but I have yet to decode Chris's signals. One potential issue is that this heading is directly over my house (and several other houses in the neighborhood, as well as a fish factory some 2km away).
I have the same issue with this fish factory on 2 metres. On that heading is directly toward ZL, and my noise floor increases dramatically in that direction.
I've been looking at Justin G0KSC designs of LFA antennas. These seem to offer superior noise rejection from nearby noise sources due to the utilization of a Loop Fed arrangement. I like experimenting with antennas, thus I think I'll give one of these antennas a build to evaluate performance. Below is the comparisons I made with G0KSC designs vs my YU7EF.
Below is both antennas modeled in MMANA - Note - Justin specifically says on some of his designs that the true SWR of the LFA arrangement on some antennas cannot be accurately modeled without using the NEC4 engine (something MMANA does not have). Most of his 6m designs however model perfectly fine in MMANA.
YU7EF EF0606 Free space
Gain = 9.63dBd
F/B = 27.9dB at 180 degrees.
This antenna has a distinct rear lobe which is not uniform. The elevation plot is also interesting. There are lobes present, for instance at 76 degrees which is 15dB down from the main lobe and off the back (21dB down at 135 degrees). Why is all of this handy? These elevation plots show us how much noise (read unwanted signals, TVs, solar inverters, DSL etc) are being picked up by the antenna from nearby sources.
A major stumbling block however is that all of these plots are in free space... what about over real ground? In my reading various articles and blogs, comparisons are made between antennas which are at different heights above ground. Sometimes an operator will say "my old yagi worked better for DX, even when it was closer to the ground".... This needs to be taken into account. For any yagi to be truly compared it needs to be on the same mast, at the same height, or very close to it.
I'm not too fussed... as long as performance is not degraded to the point where I notice it, I don't care. Nevertheless, I want to be able to increase my performance of my antenna system, at least in theory that is what the whole point of this article is about.
Lets have a look at YU7EF over real ground (plotted at my antenna height of 7m AGL). There will be some variation as the antenna sits above a shed roof which looks like a ground to the antenna.
Notice the main lobe in the elevation plane now sits at 11.5 degrees. There is a second lobe at 36 degrees (6.3dB down) and a third at 80 degrees (almost unity gain). Remember! The elevation plot in MMANA shows on half of the antenna (i.e. the flat horizontal line X axis is the centre point of the antenna - not the GND! Therefore if you flip the antenna on the X axis 180 degrees you will find the same pattern, or similar). Why is this important? Your yagi also has a main lobe at -11 degrees too - possibly beaming directly into your house!
If we now raise the antenna to the top of my mast (9m AGL), the main lobe starts to shift downwards to 9 degrees, and the 2nd and 3rd lobes gradually start to break up. This is another reason why antennas at different heights seem to "work better" than others. Your main lobe could be looking into that elusive E layer cloud that your old antenna didn't.
You'll also notice ground gain comes into play, and our antenna now has a gain of nearly 17dBi, or 14.85dB over a dipole. Getting back on track now... let's look at G0KSC and if the claims of a "lower noise" system is true, at least in theory.
I decided to model G0KSC's 6 element on a 6.4m boom design. This is a comparable yagi to my YU7EF which is on a 7m boom. Apparently this design is over 7 years old now, and he has newer designs available. I'm yet to receive an email back from him yet on these designs. Below is the free space MMANA plot.
Figures claimed on G0KSC's website, MMANA calculations are in bold:
FG = 11.63dBi at 50.150MHz
FG = 11.55dBi at 50.150MHz
F/B = 32.27dB at 50.150MHz
F/B = 28.2dB at 50.150MHz
I suspect the F/B is different due the modeling engine in the software. G0KSC includes EZNEC Pro/4 models of all his antennas, and these confirm the pattern and FG I see in MMANA - i.e. close enough.
Of particular interest now though is the pattern. The rear lobe is slightly better. in the elevation plane, the upper lobe at 76 degrees is now 17dB down, however off the rear of the antenna, it never falls below 26dB and peaks at 37db down! A marked improvement over the EF0606. Now over real ground at 7m.
The elevation plots look very similar to the EF0606. Off the rear of the antenna is a marked improvement however, and even the third lobe (straight up and down) is at least 2dB better than the EF0606.
Conclusion:
Whilst this is a very basic theory based comparison of the yagis, the proof will be in real on air use of them. I do suffer from rain/dust static at times too, and it's proven that a loop fed system greatly reduces noise pickup. If you live in a country area, you may not notice a difference, however a residential area like mine, the difference may be substantial.
In Part 2 of this post, I think I will build the yagi and evaluate performance "on air". For this to happen, I may need to wait for Justin to email me back with his newer designs. He last told me - "Let me know what size you are looking at, a lot of the designs are 10 years old on G0KSC, a 5el 50MHZ LFA is now over 12dBi for example".
Reference:
http://www.g0ksc.co.uk/intro-lfa.html
