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<P><STRONG><FONT face=Verdana><FONT size=5>The 'Loop Fed Array' Yagi Antenna
Feed System</FONT> </FONT></STRONG></P>
<P><FONT face=Verdana><STRONG>Justin Johnson G0KSC</STRONG> makes public details
of the LFA ‘Loop Feed Array’. </FONT></P>
<P><FONT face=Verdana>The LFA Yagi is not fed with a dipole as traditional Yagi
antennas would be. Instead, it is fed with a rectangular loop which is laid flat
on the boom of the Yagi in place of the dipole driven element. </FONT></P>
<P><FONT face=Verdana>The LFA is not ‘The Holy Grail’ of directional antennas
and will not give any additional forward gain for any given boom length.
However, it has a new and unique combination of properties which can be tailored
to provide improvements in many areas over a traditional dipole-fed Yagi.
</FONT></P>
<P><STRONG><FONT face=Verdana>LFA Yagi Advantages </FONT></STRONG></P>
<P><FONT face=Verdana>The advantages of the LFA concept are as follows:
</FONT></P>
<UL>
<LI><FONT face=Verdana>Superior front to back ratio </FONT>
<LI><FONT face=Verdana>Exceptionally clean patterns </FONT>
<LI><FONT face=Verdana>Ultra-wideband gain and matching characteristics
</FONT>
<LI><FONT face=Verdana>A new definition of the ‘ideal’ boom length </FONT>
<LI><FONT face=Verdana>Flexible feed point arrangement </FONT>
<LI><FONT face=Verdana>Direct 50Ω feed </FONT>
<LI><FONT face=Verdana>Optimization of loop dimensions. </FONT></LI></UL>
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<TD><FONT face=Verdana><IMG height=233
src="http://www.southgatearc.org/news/may2009/images/DSC_0796.JPG"
width=350 border=1></FONT></TD></TR>
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<TD>
<DIV align=center><FONT face=Verdana color=#666666>A 5 element 50MHz Yagi
in use on my tower </FONT></DIV></TD></TR></TBODY></TABLE>
<P><STRONG><FONT face=Verdana>Superior Front to Back Ratio </FONT></STRONG></P>
<P><FONT face=Verdana>Quad beams are renowned for their readily achievable clean
patterns, with superior front to back properties over Yagis (when comparing like
numbers of elements in small arrays). <BR>The LFA Yagi seems to have picked up
this desirable characteristic. In models created so far, front to back figures
of between 30­-40dB have been readily achieved in Yagis of 7 and 8 elements
without distorting the forward pattern or indeed creating any unwanted
sidelobes. In addition, 3 and 4 element patterns look very similar in shape and
performance to quad alternatives. This is one of the benefits of the LFA, as
traditionally fed OWA antennas tend not to model well unless at least 4 elements
are used.<STRONG> </STRONG></FONT></P>
<P><STRONG><FONT face=Verdana>Exceptionally Clean Patterns </FONT></STRONG></P>
<P><FONT face=Verdana>Another very nice ‘quad-like’ characteristic of the LFA
Yagi is side lobe suppression. This is of particular importance when short boom,
multi-antenna arrays are being used for EME work. It can be quite a challenge to
eliminate rearward lobes in the elevation plane which will lead to noise being
picked up from residential locations (and perhaps causing interference in these
locations too). The LFA Yagi provides the best benefits of both small quad and
Yagi antennas in one design. The LFA concept also offers more design variables
that can be used to tailor the performance to suit individual requirements.
Although it may require much more antenna modelling, there is also a good
possibility that the design objectives can be surpassed rather than merely
achieved.<STRONG> </STRONG></FONT></P>
<P><STRONG><FONT face=Verdana>Ultra Wideband Characteristics
</FONT></STRONG></P>
<P><FONT face=Verdana>In modelling of both 10 and 11 element LFA Yagis for 2m I
have achieved over 15dBi forward gain along with more than 34dB front to back
ratio, with no significant sidelobes or unwanted forward lobes and a VSWR of
less than 1.1. This is exceptional performance indeed, but even more exceptional
considering how well this performance is maintained across a very wide bandwidth
between 144MHz and 145MHz. This Yagi provides the nucleus of a very effective
array for the serious DXer and EME enthusiast. Figure 4 shows similar wideband
VSWR performance for the 50MHz LFA Yagi. </FONT></P>
<P><STRONG><FONT face=Verdana>A New Definition of the ‘Ideal’ Boom Length
</FONT></STRONG></P>
<P><FONT face=Verdana>As any Yagi experimenter will know, for any given
frequency and number of elements there is an ‘ideal’ boom length that provides
the best balance between forward gain and front to back ratio. Shorter boom
lengths will generally provide higher F/B but at the expense of forward gain
(though if the boom is made even shorter the F/B will begin to drop as well).
With longer booms, forward gain increases but F/B ratio drops away or becomes
much harder to achieve. An interesting characteristic of the LFA is that the
‘ideal’ boom length is greater than for a conventionally fed Yagi with a similar
director structure, and this tends to give both more gain and improved F/B
ratio. This is in part due to the size of the loop and the extra space it takes
up on the boom – behind and in front of the loop, the spacings of the reflector
and first director remain similar to those of traditional Yagis. When the loop
is made more square in shape, so that it occupies even more space along the
boom, both the forward gain and the F/B capabilities increase together. Using
the 50MHz 5 element Yagi as an example, the loop length (along the boom, the
X-axis dimension in the models) is around 45cm and therefore this extra length
must be added to the boom. I have achieved 11.3dBi forward gain and over 20dB
front to back from a boom length of a little over 5 metres, which thus becomes
the new ‘ideal’ boom length for such an antenna. </FONT></P>
<P><STRONG><FONT face=Verdana>Flexible Feed Point Arrangement
</FONT></STRONG></P>
<P><FONT face=Verdana>Another powerful aspect of the LFA is the flexibility of
the feed arrangement and how it can alter performance of the Yagi. From the
limited experiments made so far, it has been established that if the loop is fed
at the rear centre (nearest to the reflector, as shown in <STRONG>Figure
7</STRONG> ), better wideband performance can be achieved than by feeding at the
front centre. Although the feed point cannot simply be switched from one
position to the other without re-optimization, a front feed can provide a slight
increase in forward gain and F/B; the VSWR bandwidth is narrower than for rear
feed but still better than seen in comparable traditional Yagis. As stated
above, the length of the loop along the boom can also be varied to change the
LFA’s operational characteristics. More experimentation is required to establish
which is the best way to feed this antenna. </FONT></P>
<P><FONT face=Verdana><STRONG>Direct 50</STRONG> Ω<STRONG> Feed
</STRONG></FONT></P>
<P><FONT face=Verdana>As with all of my designs for the amateur builder, there
is no need for any matching unit, coaxial stubs or any other tuning arrangements
at the feed point. A simple 1:1 balun or coaxial choke is enough. Fine tuning
can be achieved by moving the ends of the loop in or out (very much as one would
to tune a folded dipole). <STRONG>Figure 8</STRONG> shows another example of the
LFA’s excellent VSWR bandwidth. Note that the narrow dip which is so
characteristic of the dipole fed Yagi has gone. For the practical version of the
FD0605H 50MHz 5el Yagi (Figure 3) I tried a 1:1 voltage balun (branch feed with
¼ wave and ¾ wavelengths of coax). This was successful but the ends of the balun
must terminate upon the loop itself. Any connecting wires will de-tune the
antenna and increase the VSWR. </FONT></P>
<P><STRONG><FONT face=Verdana>Optimized Loop Dimensions</FONT></STRONG></P>
<P><STRONG></STRONG><FONT face=Verdana>As already noted, the loop dimensions
offer more design variables that can be used to help optimize the entire Yagi
array. There are still many more tests to be conducted. The loop circumference
is close to a full wavelength, but modelling experiments have already
established that by altering the shape of the loop, the F/B ratio and forward
gain can be balanced one against the other. The typical width of a loop (the Y
dimension, parallel to the parasitic elements) for 50MHz would be around 10cm
shorter than the first director and the loop length (X axis, along the boom)
between 35cm and 60cm for best results. When starting a model for 50MHz, I
typically use 2.5m width and around 45-50cm length, although with certain boom
lengths a smaller loop length of 35cm has also returned good results. Scaling
those dimensions for other bands will give a good starting point. For example,
with 5mm wires at 144MHz, the loop length I am using is around 16-18cm along the
boom. </FONT></P>
<P><FONT face=Verdana>To fine tune the antenna once it is in place, I recommend
making the end sections of the loop from a smaller size of telescoping tubing,
so it can be slid in and out to alter the total circumference of the loop.
However, it is important to note that part of the side lobe suppression is
delivered by the anti-phase currents along the ends of the loop. To reproduce
the computer models as accurately as possible, the corners of the loop should be
formed as small radius bends. If these ends are formed as half circles (to look
like a large folded dipole), the side lobe suppression will not be anywhere near
as good. </FONT></P>
<P><STRONG><FONT face=Verdana>Ultra low temperature antennas for EME
</FONT></STRONG></P>
<P><FONT face=Verdana>Another break-through is the very low sky temperature of
these antennas which is of particular importance for EME work. The popular
VE7BQH list (Google search) provides a list of 4 stacked arrays from around the
global and compares attributes such as gain, G/T, sky temperature (measured in
degrees Kelvin) and more. <BR>The LFA Yagis, while not appearing on this list as
yet, show a much lower temperature than comparable arrays. In fact, a 4 by 13
element array for 144MHz gives a lower sky temperature than any other array
listed. Considering that an antenna's sky temperature drops the larger the array
becomes and that the VE7BQH list has antennas over 8 wavelengths long, this
attribute is quite an achievement.</FONT></P>
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<TD><FONT face=Verdana><IMG height=382
src="http://www.southgatearc.org/news/may2009/images/13el.jpg"
width=350></FONT></TD></TR>
<TR>
<TD><FONT face=Verdana color=#666666>The incredible pattern of a 13
element 144MHz Yagi with extremely tight F/R ensuring low QRM pick up in
city locations</FONT></TD></TR></TBODY></TABLE>
<P><STRONG><FONT face=Verdana>New Low-Noise Antennas for 'Urban' locations
</FONT></STRONG></P>
<P><FONT face=Verdana>One problem that affects many Hams is the tendency for
antennas to pick up man-made noise in city locations. One of the new breed of
LFAs are an ultra low noise Yagi which drastically reduces noise pick up form
any direction other than the direction the antenna is facing. </FONT></P>
<P><FONT face=Verdana>This is achieved by modelling the massive amount of Front
to Back possible with an LFA (without generating unwanted side lobes) Fig2
displays the plot of a 13 element LFA showing the characteristics of the noise
noise LFA </FONT></P>
<P><STRONG><FONT face=Verdana>Conclusion </FONT></STRONG></P>
<P><FONT face=Verdana>I believe I have only scratched the surface of the LFA
Yagi concept. In a relatively short time it has enabled me to create some very
good models which are now beginning to show proven practical results, so I am
sure that with more time and many more minds, the LFA concept can produce some
remarkable antennas. I am also working on a number of models for 70cm and 23cm
that have equally impressive predicted performance – perhaps these will be the
topic of a later article! </FONT></P>
<P><FONT face=Verdana>By offering more design variables than a traditional Yagi
(the width, length, positioning and feed point of the driven loop), the LFA Yagi
offers much more flexibility in design and optimization, so that more of the
desirable performance objectives can be achieved simultaneously. Overall, this
will result in superior performance.</FONT></P>
<P><FONT face=Verdana>At last it seems possible to achieve more of the
characteristics of the legendary quad antenna within the compact and more
visually pleasing outline of a Yagi, while also losing some of the less
desirable properties of traditional Yagis. I am sure that, like me, many hams
will see the LFA Yagi as a viable alternative to both the quad and traditional
Yagi, and equally I believe the LFA will become a common feature within DX and
EME stations. </FONT></P>
<P><FONT face=Verdana>If you have any comments, suggestions or findings relating
to the LFA, please let me know and I will be happy to share these results on my
website, which in the near future will have a section dedicated to the LFA
designs. </FONT></P>
<P><FONT face=Verdana></FONT> </P>
<P><FONT face=Verdana>Below is a link to my download page where the original
DUBUS article can be downloaded. </FONT><A
href="http://www.g0ksc.co.uk/file-download/category/1-antenna-design-files.html"
target=_blank><FONT
face=Verdana>http://www.g0ksc.co.uk/file-download/category/1-antenna-design-files.html</FONT></A></P>
<P><FONT face=Verdana><STRONG><EM>Justin, G0KSC</EM></STRONG> </FONT></P>
<P><FONT face=Verdana>E-mail: </FONT><A href="mailto:justin@g0ksc.co.uk"><FONT
face=Verdana>justin@g0ksc.co.uk</FONT></A><BR><FONT face=Verdana>Website:
</FONT><A href="http://www.g0ksc.co.uk/" target=_blank><FONT
face=Verdana>www.g0ksc.co.uk</FONT></A> </P></BODY></HTML>