Korner 15.11 87-108 MHz FM Yagi Review

Todd Emslie



Over Friday 11th to Sunday 13th March, 2011, I traveled down to Numeralla, NSW with
Will Renton, in order to help Geoff Wolfe install his new Peter Korner design
15.11 FM yagi. We spent about 5 hours on assembly, and 1 hour for the
installation. Rushing the install will only increase the potential for mistakes.
The director elements were numbered, but unlike most elements, there is no
pre-drilled centre hole. A tape measure and a sharp eye assist with accurate
centering of elements on the boom. The 15.11 yagi is broken down into several
parts in order to minimise freight charges. For example, the boom length is
packaged in two halves. There are lots of parts to sort out, so unless you are
already familiar with building FM yagis, be prepared to set aside the best part
of a full day for installation.

Care is needed when positioning the down-lead coax and balun. Keep them in the
middle of the boom as much as possible. Do NOT let them directly rest on any
directors beside the plastic director element holders/insulators, otherwise this
would affect SWR.

It could help to add some slip-on ferrite beads (e.g.
Kitagawa RI 16-28-7)
around the coax close to the dipole box to attenuate any remaining common-mode
signals [1]. To illustrate how this is done in practice, two photos have been
uploaded to ICDX Files.

The 15.11 yagi's forward lobe directivity is quite sharp. One such example is
88.0 MHz CRI
Canberra via TS with excellent nulls of Cooma area 88.0 signals.
Certain signals were only available when the 15.11 was carefully aimed within ~
+/- 10 degrees. Ideally a rotator is preferred for this reason. For the time
being, Geoff's 15.11 is hand-rotated. The 15.11 is mounted 6 metres above ground
(2 metres above a metal tin roof, although not mounted directly above it). 20
metres of RG6 Quad coax is connected to an Amiwima brand pHEMT low noise
amplifier, mounted adjacent to the tuner. Mounting the amplifier near the
antenna balun will provide additional (~ 1dB) RF sensitivity.

Overall gain improvement on the 15.11 translates to 1 bar signal strength
improvement as indicated on the Sony XR-F1HD tuner. Sometimes there was a 2 bar
improvement, e.g. Swan Hill FM via As. As a rough guide, doubling the antenna
gain (+ 3dB) corresponds to 1 additional bar signal strength. In terms of
forward gain performance, the 15.11 is roughly equivalent to two efficiently
stacked Triax FM8 yagis. Aircraft scatter from a 660 km distant station
(Narrabri, NSW) is now present for the first time on the 15.11 yagi.

We noted that 15.11 yagi F/B ratio performance was on average no better than the
previous 12-el ATN log-yagi FM antenna. This was a real surprise considering
that the 15.11 employs a 5-element reflector section. It is assumed that signal
re-radiation from both distant and nearby objects is largely negating the
15.11's excellent F/B specified performance. Recall that maximum F/B performance
is a theoretical dB figure calculated using computer modeling. In practice,
unless the yagi antenna is tested under optimum conditions (e.g. antenna field
test site), real-world F/B performance will be lower. Various factors such as
yagi height above ground, proximity to nearby metallic objects, and surrounding
terrain, also affects F/B performance.

Most of the unwanted local FM signals in the general Cooma /
Snowy Mountain area
utilise vertical polarisation. In theory, according to the polar diagram model,
in the horizontal plane the response to vertical signals is zero. However, in
practice, vertically polarised signals are rarely 100% linear. Also, general
re-radiation from multiple sources degrades the theoretical null when using
orthogonal polarisation. Ideally, F/B tests should be conducted on signals
matching the polarisation of the receiving antenna. Assuming local signals used
horizontal polarisation, F/B performance would probably be improved. In any
case, it was disappointing to have certain channels remain blocked for DX that
we otherwise expected to become available after installing the 15.11.

The 15.11 is capable of resolving weak AS signals out to ~ 700 km. The distance
potential increases in elevated areas with at least relatively clear horizon
takeoffs. A previous receiving location (Carinya, just outside Cooma) enabled
clear As DX from Taree: 366 miles (589 km), and
Coffs Harbour: 454 miles (730
km). It appears that
Coffs Harbour signals are arriving below the radio horizon
into Numeralla, hence go undetected. Often location makes a greater difference
than receiving equipment with long-haul As DX. Some everyday aircraft scatter DX
results from Numeralla yielded:

Mildura, VIC: 420 miles - 676 km
Narrabri, NSW: 410 miles - 660 km
Launceston, TAS: 374 miles - 602 km
Coonabaraban, NSW: 335 miles - 539 km

Shorter range tropo-scatter DX was improved as well. For example, the 200 watt
100.3 MHz Macarthur Community Radio transmitter (190 miles) was audible at clear
levels via As. It also seemed to be in at weaker levels via Ts. Sydney FM
stations (200 miles) were typically 2 bars via everyday Ts. Meteor scatter pings
also appeared to be both stronger and more frequent.

For general Ts and As DX, we found the wider bandwidth / higher fidelity Onkyo
Integra tuner provided better S/N ratio performance compared to the XDR-F1HD.
The XDR offers no sensitivity / resolution advantage on weak signals compared to
the generic Yamaha / Onkyo / Kenwood switchable IF bandwidth FM tuners. The XDR
is only worth using when adjacent channel selectivity is an issue. For extremely
weak signal DX reception (narrower FM deviation talk type signals), the Kenwood
KT-6040 (80 KHz bandwidth) produced a small advantage over every other tuner
used (Onkyo T-4650 snd Sony XDR-F1HD).

For a total cost of AU $650 including airmail postage, the Korner 15.11 86-108
MHz FM yagi represents good value for the more serious FM DX enthusiast. The
15.11 is currently the largest model FM yagi commercially available anywhere in
the world.

Links:

1. https://www1.elfa.se/data1/wwwroot/assets/datasheets/flRI_dat_e.pdf

2. https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=58-546-09&toc=0

3. Korner 15.11 FM Yagi Purchase Details.