FM-DX: Körner 19.3 Yagi

Peter Körner designed a 19-element Yagi for the 88-108 MHz FM broadcast band with a horizontal folded dipole and five reflectors. As the name suggests, this is the third version of this antenna, optimized to 9-11 dBd gain and F/R between 3038 dB. The distance between first and last element is 5.1 m or 201″. I built this antenna in 2010 for FM-DX reception in a horizontal polarization.

Brian Beezley, K6STI optimized the F/R ratio further, reaching at least 36 dB over entire bandwidth. See more information on his website – Optimized 19.3. Currently, another revision of 19-element Körner Yagi antenna is in development, featuring F/R ratio of around 40 dB.

Dimensions

I converted the antenna dimensions to MMANA-GAL format: Korner19_3.maa. All elements (reflectors and directors) are made of ϕ10 mm (3/8″) aluminium tubing. Use insulated mounts for each element, like these:

Antenna pattern

The pattern depends highly on a polarization, as this antenna uses a horizontal folded dipole. Its dimensions were optimized for horizontal polarization, when both arms are in parallel to the received signal.

Construction

Materials used:

  • aluminium profiles:
    • main boom: 25×25×2 mm (4 meters) + 20×20×2 mm,
    • reflectors: 25×25×1.5 mm,
  • 1.5 mm duraluminium sheet for main and reflector boom joint,
  • 10×1 mm aluminium tubes (use 10×2 for reflectors) and insulated element mounts,
  • many screws, nuts and washers, kevlar line, u-bolts,
  • plastic box, cable glands, F-connectors, 75Ω coaxial cable.

Each tube end should be closed. The water may freeze inside an element and break it during the wintertime. I used some teflon plugs for that.

The main and reflector boom joint is made of two pieces of duraluminium sheets.

The antenna boom is quite long, so it requires additional support. I used a nonconductive kevlar boom guys to keep the influence on the antenna’s pattern as lowest as possible.

The dipole is made of 15×5 mm aluminium flat bars and ϕ15 mm tubing elements between dipole arm ends.

Personally I used ϕ16×1.5 mm tubes, because 15 mm were unavailable at my nearby store.

For the 300Ω → 75Ω matching I made a half-wave coaxial balun. All connections at the feedpoint are joined with special bi-metallic Cu/Al washers for long-term reliability.

 

Additionally I used a spray dedicated for PCB coating to protect all connections. Before closing the box, I also sealed it with silicone.

Model vs reality

All antenna patterns were measured with a modified Sony XDR-F1HD tuner and XDR-GTK software. My AR-303 rotator is not very linear, especially towards north, where it starts and ends the rotation. The multipath interference is visible as a choppy signal on the following plots. I used some signal attentuators for the strongest stations. I also measured other weaker signals during maintenance of my local transmitter site, carrying 6x 60 kW + 1x 2.5 kW @ 35 km.

  • 87.9 MHz: Radio Maryja Łódź, 10 kW ERP @ 95 km, no attenuator, peak 46 dBf:
  • 92.2 MHz: PR1 Sierpc, 60 kW ERP @ 35 km, 30dB attenuator, peak 85 dBf (att. to 55 dBf):
  • 93.9 MHz: PR2 Włocławek, 1 kW ERP @ 46 km, no attenuator, peak 52 dBf:
  • 97.3 MHz: Zet Sierpc, 60 kW ERP @ 35 km, 30dB attenuator, peak 87 dBf (att. to 57 dBf):
  • 98.9 MHz: RMF FM Konin, 30 kW ERP @ 99 km, no attenuator, peak 55 dBf:
  • 101.6 MHz: PR1 Kutno, 10 kW ERP @ 48 km, 12dB attenuator, peak 70 dBf (att. to 58 dBf):
  • 103.3 MHz: PR3 Konin, 30 kW ERP @ 99 km, no attenuator, peak 58 dBf:
  • 106.3 MHz: Maryja Sierpc, 60 kW ERP @ 35 km, 30dB attenuator, peak 87 dBf (att. to 57 dBf):
  • 107.8 MHz: PR1 Łódź, 30 kW ERP @ 95 km, no attenuator, peak 54 dBf:

Construction flaws

Reflectors made with 10×1 mm aluminium tubes are the weakest part of this construction. Frequent vibrations weaken the material at element mounts – see this video. Use 10×2 mm instead or reinforce the thinner ones. Read more: 2x 9-el Yagi detailed write-up.

The rectangular folded dipole conductors cannot be sealed with a cable gland. After two years, when I put the antenna down for a maintenance, I found some signs of water corrosion. I filled the remaining space inside dipole box with a spray foam and sealed it again with silicone. It is (or should be) 100% waterproof now. Another idea is to build the dipole using ϕ11-12 mm tubes with cable glands, instead of the rectangular flat bars.

FM-DX results

Although the reception is much more propagation dependent than just the antenna, it is worth mentioning about real-world FM-DX results. 

I am able to pick very distant signals via aircraft scatter propagation:

  • 89.3 Federalni Radio – Vlašić, Bosnia and Herzegovina (1943m ASL + 40m AGL) – 935 km
  • 96.5 Radio Beograd 2/3 – Crveni Čot, Serbia (539m ASL + 60m AGL) – 828 km
  • 99.7 HRT-HR 2  – Psunj, Croatia (985m ASL + 129m AGL) – 821 km

Once I noticed two Airbus A380s flying in parallel. It resulted in an extraordinary strong signal of 105.7 Hitradio Antenne – Steinkimmen, Germany @ 760 km (received on XDR-F1HD and 18dB AP-FM preamp with NF=4dB). High power stations produce RDS reception quality via aircraft scatter propagation mode up to around 700 km. Some examples:

The following list contains tropospheric ducting highlights received with Körner 19.3:

Regarding the Sporadic-E propagation, the highest signals I have ever measured were around 70 dBf:

Summary

Körner 19.3 is definitely one of the best 87.5-108 MHz antennas ever designed. Unfortunately, I cannot take full advantage of its superb F/R ratio due to strong multipath interference – metal roofs, high voltage lines and the worst – a nearby oil refinery with many chimneys and metal constructions.

This antenna is also somewhat suitable on OIRT (66-74 MHz), as this frequency band is empty, so the F/R ratio is not very important. For the bottom of band (below 69-70 MHz) Körner 19.3 should be directed backwards, as the reflectors will act as a directors. For the top of band (above 69-70 MHz) it should be directed right ahead.

The following video presents a full rotation of my Körner 19.3 recorded in August 2010. Currently, the antenna is installed on a higher mast – 10 m AGL, instead of 7.5 m (+ 114 m ASL).

Last update: December 2017