This antenna project has started already in year 1998. Actually planning started in '96 when we found this mast being for sale.
It should be as in "ARRL Techniques and Antennas for Low Band DXing" and "ON4UN's Low-Band DXing" books by John Devoldere, ON4UN. It should have gain of about 4 dB over single vertical and front to back ratio of about 20 dB. It is switchable to four directions plus omnidirectional mode.
Users of similar antenna: K3LR, AA1K.
Got it up and running before CQWW CW '05. Now there is real directivity and front-to-back ratio in receive. After getting some directivity I have worked close to 30 new DXCC countries. Maybe better conditions or more activity play a part in this. But they were good conditions back in '98-'99 and there was no lack of activity, while actually living on the station. Now I can make contacts with stations in same areas where I could only hear them, but not work them.
In the photo above mast erection work is in progress autumn of 1998. Juhani, OH4KBC and Jouni, OH4KZM up in the tower.
Antenna consists of about 41 m high 1/4 wave radiator element insulated from the ground and parasitic reflector and director elements. 32 m of mast and 11 m of aluminium tubing (90 x 5 and 80 x 2,5 mm) are used for radiating vertical and supporting of parasitic elements. Verticals' resonance is below 1.7 MHz. At MFJ's lowest frequency 1655 kHz impedance is 26 + j 10 ohms and reactance is decreasing.
Buried radial system has been under construction for many years now. Elevated radials don't work with parasitic elements, done that. Currently there is about eight kilometers of wire installed on the ground. A lot of radials were put down before CQWW '05 contests. Still need to add some and have to plow radials into mossy lawn where they reach the garden. I found it easiest to use shovel to make slot in grass. Wire was pushed into slot with a stick. On gravel I used small hoe to make groove for radial wire and backfilled while keeping wire on the bottom of this groove. Radials are kept close to the surface, from 1 to 4 cm deep.
This is first winter with elevated radials. Feed is directly with 50 ohm coax.
Four pieces of 23 m high top-loaded verticals with 20 meter long sloping T-sections are used for parasitic elements. These are made from 1 mm thick copper clad steel wires. Relays and coils are used to resonate these to act as director or reflector. Parasitic elements are supported with 3 - 4 mm ropes. These run through four rollers installed on the top of aluminium tube. Lower ends are fixed to trees with similar rollers up on the tree trunks. Parasitic elements went up and were tuned with MFJ-259B. Directors were resonated at 1935 kHz and reflectors at 1778 kHz as per ON4UN's book. It was easy to adjust each element to 0 ohms reactance. Real part of impedande varied from 30 to 40 ohms depending on the element. I found two relays which didn't make good connection and I changed them.
Before CQWW 2011 did some retuning. Used AA-230PRO to tune directors to 1935 kHz ecsactly, very easy with this new equipment. Used also oscillator and roller inductor to find best f/b. Did new coils on T200-2 toroids as per measured inductance. Work is still continuing.
Switchable directions: NE (Japan, Far East, Oceania), SE (Middle East, South East Asia, Oceania), SW (Europe, South America), NW (North America) and Omnidirectional.
I use MFJ-259B antenna analyzer for tuning and measuring the antenna. This one I blew up several years ago when connecting charging cable. The unit is not well protected for inverse polarity I noticed. Two transistors and two regulators have been changed. I recently found revised calibrating instructions from w8ji.com. I followed these and got analyzer running. Seem to be that I had not properly adjusted harmonic suppression before. This time I used 10 MHz open stub and internal Vz to set harmonic suppression. I have made a set of 1/4 watt 1% resistors fixed to modified BNC-connectors for calibrating. Analyzer seems to give now quite accurate results.
Got AA-230PRO and use it also. Seems to be fine equipment, a bit expensive though.
Low-pass L-network was built to match antenna impedance to 50 ohms coaxial cable. It consists of series coil and shunt capacitor. Coil is made from 6 mm copper pipe and has 12 turns with inside diameter of 7,3 cm and length is 13 cm. Capacitor is two doorknobs, 470 pF and 1000 pF, and 500 pF air variable in parallel. Variable is adjusted for 0 ohms reactance and is now about 100 pF. Omnidirectional vertical is tuned to 50 + j 0 ohms at 1832 kHz, vertical array's impedance is about 33 + j 0 ohms when parasitic elements are on-line. Need to add another circuit to match antenna to 50 ohms and maintain element resonance.
The base of vertical. Mast is insulated with green plastic. Metal box is installed with ceramic insulators. Plastic box attached to metal box contains connections of relay control cables.
New L-network.
Top of vertical. Supporting ropes for parasitic elements are visible.
Relays and loading coil for each parasitic element are installed in plastic box. New coils are on toroids.
Here can see guying. Picture is panorama that's why wires are "broken".
Ropes highlighted with grey and one T-element with black colour. This element is towards south-east.
Two pictures of top section.
This is how lower guywires attach to mast. Slight corrosion can be seen on the tower structure. Sections are hot dip galvanised, but maybe 25 years ago. Mast members are 20 mm and diagonals 12 mm solid steel bars. Each section is four meters long, so there is eight sections.
This drawing shows approximately how antenna is located. North is up. Lot is about 100 meters wide and 200 meters long. Buildings 3 and 5 are surrounded with radial bus wire where other radials connect. It is not so beautiful in real life, but most of wires can't be seen anymore. Maybe 30 % of radials were put down last autumn 2005.
OH4EA, OH4HLJ, OH4JFN, OH4JGE, OH4JP, OH4JT, OH4KBC, OH4KEC, OH4KLU, OH4KPN, OH4KZM, OH4MDY, OH4RW, OH4TY and OH5LP.
Updated: 24-01-12
© 2000-2012 Jukka Tarvainen