main site header
Back to HOME (main index) page

LF Antennas

 The 'Pepperpot'

      The main receive antenna for LF at G4AYT is an active antenna following the design by PA0RDT. As it fits neatly into a large pepper or spice pot, readily purchased from the local supermarket, it has earned the name of the 'Pepperpot'. This antenna is very effective from a few kHz to beyond topband (2.0MHz and more), often exhibiting a better signal to noise ratio than larger antennas. It also has the advantage of being possible to site it away from local interference sources. The antenna consists of a very small 'plate', in this case a piece of copper clad board 30mm X 45mm, connected to a high impedance matching amplifier. The antenna power is fed up the co-ax (CT100) from the radio room using a DC isolating box, consisting of a choke and capacitor to separate the DC and signal. The pepperpot is glued with Araldite to a glass reinforced plastic pole of 1.5" dia., the co-ax coming down the inside of the pole. The first version had a BNC connector at the base (see photo), the second version had the co-ax directly soldered to the board and a cable tie on the inside of the base (the original lid), with a liberal coating of Araldite. The original version also had the copper clad board standing off a little way from the circuit board, this was found to be a weakness and the boards were glued together in the second version, with a solder through link wire.

Full circuit details and information about Roelof, PA0RDT's, original design which is constructed on a single board, can be found HERE.

        

             The small size of the Pepperpot antenna                     Before Installation the 'lid' is taped in place                     Testing before locating in the best position

      The above photos are of the first version, the second was operational and located about 5m above the house roof on a convenient chimney. However this developed an unidentified fault during 2013 (to be investigated). As a result the original version, now hard wired and 'antenna' glued to circuit board, has been re-instated on the house roof on a 1.5m fibre glass pole by Steve of Acclaim Aerials and is working really well (thanks Steve!).

 The 'Long Wire'

      The long wire antenna at G4AYT is about 20.3m (or approx. 66ft) of thick multi stranded, well insulated, copper wire in an inverted 'L' (very approx. 2/3 vertical and 1/3 horizontal). The support on the house end is an aluminium 2" dia. pole and at the bottom of the very short garden, a 1.25" aluminium pole - this pole is well concealed in a conifer tree. The length of wire was adjusted to be a good match on 80m. After three base insulators of the vertical section the wire goes to a loading coil in a plastic cabinet (for topband, 472kHz and 136/7kHz) then approx. 4m of RG213 co-ax runs back to the radio room. Also in the cabinet is the station RF earth connection. This earth consists of 9 ground rods and two insulated radials 33.8m long, spaced to reach all extremities of the front and rear gardens. The aluminium poles are also connected to the earth system.

      The antenna works well on 80m, but naturally requires series inductance to resonate on the amateur LF bands. Experiments on topband (160m) have been carried out to find the required inductance for the loading coil. An excellent match is obtained with 21uH, 15 turns of thick insulated wire on a 115mm former. Results on 160m are proving better than with the previous much longer full quarter wave wire which, due to the small size of the garden, had to be 'bent round' to fit the available space. Experiments are currently taking place to find the correct loading inductance for the 136/7kHz band. 131 turns of similar wire on a similar former, with the variometer indoors for final adjustment, resonates the system on the 472kHz band.

      Note: Whilst the loading coil resonates the antenna at the operating frequency, it does not match the antenna system to the cable. Ideally a toroid should be used to obtain a perfect match. However, in practice the match is so close that a near perfect SWR is obtained on 160m. 

 

The Variometer/Matching

      The low power matching (only necessary on 136kHz and 472kHz bands) consists of a variometer to bring the system to exact resonance and a toroid transformer to match the 50R cable to the TX. The unit also has a built in SWR bridge and is kept indoors.

      The variometer has an approximate range of 84 - 143uH, allowing fine adjustment to keep the system SWR as low as possible.

      The toroid matching transformer has a 10 turn winding for the 50R cable and a 17 turn winding for matching. The 17 turn winding is tapped each turn from turn 6 to turn 17, giving a matching range of approx. 18 - 145R. The toroid core is a large 3C90. This arrangment allows the station RF earth to be kept separate from other earths (e.g. mains) if necessary.

      Plenty of space is available to the rear of the unit to house additional inductance should this be required for different antennas.

      The SWR bridge is based on a design found on GW4ALG's website.

      It is tempting to use the SWR meter for tuning up, this does usually work quite well, providing a dip in reflected meter reading is used and not a peak in forward reading.

      Note: Since the photograph was taken, the large outer coil of the variometer has had the number of turns reduced to 13 evenly spaced. The inner coil consists of 62 turns.