By Bob Padula, Victoria, Australia
|Some thoughts from suburbia, 15 km east of Melbourne city!
Without an effective antenna, the most expensive bells-and-whistles receivers are useless; without a decent signal, or signals degraded by locally powerline radiated interference, we may as well go out and spend the time planting beans!
This is what I have...
5 MHZ DIPOLE ANTENNA
A simple half-wave dipole, about 30 metres long, around 10 metres above the ground, supported at one end by a steel mast tied to a tree, and by an aluminium pole at the other end, fixed to the weatherboards of the house. The antenna runs north-south, limiting pick-up of high intensity power line radiation originating from 6,600 Volt power lines in the street. The feeder is a length of plastic covered power flex, bought in K-Mart, which has a nominal characteristic impedance of about 100O Ohms, near enough to the centre impedance of an unfolded dipole.
12 MHZ DIPOLE ANTENNA
A simple half-wave dipole, some three metres above the ground, about 12 metres overall length, supported at one end by a tree, and by a sewerage "stink pole" at the other end emerging from the external laundry. Feeder as for the 5 MHz dipole. These antennas are fed to a small metal box, and are terminated in banana plugs, with the sockets on the box. The box also contains a Pi-Section Coupler and Antenna Tuning Unit, with further plugs and sockets taking the signal into either of the two receivers - the National DR49, and Yaesu FRG8800.
This arrangement minimises the impact of intense, high level power-line interference fields which surround my home here in suburban Mont Albert. Orientation of the 5 MHz dipole is north-south, which reduces radiated power line noise originating from the 6,6000 Volt distribution network in the street. Broadband noise generated by the overhead Optus cable network is also alleviated.
|The improvement in signal/noise ratio is due to the use of a balanced feeder, which limits the amount of local noise fed into the receiver, due to the noise-cancellation characteristics of such a system. The Pi-Section Coupler acts as a balanced-to-unbalanced device, as neither receiver has balanced antenna inputs. I have found that a simple dipole, feeding into a balanced line, is far more effective in reduction of local noise than an unbalanced or single-wire line. |
The use of expensive coaxial feeders (75 Ohms) are a waste of time for HF reception, as there are no signal cancellation effects. Indeed, a dipole lying on the ground, or even buried in the lawn, can often give better results than a single wire antenna 30 metres above the ground! The overall aim in increasing S/N rations is to minimise signal pickup by the feeder, and only allow real signals to reach the receiver, derived from the active, horizontal element.
Impedance matching is no big deal, as we are dealing with static devices, and not looking for optimum power transfer and coupling as with active (live) transmission lines. Yes, it's important that all un-needed domestic appliances be turned off, and disconnected from their power points. It is a total waste of time trying to tune in to weak HF DX signals surrounded by intense RFI coming from PCs, FAX machines, modems, printers, dimmers, hand-capacity lamps, VCRs, microwave ovens, and proximity sensors. Movement activated security lighting is notorious in its heavy broadband RFI, whether yours or the neighbour's! If you have these devices running continuously, you will be wasting your time in trying to hear weaker signals, as the hash is transmitted both by direct radiation, and indirectly back into the house wiring. The house wiring acts as a rather good transmitting antenna, and any junk fed into it will cause problems. Avoid anything which has inbuilt chopper power supplies, whether shielded or otherwise. EMI legislation attempts to limit the amount of direct radiation from appliances but not in the level of hash fed back into the mains.
Don't forget to turn off your spark transmitter!
Bob Padula, Mont Albert, Victoria, Australia