This is a quick look at the various antennas used for DXing the AM band. Not exactly a definitive and detailed description, but a pointer to what is successfully used these days.*Wire antennas*
The most common is a BOG, which stands for Beverage On Ground. Simple antenna. Just take a length of wire from 200 feet (60m) to maybe 1500 feet (450m) long and lay it out on the ground in a straight line. Aim it in the direction pointing from the receiver to where you want to optimize reception. The simplest version requires nothing more. It can be tweaked by using a 50 ohm to 450 ohm (9:1) transformer at the receiver end. Ground one side of each winding. The 50 ohm side is to the receiver, the 450 ohm winding end goes to the BOG wire. And, a termination can be added to the far end. It is a variable resistor of 500-1000 ohms. One end to the wire, the other to ground. Adjust for best nulling off the back.
A real Beverage antenna requires both the matching transformer at the near end and the termination at the far end. The difference is it needs to be elevated 6 feet (2m) or more above ground. And, since the velocity of propagation is faster, a longer wire can be used.
The BOG seems to become less useful if the wire is too long. A true Beverage isn't quite so limited, though the directionality becomes a very sharp pattern. You might hear a 1,000 watt station several thousand miles away, but you may well never hear anything else on that particular frequency.
The classic loop antenna is a dozen or so turns of wire in a box or spiral layout, tuned by a variable capacitor. This is a narrowband antenna. The feed is either high impedence by connecting across the capacitor, or a single turn coupled link of low impedence. I've had better luck with the latter. The direct connection requires a very high impedence preamp for both isolation and gain. Another loop type is simply a large vertical loop of wire with the ends connected to a balun.
Another version is the copper tube loop or the ferrite bar loop. Both are broadband, but can be made into tuned loops by adding a winding and variable capacitor to resonate it. Broadband use usually requires a preamp such as the DX Engineering RPA-1.
All loop antennas such as these have a figure-8 pattern and can be rotated to null an undesired station.
*Flag, EWE and similar antennas*
These are wire antennas that are quite directional. The simple version is a rectangular shaped loop of maybe 20 feet (6m) high on each end by 50 feet (15m) on the upper and lower horizontal runs. The feed is a 50 ohm to 450 ohm (9:1) transformer in the middle of one vertical end. In the middle of the other vertical end is a termination resistor nominally 500-1000 ohm. Nulling can be done by adjustment of the resistor. This antenna is directional away from the resistor termination end.
Another variation is to make two identical 9:1 transformer feeds. One goes to the receiver, the other to a smaller variable resistor. Or, both ends can be run to a phaser such as the DX Engineering or Quantum types.
There are a number of different wire layout configurations, but all are pretty similar. Feed on one end, null adjustment on the other. A Superloop is another name for one of those.
Recently I took a 3 foot (1m) copper loop and put the toroid pickups on the two lower corners. I ran those to a DX Engineering phaser. It was able to null in every direction without moving the antenna. Setting the antenna in line with the desired null direction did work better overall. Null depth was around 40db.
There are a few types in use these days. A 10 foot (3m) length of wire or aluminum tubing is common. It will require a high ratio matching transformer and a preamp. Larger verticals are also used, up to 50 feet (15m) or so. They should be used in conjunction with some sort of ground, be it driven rods or radials. One good use is paired with a loop antenna and phaser. It can make a good cardiod pattern.
Commercially there are a number of vertical whip and preamp combinations. Some work pretty well.
Over the years I have had opportunities to use large broadcast towers as receive antennas. Without exception they have not been great. The noise pickup and omnidirection characteristics have made them less useful than I hoped.
Many antennas require a ground of some sort to work correctly. BOGs, verticals and some variants of the EWE antennas all need a ground. Just driving a copper clad rod into the dirt really isn't enough. Multiple rods separated by a few steps can be better, or a number of shorter wires laid out in all compass directions can be good. For parked portable use, several radials run out and connected to the vehicle body can work.
Many failed antenna experiments can be traced to bad grounding. For example, the ideal Beverage antenna would go across a wide beach and terminate in salt water. Rather hard to arrange in most places, unfortunately. In many of these antennas, the ground connection is the other half of the antenna and should be built to do it justice.
|Last update: 01 July 2008|