USING THE ACTIVE ANTENNA WITH RECEIVERS LACKING AN ANTENNA CONNECTOR

A common problem encountered after installation of an active antenna is the lack of a suitable coaxial ,input connection on an AM receiver to be used with the active antenna. This is a problem with many portable radios and home stereos with AM coverage. Most stereos use a built in ferrite loop antenna. Often this loop is ineffective, as the receiver is mounted close to other equipment that produces high levels of interference in the AM band. Sometimes a high impedance input for an external wire antenna may be present, but often this gives noisy reception and may be worse than the built in loop. And, many apartment and other multiple dwelling situations have high ambient RF noise levels at AM frequencies

While a shielded RF input connector, as found in most communications receivers, is ideal, the active antenna may be connected to an external coupling loop that is magnetically coupled to the AM loop antenna in the receiver. This takes the form of a small coil of wire connected to the active antenna cable. Typically this may be 5 -25 turns of insulated wire (#22 to #24 is ideal wire size) wound on a 3 to 4 inch round form, such as a section of a plastic bottle, piece of plastic pipe, cardboard tubing, or any suitable cylindrical shape.. The loop is connected to the coax cable from the DC block that normally would feed the receiver. Place the loop within a few inches of the AM loop antenna and experiment with its position for best signal pickup. This has been found to work well at all frequencies from 100 kHz to the top of the AM broadcast band (1710 kHz) and probably somewhat higher. Most receivers tuning above this limit have external antenna inputs for shortwave frequencies above 1700 kHz and the the coupling loop will not be needed. Note that the AM loop will still pick up some noise components from the receiver surroundings, but the much stronger signals from the active antenna will help to override much of this noise
Coupling Loop with Attached F Connector.
This loop is 4 inches in diameter and has 24 turns #24 PVC covered hookup wire. An F Connector is shown that is used to connect the antenna output cable from the DC block to the loop. The number of turns is not critical, but we wanted to ensure performance at 150 kHz to enable an old Zenith portable to be used for long wave DX reception of European AM stations. As little as 3 to 5 turns would be sufficient for AM broadcast reception. Below are two photos showing the loop being used with two older receivers having no antenna inputs. Note that in each case, no direct connection to the receiver is required. You only have to adjust the loop position for good reception.
   

With 1963 Model Zenith 3000D Transoceanic

Ferrite Loops (WaveMagnet) for LW and AM on top

   

Motorola A11W 23 Vacuum Tube AM Table Radio

Ferrite loop antenna is in back of set

 
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