Category Archives: Receiving

W4KGH SWL Receiving Antenna Technical Details

I’ve had a few people ask me for specifics on how the SWL Receiving Antenna works. It’s quite simple, actually: A specially wound transformer is used to match the antenna to the radio for maximum signal transfer. Now, you can just take, say, 50 feet of wire and hook it directly up to your receiver and it will work, but much of the received energy will be lost in the wire before it gets to your radio. The reason for this is a mismatch of the impedance (the RF equivalent of resistance, measured in ohms) of the antenna itself to the impedance of the receiver. Simply put, this mismatch can cause up to 90% of the received energy to be lost in the antenna wire. Not a good thing. The solution is to match the antenna impedance as close to the radio impedance as possible. This is done by dividing the antenna impedance by 9 using the transformer shown below.

Diagram of impedance matching transformer

Impedance matching transformer

Photo of W4KGH SWL Receiving Antenna

The W4KGH SWL Receiving Antenna

Typically, the impedance of an end-fed wire is quite high. It can be several hundred to several thousand ohms. The impedance of the antenna connection at the radio is typically 50 ohms (sometimes 75). You can see that if the antenna impedance is 450 ohms, this would be a 9:1 mismatch. This would result in the loss of much of the received signal. The transformer would reduce 450 ohms to 50 ohms for a perfect match (450/9 = 50) at the receiver. This would result in most of the energy received at the antenna being transferred to the radio.

In the W4KGH SWL Antenna matchbox, terminal A goes to the antenna wire. Terminal B is either connected to a ground rod or jumpered to terminal C which is connected to the coax shield. Which method is used depends on how much background noise is present at the listening site. One or the other configurations will be found to reduce noise. Directly grounding terminal B will also eliminate static buildup on the antenna wire that can damage the radio. This is much safer than connecting a long wire directly to the radio.

Bottom line: The matching transformer allows for more efficient signal transfer to the radio, resulting in the ability to pick up much weaker signals than an unmatched long wire. Additionally, by grounding one end of the transformer secondary winding, static charge on the wire is eliminated protecting the radio from damage.