Category Archives: Noise

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.

Using the W4KGH SWL receiving antenna

The W4KGH SWL antenna does not require any assembly; it is ready to deploy.

Your site layout–or your personal preferences–will dictate how to hang the antenna. Hanging the antenna horizontally will take up the most real estate and is usually not feasible in an urban setting or a small subdivision lot. A vertical installation can work if you have tall trees on your lot or live in a high rise apartment or condominium (drop it from a sixth story window, for example).  An inverted vee or sloper configuration is usually practical just about anywhere. The idea is to get the end of the antenna or the apex of the vee as high as you can.

My personal preference is the sloper. This configuration has an omni-directional receive pattern closest to that of a vertical antenna, especially at lower to mid HF frequencies. It also allows you to keep your ground run short and close to the end of the antenna.

How you configure your ground will depend on conditions at your location. The antenna matchbox contains a 9:1 impedance matching transformer with both the primary and secondary grounds connected to separate studs on the matchbox (B & C in the photo). Normally, you would want to short these two connections and connect them to ground. You could also leave this connection ungrounded at the antenna end and ground the receiver end. The problem with that setup is that the coax shield can pick up noise, since it can act as part of the antenna. If you have a “radio quiet” location, this won’t matter, but few locations are free of man-made noise these days.


Matchbox Connections: A-Antenna, B-Secondary Ground, C-Primary Ground

Another configuration is to remove the short and ground the antenna side of the transformer (B). (This is the best connection to keep static charge from building up on your antenna.) Then, you can try grounding the receiver end or letting it float (no ground) to see which gives you the least amount of noise. A good way to test this is to tune in a relatively weak signal and see which configuration results in the best reception. Whatever you choose, there must be some sort of ground connection to stud B–even if just a counterpoise wire–on the matchbox or you’ll suffer severe performance degradation.

Wherever you make your actual ground connection, be sure to use at least a 6 foot ground rod. You can get these at your local hardware store or any big box home improvement store.

As always, I’m available for support if you need it. Just use the secure contact form.