#radio #electronics #notes
> [!note]
>_This originally appeared at northcountryradio.com, as **Reduction of Stray Noise Pickup**. I did not write this!. The web site no longer exists and you need to use the wayback machine to find it, and you can [find it here](https://web.archive.org/web/20181006050708/http://www.northcountryradio.com/Articles/actant3.htm). I had a problem reading this as it was white bold-ish text on a black background and the english was hard to follow._
>
>_The original web site is down, the domain is unavailable, and all there is to go on is a P.O. Box in New York. I'd ask for permission to re-publish, but who I'd actually ask is unclear. I'm going to take the approach the wayback machine does and just say this is an archive and formatted for legibility._
>
>_I will include the original text as well as one that's slightly re-written for legibility_
>
>_If this is yours and you would like me to remove it, contact me and we'll figure it out. _
> [!success] Hurray, I don’t need this
> I’ve isolated my noise problems for now to bad networking. Hurray. You can read about that on my [[KiwiSDR Project]] page. I am going to keep this here as the internet archive version is still a difficult read and this could come in handy for someone.
Edited by Me Version:
### Minimizing Noise Interference in Active Antennas
After installing an active antenna, you might encounter excessive noise pickup, even if the location was pre-checked with a portable receiver. You may notice that when you disconnect the antenna from the cable, the receiver becomes completely quiet. This typically indicates that the noise is not penetrating the cable but is being picked up by the antenna through the cable shield.
The solution is to isolate the cable shield from the antenna and provide a noise-free ground. Since the antenna often derives power from the same cable that carries the received signal, this might seem tricky. A separate battery supply for the antenna or a separate RF coupling transformer can work, but these options add complexity.
A common solution is to use a 1:1 choke balun, which introduces a high impedance in series with the cable shield at radio frequencies while still allowing DC and RF signals to pass. You can create this balun by winding 20 to 30 turns of miniature coaxial cable (like RG-174/U) around a high-permeability ferrite toroid. This assembly is then connected to the cable, isolating noise on the shield. An earth ground (using an 8-foot ground rod) is connected to the antenna side of the choke, ensuring any shield noise is routed away from the antenna.
This method is particularly effective for reducing noise at AM and lower frequencies (below 3 MHz), which often comes from sources like fluorescent lights, dimmer switches, and power supplies. It may not be as effective against radiated noise, which requires placing the antenna far from noise sources.
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Original:
### REDUCTION OF STRAY NOISE PICKUP
**One of the problems encountered after installation of an active antenna might be excessive noise pickup, greater than was expected. Even if the mounting location was first checked out with a portable receiver, disappointing results may be found after the active antenna is installed in the chosen location. When the antenna is disconnected from the cable, the receiver inside the house goes dead quiet. This rules out noise pickup due to the noise penetrating the cable. Usually what really is happening is that the noise from inside the house or other receiver location is being brought out on the shield of the coaxial cable, where it is picked up by the active antenna, as it appears in series with the effective antenna to ground impedance of the antenna. The cure is to isolate the cable shield from the antenna and provide a noise free ground for the active antenna.**
**However, since the active antenna often derives power from the same cable that is carrying the received signal back to the receiver, this may seem to not be possible. Using a separate battery supply for the active antenna and a separate RF coupling transformer is one solution, but this defeats the convienience of a single feedline for both received RF and DC power. It also adds complexity.**
**One solution which generally works well is to use a 1:1 choke balun that places a high impedance in series with the shield of the cable at radio frequencies but will still allow the cable to pass DC and RF. This balun is easily made from miniature coaxial cable such as RG-174/U and a small (1 inch dia) toroid made of a very high permeability ( u= 2000 to 8000 ) ferrite. The cable is wound on the toroid, with 20 to 30 turns and brought out to connectors on each end. The assembly is mounted in a plastic box to electrically isolate the input and output connectors. Since the ferrite does not appear in the field inside the cable, there is no effect on RF signals. However, the outside of the cable sees a very high inductance (10-50 millihenries depending on material and number of turns). This effectively disconnects the cable shield from the active antenna as far as noise on the external shield is concerned. Next, an earth ground is connected to the cable shield on the side of the balun connected to the active antenna. An 8 foot ground rod with a short (1 meter or less) ground lead usually is adequate, as the active antenna is a high impedance device. DO NOT GROUND anything else to this earth ground. Now there is no path for the cable shield noise on the receiver side to couple to the antenna except through the high impedance of the balun choke, and any noise that does is routed to ground on the antenna side of the choke. This effectively disconnects the cable from the antenna as far as conducted shield noise is concerned. The better the ground the better the results. It also helps to raise the active antenna 2 meters or more above ground.**
**Most noise situations are worse at AM and lower frequencies, and this technique works very well in reducing noise from flourescents and dimmer switches, and switching power supplies. This noise is generally worse at low frequencies (<3 MHz) and is mainly conducted noise. This technique is not as effective on radiated noise pickup. The only cure for radiated noise is locating the antenna a good distance from noise sources**