Hi Guys,
This is my first posting. I think it would be useful if I gave you a little bit of background. I'm mailing from Ireland. Part 15 rules as they apply in the US, don't apply here.
I imported a second hand TH5 AM tx from the US. I haven't used the transmitter for a while but had some free time today and relocated the TH5 and the 3 metre indoor antenna in the attic and switched it on. The reception seems better than before, less interference, however I noticed that it is broadcasting up and down the AM (Medium Wave) band. At first I thought there was a fault in the analogue radio receiver, however another radio receiver, this time with a digital display, shows that despite transmitting on 1610 (1611 kHz over here in Europe) the receiver also locks on 711 kHz and 1386 kHz. The problem is not replicated in the car in the driveway outside the house however.
There is only one (yes 1!) legal station broadcasting on the AM (Medium Wave)band in the whole country, so I'm not worried about interferring with a legit station, but more curious as to why this might happen. Would it be more a problem with the receivers than the transmitter????
Any ideas?
Thank you in advance.
Pat.
Welcome to the forum. We already have become acquainted somewhat through Big D Radio.
This very likely is happening just due to the RF right around the area of the transmitter. I have one too. In the house it can be picked up on 1520 , but once I am outside it stops. On 1610 with the wire antenna it goes maybe 1/4 mile to 1/2 mile. The operating frequency is set to 1610.
Alan
To expand on Radioboy's reply, those extra signals at various frequencies are audible on receivers inside the house due to the very close proximity of the transmitter and receivers due to several possible causes, none of which should be a concern.
1. All part 15 transmitters have some very low level spurious emissions due to imperfect output filtering. They are legal because the FCC requires spurious emissions to be only 20 dB below the carrier frequency. When a receiver is very close to the transmitter, low-level spurious emission can become audible.
2. When a receiver is in very close proximity to the transmitter antenna, the very strong carrier-frequency signal can cause receiver overload. Receiver overload can manifest itself by distortion in the receiver when tuned to the carrier frequency and/or by the appearance of the signal at other frequencies. The glossary at http://www.dxing.com/radterms.htm#net defines overloading as follows:
overloading: When strong signals in a frequency range interfere with proper operation of a receiver, creating false "ghost signals" on various frequencies in the frequency range.
3. By relocating your transmitter to the attic, and assuming you are relying on the AC wiring for the ground connection, you have extended the RF ground path down into the house through the AC outlet ground wire. This causes a large amount of RF to be radiated from the AC wiring which increases the signal to radios in the house, but also increases the opportunity for spurious signals to be created by any device connected to the wiring that presents a non-linear load to the RF currents flowing in the wiring.
You have already verified that the strange signals are not present on your car radio. As long as the spurious signals and harmonics (integer multiples of the carrier frequency) or not audible at a reasonable distance from the transmitter, say 50 ft or so, you don't have an off-frequency interference issue.
Guys, thank you very much for your replies.
Alan, good to connect with you again.
PhilB, I'm not very technically minded, but your detailed explaination is very clear to me and of great help to me. Thank you so much.
Kind regards
Pat
AM or FM transmitters, especially when putting out some power overload the tuner in radios in very close proximity to the antenna either imaging all over band or even knocking off all stations. This happens on cheap radios that can't reject strong signal overload. Better radios like sangeans for example will do this a lot less. Car radios are so much better and reject this. When you move the radio further away or go outside you are far enough away so this stops.
So now you know, if you have a too powerful a transmitter like the chinese FMs that have 10 watts or higher you can be interfering on a lot of radios in other peoples houses! Or communications etc.