Topband: Birdies

[Lee K7TJR]

     The subject of birdies on top-band is a lot more complex than it may seem on the surface. The ones most often seen in the US of course are the BC birdies on 1810, 20, 30, etc. These can come from multiple sources. One of the sources can be mixing right at the BC transmitters themselves and they rear their ugly head occasionally. They usually have to be tracked down as many have pointed out. Another source that may need to be tracked down is a birdie created as the result of local mixing in dissimilar metals usually low level and close to the receiver location.
     Another source of the birdies can be the receiver itself. This can happen even in a so-called high dynamic range receiver depending on a lot of circumstances. Another source of the birdies can be any additional preamplification added to the receiving system. Both the receiver and a preamp will generally have a 3rd order IMD spec that shows what level a birdie will be as a result of input signal levels. While it is indeed true that a high dynamic range receiver or preamp will help eliminate these birdies it is important to realize that the specification in itself shows that neither the receiver or the preamps are perfect. They will make distortion and it will be a matter of degree depending on their input level. Generally speaking the IMD spec will be related to the 1dB compression level of the amplifier (meaning the RX or preamp) This spec is often measured using a single tone signal and can be typically 20dBm or more. Here is the kicker that is most often overlooked. Yes you may see that none of the BC signals that make the birdie are any where close to the 1dB compression(clipping/distorting point) however there are what, 100 channels of broadcast signals. As far as the amplifier is concerned it has to expend power to amplify each and every one of those at the same time and they all add together possibly exceeding the output power capability of the amplifier. Not only just in the BC band but any signal that is in the system bandwidth. In the case of Beverages I have looked at the signals on a spectrum analyzer and find even 100+ MHz 0dBm levels. Any amplifier/receiver that is not filtered in the chain must pass these signals without distortion. Once a combination of them  exceeds the power output capability of the amplifier they all can be mixed or distorted. Any broadband preamp or receiver input is faced with handling a lot of RF. This in some cases with modern preamplifiers can be any signal from 100kHZ to 100 MHz. Thats a lot of RF for one amplifier to process.
    My point here is that in looking for the cause of the birdies at my station I found that a high pass filter intended to reduce BC stations in front of the entire system only reduced the birdies. The only thing that seemed to almost completely eliminate them was to use a good band pass filter in front of any preamp receiver combination. YMMV
Lee  K7TJR  OR