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. Ye
s 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 fo
r 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
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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