Once upon a time there was a contest ....!
For many years the OZ9KY contestgroup has been participating in the VHF/UHF contesting.
Having on of the most exposed - and best - QTH in this country, and running high power,
big antenna systems and sensitive receivers/preamplifiers we have experienced our share
of interference problems.
To improve the situation, we must understand the problems involved and we must be able to do some sort of measuring of signals and equipment. In this note I'll describe some of my observations along with measurement on a number of transmitters and receivers.
How to interpret interference.
When we experience interference or "splatter" we tend to blame the other guy - after all the problem wasn't there until he started. But of course we have to realize that both the receiving as well as the transmitting side may be blamed and often both are. No transmitter is perfect. Besides the wanted signal it may put out unwanted signals like: spurious (discret frequencies with or without modulation), sideband noise associated with and adjacent to the wanted signal and wideband noise.
The receiver on its side may suffer from overload from simply not being able to handle one or more of the signals, the selectivity may be insufficient to keep faraway >20 kHz stations out of the IF amplifier and the phase noise from the local oscillator may end up in the passband through reciprocal mixing.
These are all well known and described in the litterature.
Limits for transmitter broadband noise.
When you are located on a hilltop you will have line-of-sight to a lot of other contesting
stations. A drawing will show the actual exampel.
- measurements of real life signals. It's often assumed in calculations, that stations with line-of-sight have free space attenuation between them. To check this hypothetis a number of real life measurements are being made. More later.
Direct measurement of transmitter broadband noise.
You need a spectrum analyzer with more than 100 dB dynamic range in order to directly measure transmitter broadband noise. This is much more than the 60 to 70 dB dynamic range of an "ordinary" amateur surplus spectrum analyzer.
But with SSB transmitter's there is an alternative - measure without modulation.
Limits of transmitter IMD and spurious signals.
Close to the transmitter frequency the IMD level is nowhere near -100 dB. Typical 3'rd
and 5'th order products may be 26 to 36 dB down from PEP. But at >20 kHz we ideally need
the -100 dB, and this is what we should aim for!
Direct measurement of transmitter IMD and spurious signals.
Measuring IMD and spurious in a 60 dB range is relative straight forward on a
good spectrum analyzer. It's possible to "overload" the analyzer by 10 dB -
which works in most cases - to get 70 to 80 dB range. Any more "overloading"
may saturate the IF section, and should be approached with caution
But measuring in a 100 dB range during modulation is TOUGH.
However by using an x-tal notch filter to reduce the SSB signal while passing the
sidebands it's possibel to get down to -100 dB, at least for some of the side
Limits of receiver single-signal selectivity.
As shown in the example a situations with an interfering signal of -40dBm at
the antenna is not uncommon. (If you want to speculate in worst cases there can
be more of them - and stronger) As contest stations often uses a preamplifier
at the antenna to increase sensitivity, the signal down at the receiver is even
higher. With a "well tempered" preamplifier (ie not too much) we end up with
-30dBm at the input of the transceiver/transverter.
Indirect measurement of receiver selectivity and phase noise.
A way of checking receiver performance - both blocking and selectivity -
is by taking an
with an output of 5 to 10 mV (-33dBm to -27 dBm), and using this as a
signal generator with very low phase noise.
The simple way to check a receiver:
The simple way to measure the selectivity:
Specifications for transmitter and receiver.
For the "big gun" contest station: (1kW, preamp., big antennas)
For the "small pistol" station: (25W, no preamp, single yagi)