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 Isotron 80/40 Combination
Our Most Popular Unit!
www.isotronantennas.com
 
THE BEST WAY TO OPERATE HF
 
EASY INSTALLATION
EXCELLENT PERFORMANCE
DURABLE CONSTRUCTION
CC&R FRIENDLY (XYL ALSO)
 
Bandwidth and Interference
 
With the HF bands being in a slump for so long, many have found it advantageous to increase the power output of their station. This increase in power is normally appreciated by the receiving station.
 
This has made a disturbing situation. Regretfully there are a number of operators that drive their amplifiers much too hard. How can you determine this?
 
I have heard conversations where one or both parties have a slight distortion on their audio. The two in QSO seem to be oblivious of it. I imagine that the poor quality of digital audio as with cell
phones have made this common place. However, on the air it is unacceptable. Why?
 
It is not only unpleasant to listen too, it causes severe interference to other stations trying to use the same band.
 
Most transceivers will not allow this to happen. However, if you add an amplifier it can happen quite easily. Most amplifiers do not have the capability to determine distortion. The ALC function will not do this. It is up to a good operator to determine the limits on driving the amplifier from the transceiver.
 
A linear amplifier basically takes a signal from the transmitter and amplifies it producing the same signal out, only stronger.  The linear amplifier can only handle a certain amount of drive
power from the transmitter. If you stay within this parameter, your signal will be stronger, but have a good quality to it and produce little IMD's.
 
What are IMD's? "Intermodulation-distortion".
 
What does this do? It will increase the bandwidth of your signal. In most cases it will not increase your signal strength. You have already saturated to that point.
 
In practical terms, we should be able to be 3 KHz's away from another station with no interference (IMD). However, over driving can cause an increase in IMD's. This means the signal produced will have audio fluctuations that far exceed 3 KHz. If a station is set
at 14.300 MHz, it will interfere with stations far beyond 14.303 and 14.297 MHz. Some stations are so badly distorted that you can not use a frequency 10 KHz from the offending station.
 
Using an amplifier requires being responsible to operate it properly. It will produce severe IMD's. With the increase in better propagation coming, we will find the bands less usable if this
distortion is a practice among operators. There are operators that do not care. It does little good to advise them. However, those that want good operating practices need to be informed so their signals will be clean and allow more room on the band for others to operate.
 
73,
Ralph WD0EJA

April 2021

 
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