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A
preamp is of no value
By Bill Smith
hcdx list, July 28, 2000
Preamps to 30 MHz are correctly used to
1. increase the signal amplitude (loudness) or
2. overcome combining and distribution loss.
Example 1. best applies to inefficient receiving antennas
such as the many loop and traveling wave designs.
Example 2. typically applies to feeding multiple receivers
with a single antenna such as during DXpeditions or similar distribution
situations.
Passive splitters typically lose 3 dB per port; that is a two way
splitter will deliver 3 dB less RF to each receiver than if the
antenna was directly fed to the receiver.
Four way passive splitters typically lose 5 dB and so on.
The human ear commonly will just detect a 3 dB amplitude
change.
Unless the receiver is broken, or of poor design quality, seldom,
if ever, will a preamp improve signal-to-noise (s/n or s/n ratio)
performance below 30 MHz.
A preamp used thus will increase the signal amplitude -- and the
background noise -- by the same amplitude, but it will not
improve the signal readability over noise.
Try this very simple test.
Disconnect the antenna from the receiver and listen to the background
noise. What you hear is noise generated within the receiver.
Now connect the antenna. Did the noise level increase?
If so, the receiver is operating to its ability to hear any signal
arriving on the antenna. The receiver's ability to hear the weak
one is entirely limited by atmospherics and noise picked up by the
antenna.
A preamp is of no value. It increases both the signal and the noise
by equal amounts. There is no improvement in signal to noise
performance.
Do not be enthused nor spend money on a HF or SW preamp just
because the manufacturer claims a 0.5 dB 'noise figure' or some
such number.
Below 30 MHz a five to eight, or ten, dB receiver noise figure is
more than sufficient.
Fact is, a preamp may actually harm your receive capabilities by
introducing noise, mixing and intermodulation products that could
easily cover weak signals otherwise receivable.
A resonate or more efficient antenna will return many, many
more dividends in the earphones than any expensive, highly touted
preamp. The best dollar ever spent on a listening system is on the
antenna itself.
Another fact, unless you want to listen below 2 MHz, a firewall
filter eliminating or greatly reducing signals below 2 MHz may improve
your receiver internal performance on shortwave. This is especially
true of some solid state receivers.
Finally, you purists reading this, please save your flames. This
is not a textbook on s/n et all. It cost the gentle reader exactly
what he or she paid for it.
73 from Bill Smith, W5USM "Shortwave Since 1950"
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antennX