[HCDX] HF Preamps~ Do I Need One?

[W5USM@xxxxxxx]

A well respected MW and SW DXer posed this question today in an e-mail.  
Thinking an answer through I thought may be it was useful to share some 
thought on the subject.

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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