Pre-amps

[Glen Zook]

Preamplifiers are almost never really needed on HF since the ambient noise level is almost always greater than the noise figure of the receiver.  The easiest way to test the receiver is just to attach an antenna.  If the noise increases then you do not need any further amplification.

Operators today think that they have to have a high "S" meter reading.  "S" meter readings these days are meaningless.  Quite a while back the various manufacturers got into claiming their receiver was more sensitive than the next manufacturer's receiver.  However, all that they did was to reduce the signal level required for an S-9 reading and to do away with the accurate "dB over" readings.  the "olde tyme" standard for an S-9 reading was 50 microvolts of signal.  At this level a "20 over" reading requires 500 microvolts, a "40 over" reading requires 5,000 microvolts, and a "60 over" reading requires a whopping 50,000 microvolts.

I have personally measured the signal levels required for an S-9 (and "dB over") readings on a lot of "modern" equipment.  Quite a number of the units have been "set" at the factory to read S-9 at a level of 10 microvolts or less.  Many of the units read "40 over" at 100 microvolts or less signal input.  The result is that way too many operators these days expect to receive at least an S-9 reading and most of them are insulted if they don't get at least a "20 over" reading.

All of my receivers have their "S" meter calibrated for a 50 microvolts signal level and they are pretty accurate in the "dB over" category.  The result is that many stations don't even make an S-9 reading on the "S" meter, let alone a "20 over" (or even higher) reading.  Unfortunately, if I give someone an actual report of what the meter is reading they are usually very insulted!  Therefore, I don't even like to tell the other station what their actual signal level is.  Basically, I use the "olde tyme" signal reporting method that we used when "S" meters were not installed on a lot of the receivers.  If I don't have any problem copying then they are a definite "S-9".

I do have a better than average antenna system and can receive stations that a lot of other amateur radio operators in the area have problems receiving if they can receive them at all.  There are photos of my antennas at http://k9sth.com/uploads/Antennas_At_K9STH.pdf and I live 1/2 block from the highest point in the city.

Way too many operators think that a high "S" meter reading is what determines how well a station can be received.  But, the "S" meter reading doesn't mean anything!  It is the signal to noise ratio of the signal that determines how well you can receive.  On "weak signal" VHF/UHF frequencies often the received signal does not even move the "S" meter.  However, the signal is perfectly "Q-5" (can be understood without any problems at all).  Fortunately, most "weak signal" operators know that the "S" meter readings are really meaning less and are used to very low "S" meter readings.

Using a preamplifier on HF generally results in an overloading situation when strong signals are present.  This produces "splatter" and other spurious products in the receiver which, unfortunately, the operator often blames on the transmitting station when the actual cause is the receiver.

Operators need to really learn how to use their equipment.  If another station is overloading the receiver (you are having "splattering" and other interference) the first thing to do is to turn off the AVC/AGC, run the volume control up and then use the r.f. gain as the volume control.  In the vast numer of situations the splatter will disappear and the desired signal will come through fine.  It is just that the "S" meter won't be moving (often it will be "pinned" because the r.f. gain is turned down).

Glen, K9STH

Website:  http://k9sth.com


--- On Mon, 12/7/09, Bob Jackson <bob at NOFROWNS.NET> wrote:

Are there any stand alone HF pre-amps out there for use with xcvrs? Some club members have older rigs whose front ends aren't too hot.


      

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