Syllabus Sections:-

RF. amplifier and pre-amplifier

3J1   27 Recall the operation of the RF amplifier.

The operation of the RF amplifier is to increase the RF voltage of the input signal, from the antenna, from a few micro volts to say a few milli volts.
It must have very low internally generated noise as the sensitivity of the receiver will be compromised.
An effect called Radio Receiver Blocking can occur when a near by strong signal  on a close frequency to the wanted frequency is received which reduces the sensitivity of the receiver.
The use of a high quality RF mixer should be used in the design of the RF amplifier to have the best overload ability so that it can cope with any overload..
The high quality mixer has a high quality local oscillator input level which enables the mixer to cope with the high level signals that can give rise to problems with overload.
An example of an RF amplifier circuit is shown in
page 58 Fig 9.4 and there is also a full explanation as to what the components do in the text above the diagram.

3J1   27 continued Understand that external RF preamplifiers do not always improve overall performance and will reduce the dynamic range

By adding a pre amp at HF you may overload the receive circuit of the receiver and the higher levels signals produce the harmonics and distortion. If the receiver has already got internal noise then it will just increase it making it far overpower the rx of the unit to the point where No useful signals can be heard.

The effect is to reduce the dynamic range equal to the gain of the pre-amp. Thus it is often better to have a mixer and RF preamp stage and keep the dynamic range.

If your receiver has a dynamic range of 70dB that is the range between the weakest signal your receiver can handle and the maximum signal - if you add an RF amplifier of 6dB gain then that original dynamic range could be reduced by 6dB and the performance is degraded and this is why you see transceivers with a switch to switch in or out the amplifier.

3J1   27 continued Understand why at HF this loss can be as much as the gain of the preamp but that at VHF and above a low noise preamplifier is beneficial.

There is an something to be aware of when adding RF pre-amplification gain it reduces the dynamic range of the radio.  When you use a  preamp and the noise floor in a receiver goes down, so the preamp is likely improve the Signal to noise Ratio because it is revealing signals that were below the original noise level. Thus with a preamp switched on if the noise floor does not drop or if it  goes up it means that you were already receiving at the atmospheric noise floor level and you have just added unneeded noise and, decreased the dynamic range of your receiver by the gain of the preamp. 

However on VHF/UHF this is not the same as these bands are normally much quieter than on the  HF frequencies, so if the frequencies to be used are very noisy its best not to use a preamp however if the frequencies to be used are quiet then it is acceptable to use a pre amp as it will not overload the receive circuit.
 Remember if you add a pre amp to a good receiver it may make that receiver WORSE !
!

3J1   27 continued Understand why most benefit is gained by locating the preamplifier at the antenna

If using a preamp is needed then locate it at the masthead, as the signal travels down the coax / feeder it will be 
attenuated the signal  but the attenuation of the signal by the feeder will have less effect on the receiver because the signals have been boosted in level before reaching the receiver.
Try to make sure that any preamp used has at least the same noise figure as the receiver if not better. It is also worth noting that by adding a preamp you may also overload the the receiver with strong signals .






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