Category: Receivers

A recent posting on social media kicked off some discussion about infinite gain preamplifiers, mostly in the context of an unloaded very short vertical.

Over the past couple of years I’ve had a number of comments and questions about active antennas, instigated by my ARRL book, Receiving Antennas for the Radio Amateur.

The “main ingredient” of an active antenna (in this discussion, we’ll center on the very short WHIP), is the preamplifier, which generally takes the form of an FET source follower.

A true source follower (or ideal cathode follower) is theoretically capable of INFINITE power gain). In practice, modern FET input op-amps have an input resistance on the order of a teraohm or so, and an input capacitance of about a picofarad.

Although we can’t QUITE get to infinite power gain with a real FET (or FET input op amp), we can get EXTREMELY high power gains. Assuming an output (source) resistance of 1Kohm and an input resistance of 1 teraohm, a voltage follower will have a power gain of 10^21:1…..not too shabby. (This is assuming essentially a DC signal, where the input parallel capacitance can be ignored).

At this point in time, there has been no mention of noise… but it is key to the problem. Continue reading The need for infinite preamplifier gain?

The article The oft asked question of how much an LNA improves a 70cm weak signal station solicited some comment on optimal configurations. This article deals with the notion / Rule of Thumb that optimal LNA gain is just sufficient to offset line losses.

This article explains with graphs the relationship between Signal / Noise degradation (see Signal to noise degradation (SND) concept) and LNA gain in the configurations discussed in the original article. See The oft asked question of how much an LNA improves a 70cm weak signal station for documentation of the scenario assumptions.

The critical value for SND is a personal choice, but for the purpose of this discussion, let’s choose 1dB. That is to say that the S/N at the receiver output is less than 1dB lower than the ultimate that could achieved with the antenna system given the external noise environment.

The total line loss in the example configurations was 2.6dB. The model assumes that LNA Noise Figure is independent of LNA Gain, though in the real world, there is typically some small dependence.

Often the choice of LNA Gain drives the choice of a single stage or two stage LNA, which has cost implications.

Terrestrial external noise – 495+5K

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 15dB LNA gain above which SND improvement is slower. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station – Rules of Thumb

A recent online post seeking opinions on the chap’s 70cm weak signal configuration is an interesting subject for study, and one that should be of interest to many weak signal DXers.

This article focusses on just one question in a quite similar configuration, what is the advantage given by the LNA?

Study configuration

The scenario will be evaluated for both terrestrial and satellite paths.

Above is the assumed ambient noise environment, it has great bearing on the results. More on that later. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station