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Ideally, you should place attenuators between the LNA and receiver, this ensures the best overall Noise Figure.

However, there is a trade in LNA design between strong signal handling and Noise Figure.

Most cheap LNA's don't have a sufficiently good enough strong signal handling performance, and they produce Intermodulation products, which typically show up around the MW broadcast band and either side of the 160m Amateur band.

The Intermodulation performance of an amplifier is usually specified by the second and third order intercept points that are typically measured at the amplifier output. This is a theoretical value where the Intermodulation products rise at a faster rate than the original signals, and if you plot all these levels against each other, you will find points at which they intersect. Once you have such a graph, you can then work backwards, to see what maximum level the input signals can be, before the Intermodulation products start to rise above the receiver noise floor.

I use a copy of the DXE-RPA1 pre-amplifier and have measured the IOP2 at +92dBm, and the IOP3 at +45dBm, and this is just about acceptable when connected to a decent antenna.

For comparison, a single PGA-103+ amplifier (still decent, and better than most) measured an IOP2 at +48dBm, and the IOP3 at +36dBm, but this does produce noticeable IMD products.

The Kiwi typically has a Noise Figure of 14dB at 30MHz, which is just about adequate. If you add an LNA with a better noise figure, that will improve the sensitivity on the upper HF bands. But every dB of attenuation, you place before the LNA, will degrade its Noise Figure by the same amount. Placing attenuation after the LNA, does not degrade the system NF to the same extent. There are various cascade Noise Figure calculators on the web, if you wish to investigate this for yourself.

Pre-amplifiers should ideally only have enough gain to overcome any system losses. As a simple test, if you connect a 50 ohm load in place of the antenna, and note the level of the noise floor at 30MHz, then connect the antenna back up, you should aim for a 6dB increase in the level of the noise floor. Any less than this and you are missing some signals, and any more than this, you are using up the KiWi's valuable dynamic range, which is not desirable.

Regards,

Martin

Personally, I think the internal attenuator is more trouble than it is worth.

An external attenuator controlled by the KiWi, as a separate accessory, may be a better option for those that need to handle day / night propagation changes.

However, in most cases, an amplitude / frequency slope equaliser and / or Broadcast band notch filters can fix most problems, without having to dynamically change the overall gain distribution once it has been initially set. Having a fixed system gain, makes it easier to observe changes in propagation and noise levels.

Regards,

Martin

My experience of building filters like this, is that the values provided by the various on-line calculators for self-supporting air spaced inductors wound from wire, is that they are not that accurate.

Even when you have wound them, and tweaked than so that their measured values are correct, you will need to adjust them again once they are installed in the circuit, in order to achieve the required response curve.

To do this you ideally you need a VNA, or Spectrum Analyser and noise source, so that you can bend the coils in real time and observe the effect.

SMD components can produce reasonably repeatable filters, as their construction and tolerances are much less affected by external factors.

However, if you don't have access to the test gear required to set it up, then there are plenty of cheap kits, or ready built, FM Band Stop filters being sold on AliExpress, starting at less than $5 USD, which is pretty much just the component cost if you were to buy the parts alone.

https://www.aliexpress.com/item/1005007644615468.html

Regards,

Martin

Regarding the DX Engineering RPA-2 pre-amplifier.

It's a good enough device, but somewhat expensive. I have used cloned copies of the RPA-1, which has a very good IMD performance, but the noise figure is not that brilliant, at around 6dB.

As your signal levels are quite low, I think one of the PGA-103 based pre-amplifiers that are available on AliEpress, such as this one, would be a good option.

https://www.aliexpress.com/item/1005005241002955.html

But you may need to add a 10dB attenuator on it's output, to bring the signal levels down to a more suitable average.

There are many other pre-amplifiers on AliExpress, that may work just as well or perhaps even better. But as most of the MMIC chips used in them only tend to be specified from about 500MHz upwards, I don't know how well they would perform on the short wave bands. From experience, the PGA-103 works well below 50MHz, and it's gradually rising input impedance, makes it suitable for use as an active antenna too.

Regards,

Martin