The chip is designed for usage in 50 Ohm systems and the surrounding circuitry is fairly trivial and well described in the data sheet .
The general connection pattern for all MMIC chips follows this schema:
A RF preamplifier for the small receiving loop antenna
Improving the performance of a small receiving loop is normally done by adding a low noise preamplifier close to the Antenna. Several designs have been published for such preamplifiers to be built with discrete components, I have chosen to design one around the MMIC MSA0270 silicone bipolar amplifier which is specified with a 12db nominal gain up to 2.8GHz. Of course the chip is a bit of overkill for this purpose but I have scored a small quantity of these on eBay so this should be a nice project to experiment with them.
The resistor for the MSA0270 is calculated as
R = (Usupply - 5V) / Iamp
We desire to run the amplifier at 25mA current and plan to feed it with 12V, R must be (12V-5V)/25mA = 280 Ohm (or closest value).
The amplifier itself is built directly on a piece of single sided copper PCB into which I have cut a couple of incisions with the dremel drill and a router bit. Ideally, the board should be double sided and the dimensions of the stripline should be calculated according to the PCB material to achieve a precise 50 Ohm conductor. For this case, as the receiving loop anyway operates up to 7 MHz only, the dimensions of the stripline are uncritical.
The two capacitors are 100n ceramic types.
As the amplifier is fairly sensitive, it should be enclosed in a RF tight enclosure. Special attention should be given to the incoming 12V power line as this might carry stray RF. It can either be fed through a passthrough capacitor or, if the enclosure is not suitable (like in my case), should carry a blocking capacitor directly at the power socket.
Measurements
Current The circuit draws 25mA of current, as designed.
Amplification .. This can be measured with the miniVNA in transmission mode but an attenuator should be inserted between the DUT output and the amplifier input in order to compensate for the expected amplification. The first test showed an unexpectedly strong output signal (20db over input) with a very strong frequency dependency, apparently originating from self-oscillation. An additional buffer capacitor and a better RF choke have remedied the issue.
The following measurement has been taken with a 20db precision attenuator between the VNA generator and the amplifier input.
As can be seen, amplification goes well up to 180MHz and probably higher but at some point the inadequate PCB layout and capacitor quality will degrade performance. For its intended use in the HF band, the response is completely sufficient.
Practical results
My first test has been with the amplifier feeding into my trusted Softrock40 which has been found to produce very reliable input power measurements (need to write about this!) The receiving loop was kept in the basement where so far no reception was possible. On 40m stations all throughout Europe have been heard succesfully with the amplifier enabled.
As this setup only has a low quality sound card (16 bit), the noise floor (-110dbm) is too high to visualize a contribution of the amplifier. Further data will be provided when I have tested the amplifier with the 24bit/96KHz setup.
On the "classic" transceivers several stations have been heard, which have not been audible over the (matched, short) attic dipole. Especially at night, a direct side by side comparison on 80m has shown stations crystal clear which otherwise were drowned in QRM. The S-Meter showed the signal at 2 S-meter steps stronger on the loop. I would love to attribute that to the loop but am confident that the attic dipole is not performing well on low frequencies with a wet roof. Once the outdoors antenna can be used again, I will update with a comparison.