A little while ago I purchased a Mini-Whip set (the outdoor unit and the bias-tee) from Chine. Quite inexpensive, and now when the weather is improving (some days, it is April, after all) I started an experimental set-up.
I was aware that I should avoid noise from entering the antenna via the outside of the coax, so the first step was using a common mode choke (CMC) at the antenna, with a low cost (green) EMI toroid from China.
The antenna is mounted on top of a 4m long telescopic fibreglass mast, intitially just attached using duct tape, placed at a relatively low noise point in the garden with more or less optimized distances to all surrounding houses. This is sufficient for the testing and can be changed later. The power supply is a battery, just to avoid noise entering directly the system via mains power.
The result was quite disappointing, but not entirely unexpected up to 7MHz there was a substantial noise floor, reducing the signal-to-noise ratio (SNR) when compared to a multiband dipole I have in the garden, and the noise level on medium and long wave (MW, LW) was killing all signals less than S9 Even then, and this was encouraging, more than half of the MW frequencies showed audible stations before sunset.
Next step: Adding a "SW" CMC just next to the bias-tee. This was using a FT 240-43 toroid core with 2x10 windings of RG-174 coax. Progress: The noise was reduced on SW, now the Shannon VolMet on 5505 was audible with a decent signal, not quite as good as with the dipole, but a definite improvement. MW was still quite noisy, and LW not good enough. Very few NDBs (Non-directional beacons, mostly situated at airports) were audible in the 300 - 500kHz band. signals on 472kHz were still drowning in noise.
Yet another step: A second LF CMC was added (one more of the green Chinese toroids) was added, making MW quite usable, though not perfect.Several NDBs popped out of the noise, and with better filters in a receiver there would probably be many more to find. The usable frequency range was extended to 400kHz up to at least the 30m amateur band at 10MHz. The Shannon VolMet now has a SNR comparable to the dipole. Good progress.
This is the state of affairs at the moment. There is still too much noise for my taste, but at least I can work on removing noise coming from my own house.
There is more to try:
1) adding a second (MF/SW) CMC at the antenna. This is not too difficult.
2) Adding a ground connection directly to the ground of the Mini-Whip PCB, "bypassing" the CMCs at the antenna. This will take a bit longer because good weather is necessary if I want to solder outdoors. As an alternative the antenna could be dismounted and the soldering could be done indoors. Further, a 2m ground rod should be hammered down.
Still more to do with this receive antenna system, but it is a decent start
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