It’s always nice to find that other people’s findings match your own. PA0RDT is famous for his design of the mini-whip, a very small but effective long- and medium wave antenna. Roelof found that the magnetic component of a radio wave could more easily go through obstacles, while the electrical component was more contained within a closed space, like a house. This means that the electrical component of noise generated by appliances in the house stays inside – mostly – while the magnetic part travels outside the walls. Thus his mini-whip design picks up the electrical component only and amplifies that. If the antenna is placed some distance from interfering obstacles it will result in a strong, but relatively noiseless signal. I build one and can say it really works.
But amplification comes at a price: IM and overload. Passive loop antennas have non of the afore mentioned problems. Since the beginning of my radio days I have always had a one metre square loop antenna for MW and LW and deep in my heart I longed for one. So I build one again, but this time bigger: 180 cm square with the corners clipped. Tuning is done remote with a BB112 varicap. The circuit was once published by the Benelux DX Club and I’ve had it for over 25 years, but never build it to this day.
Because of bad weather I could only put it up last weekend and boy, what was I disappointed. It seems it picks up all the television noise from the whole neighbourhood, with added noise from within my own house. The mini-whip is clearly better than the loop, so PA0RDT is right. But using another laptop power brick helped a bit and after firing up Argo it seemed that despite the noise the loop was still proving itself useful.
The loop tunes from 136 kHz to just above 400 kHz, which covers most of the NDB frequencies. Even though long wave beacons are on the decline there are still a lot of them. I heard some 30 new ones over the weekend, both in the daytime and at night. Argo is a great tool, sometimes beating my ears in picking up signals. Here some screen shots.
Locator “O” on 201 kHz and PQ on 202 kHz, both unidentified.
An odd one heard on 220 kHz: BRBA5. Notice that the dash in the letter “R” is longer than the other dashes. Ears won’t notice this, but with Argo you can see it.
Three beacons on the same frequency of 380 kHz: LM, OB and sandwiched in between YU from Hualien (Taiwan’s east coast).
NDBs are fun to DX, but my goal is to do some 2200m DX. Without any voltage applied to the varicap the loop is tuned to 136 kHz, which means I can leave it on all night and hope that I can detect some signals from Japanese hams, or maybe the Philippines. Unfortunately Chinese hams are not allowed to use 2200 meters and I don’t know of any Taiwanese hams operating this low. Help! I think I am becoming “low-nely.