Hi,
For reception of the NDB-Band, down to even 10kHz (Alpha beacons), I was searching for the best reception antenna.
I use broadband loops for shortwave (horizontally mounted on an insulated pipe, top of 1.5" /50mm fiberglas) - that works well. However the loops loose sensitivity quickly (also if mounted vertically), if frequency is lowered below the cut-off frequency.
So what I needed:
- directivity, at least down to 250kHz hopefully even lower in frequency
- high enough output of the system
- free of common-mode noise
- steerable in direction, or at least switchable in direction
I use 170m Beverages, but it is always a lot of maintainance involved to keep everything perfectly working. According to Murphy, problems with the wires occur, when weather is terrible, the fields are in deep mud, and temperature is just above the freezing point.... you know what I mean...
So in the meantime I am testing the following:
- Small fiberpoles with attached vertical dipoles, 2x2.5m and a push-pull, high impedance amplifier and feedcable driver in the middle of the vertical dipol
- homebrew phasing network to phase and adjust the output of several vertical dipoles in the shack
- Large spacing between the vertical dipoles (100m = 0.1 x wavelength on 300khz)
If you think, that verticals are noisy, try two of them phased for optimum pattern - you will be astonished, how much better signal to noise ratio can be. The improvement over a single vertical is just incredible.
I regular post my overnight reception results in de DGPS portion of this forum
Several tests still to be done:
- Is it possible to change the vertical dipoles into just groundplanes without getting too much troubles of common-mode noise ?
- I also used a large EWE (20m x 60m x 20m) on 300kHz with great result. However this beast is really large - not easy to install it in many different directions. Also such EWE antenna is just the same as two verticals with the horizontal topwire being the phasing line. However I like to know, what is the smallest EWE, which has still enough output on 300kHz, if you do NOT use any pre-amp and just use passive broadband matiching with matching transformer... I think the efficiency must be in the area of -30dbi, hopefully better. I guess, that a 300kHz EWE cannot be much smaller than what I used recently (20x60x20m). At least you have to have the two vertical parts at a large spacing - otherwise it will become difficult to achieve any directivity without a large signal drop...
I will post my findings here for the community...
If you have any further suggestion, let us know ...
Ulli, ON5KQ
http://on5kq.ddns.net:8073my online web-receiver, during the winter set-up with Beverages, daytime with horizontal loops at 11m