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Author Topic: Best broadband receiving antenna below 500khz (longwave, NDB, DGPS)  (Read 1614 times)

Offline Uli_Bel

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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:8073
my online web-receiver, during the winter set-up with Beverages, daytime with horizontal loops at 11m

Offline IQ_imbalance

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Re: Best broadband receiving antenna below 500khz (longwave, NDB, DGPS)
« Reply #1 on: February 21, 2018, 1452 UTC »
I had pretty good luck with NDBs using my H-800...but that was with my PCR-1000, not the Afedri (which is sensitive to BCB overload).  Might be worth looking at LF Engineering's dedicated LF antennas, depending on your receiver.
LOG/NE-SW unterminated BOG
AFEDRI SDR-Net
Central MD

Offline RobRich

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I used to routinely listen to European longwave broadcasters using my 148' loop-on-ground antenna.

https://www.hfunderground.com/board/index.php/topic,29940.msg114696.html#msg114696

"Used to" because many of the LW broadcasters have shut down.
Tampa, FL USA | US Map Grid EL88
Kenwood R-600 + R-2000 | 2x RTL-SDR V3 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
Ameco PCL-P | MFJ-784B | MFJ-901B | 2x MFJ-1020B | 2x Timewave DSP-9
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals

Offline ThElectriCat

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I haven't experimented much with active loops, but I am usually a fan of small (relative to wavelength) loops over wire antennas, even loops of the untuned vareity. I get a much higher noise floor on wire antennas, and switching to the loop does not lower the signals any more than it lowers the noise. That being said, I also have an ac powered receiver, and operate at my house. if you can get away from the city, the wire may preform better.  as far as active antennas go the best source of info I have ever found is this site
http://www.vlf.it/
it is pretty much devoted to stuff below 22 kilocycles, but almost all of their "receivers" are actually preamplifiers, amplifying audio range rf signals. these designs could probably be scaled up to higher frequencies. whatever you do, if you build a preamp, make sure it has a good rolloff in gain above 500 kilocycles, or even weak AM stations will clip and intermodulate your front end
In another life, I could have been a telephone engineer.

Offline Looking-Glass

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Was told a couple or more years back by a Scottish SWLer that Wellbrook Loops from the UK are the way to go for LW DXing, not cheap in price but very efficient... ;D
« Last Edit: April 08, 2018, 2304 UTC by Looking-Glass »
Hermitage Flat, NSW.

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Yaesu FT-1000D, Yaesu FT-2000D, ICOM IC-736 HF/50MHz, ICOM IC R75 & Tecsun S-2000 to 450 feet of wire, and a multi band vertical of dubious reliability.

Offline ThaDood

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         I've been wanting to try the WOG (Wire On the Ground.) A friend of mine swears by it, and having it on the ground reduces the QRM noise levels. just don't lay it out where you have buried power lines, or under the power lines above. Unfortunately, being April 2018, we are now getting into constant lightning static noise season, but this November I know that I'd like to give this a try and lay out about 200ft of EXT cord and use that as the WOG for the winter.
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline RobRich

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I used to run an 150' grasswire built with RG-6 coax fed with RG-6 coax. Center of the feedline coax was to the center conductor of the antenna coax with the far end shorted back to shield. The shield side of the feedline coax can go to a ground rod, a couple of perpendicular shorteneed radials, etc. I did not bother with a transformer.

That said, using just wire for the primary antenna element should suffice as well. I just happened to have lots of RG-6 coax to use.

I did not check longwave performance at the time, but it worked fine for my purposes for MW BCB through much of the HF spectrum. I did have a preamp available, but I do not remember using it much, especially at lower frequencies.
Tampa, FL USA | US Map Grid EL88
Kenwood R-600 + R-2000 | 2x RTL-SDR V3 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
Ameco PCL-P | MFJ-784B | MFJ-901B | 2x MFJ-1020B | 2x Timewave DSP-9
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals