HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on October 03, 2014, 2327 UTC
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Any suggestions for a directional antenna for NAVTEX reception, 518 kHz (and the 490 kHz local language transmissions as well)?
jFarley - What about modifying your resonant loop for 518 kHz?
I'd like to get something up for the winter DX season, and finally bag some of these distant NAVTEX stations.
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I put up a new "double kaz" delta loop for North - South reception recently. Though intended for MW reception, the signal gain really falls off towards the lower end of the MW band, but otherwise it's highly directional with good rejection off the sides. I just bought a preamp which should help on the lower frequencies. I'm looking forward to see if it'll pull in some new stuff on 490/518 this winter.
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Hey Chris;
There's really 3 considerations here (arrived at while walking the dog...in the rain...uphill both directions...)
1) The amp. I don't see why it can't be used down here; I'm pretty sure the transformer/inductors will have enough oomph around 500 kHz. The cores are FT50-77, and should be fine down there. It is designed for a balanced center-tapped winding, though.
2) The winding. The amp has a pair of built-in NTE618s in back to back configuration, which give an effective tuning capacitance of around 10 to 250 pF. This was designed primarily for the 2000 to 8000 kHz range. This dictates the required winding inductance which at C = 200 pF, F = 490 kHz needs to be around 530 uH. That is a lot of inductance for an air core loop. To maintain directionality, the total length of the winding conductor should be a small fraction (10-15%) of the operating wavelength. The aspect ratio (the length of a side divided by the depth) needs to be less than 4 or 5, so they say.
3) The system loaded Q. This is required to establish a useful Effective Height (the relationship between the incoming signal strength and the terminal voltage of a parallel tuned tank). The tuning diodes have a specified minimum Q of 100 at 1 MHz; I tend to see better than that on the average. They should be okay, not great, at 500 kHz. The winding will need to have a very good Q - maybe around 500? - to keep the system Q up. It will be big, and maybe not the easiest thing to swing on a rotor.
The loaded Q also needs to be high to help protect against IM products from strong MW stations. The loop needs to act as its own bandpass filter.
I am wondering (and I've thought this in the past) that the best implementation would be ferrite rod loop, at least for a quick and cheap attempt. To make a center-tapped (balanced) 500 uH winding on a rod, using Litz since it would be effective down at these frequencies, shouldn't be that difficult. This rod and the amp should be fairly light, and easy to pigglyback on any rotor you already have up and running. I have been meaning to give this a try, but I just haven't gotten to it yet.
Re directionality: loops will obviously give, at best, a cardioid response pattern.
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I was also very pleased with the Kaz antenna, and the whole terminated flag family of antennas. The standard Kaz with 40' length and 10' height does have a low signal level at the bottom of the BC band, so I used the 'SuperKaz', which is 112' long and 28' high. It gave more than enough signal strength down to 30 khz, which is as low as my radios will tune.
Currently, I am using a pair of what I call half-Kaz delta antennas. A 56' horizontal wire, then straight up for 28', then sloped wire back to the feed-point. The termination resistor is located at the bottom of the vertical wire. A 12:1 balun on a Type 73 ferrite at the feed-point works well with 75 ohm coax. The front-to-back of these are impressive from 300 khz up to around 3 Mhz. Above that they become much less directional. This works well for general shortwave and pirate listening. The signal strength is good, I don't use an external amp with them.
I have recently built some chokes that really cut out the local man-made noise and hash. Type 75 ferrite works well from 300 khz to about 10 MHz. I am using Type 77 ferrite to clean up the hash below 300 khz. I am now able to hear WWVB on 60 khz better than I have ever heard it before, day and night. I use a Type 31 ferrite choke to clean up the frequencies above 10 MHz.
Another nice thing about the Kaz and terminated deltas is that only 1 (non-conductive) 30' support is needed. The bottom ends can be low. I have mine at 6' to allow for easy mowing.
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jFarley - Thanks for the info. Any recommendations on the minimum strands/gauge Litz wire I should consider using?
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I'll crunch #s and see if'n I can figger it out this weekend.
In the dim recesses of my brain, I seem to recall making up a prototype (at least) of a ferrite rod antenna which I used for some LW TA DX, and that it worked pretty well. It was a while ago, but this is an interesting thing to think about. I am especially excited about it's potentially compact size, and its ability to easily hitch a ride on an existing rotator.
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full wave yagi...just sayin
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I love my SAL-30 for the DF stuff, but a DKAZ is probably the best bet for MF. I have yet to have the NAVTEX/MF DSC reception I had last winter since took down my DKAZ. After consulting with KAZ, I have 3 new DKAZ's going up soon, one for 500 khz and two specifically for 2 mHz MF, one NE and the other NW. The 2 mHz DKAZ will also be perfect tropical band antennas up to 6 mhz or so.
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Okay!
I found an R33-050-750 rod in the junkbox which had about 50 turns center-tapped of 7/44 Litz on it; this is pretty fine as far as wire gauge goes. I hooked it up to the standard amp and fired it up here in the basement. Bingo! Tunes from around 330 to 1110 kHz, and is showing a Q approaching 300 at 518 kHz. This should do the trick. Regionals such as WILL 580 and WTMJ 620 both peak sharply and are giving reported strengths of greater than S9+15 with the rod here in the subterranean shack.
I found a couple of large pot core inductors which were wound with Litz, and there is more than enough wire on each to give the rod a proper winding. I don't know the gauge, but the wire OD is about 15-20 times that of the 7/44.
I hope to rewind the rod, and get it outdoors on the rotator by the weekend. I am looking for higher Q and and a tuning range which will cover some of the TA Euros.
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jFarley - Sounds promising!
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Short and sweet, using the current amp with the "standard amp" with tuning diodes is not a realistic option. For two inter-related reasons.
1) The amp exhibits minimal voltage gain, and is actually pretty close to 0 dB voltage gain (it does have a very large power gain, and simply matches the coax impedance to the high resonant tank impedance). When I originally put this all together way back when, it worked, and I did not pursue and understand some things to the degree I do now. This amp works great for resonant loops which have a sufficiently high Effective Height due to the turns count, the loop area, the wavelength, and the loop Q. Such a loop simply will not need voltage amplification.
2) The above will most likely never be true for a small ferrite loop tuned by tuning diodes. There just is not enough Q in the typical diode down at NAVTEX and below, and while the ferrite rod gets you a lot of inductance in a small package, the downside is a small turns times area product.
The bottom line is that the ferrite loop will need to be tuned by a mechanical air-variable cap, and the amp following must add some voltage gain to the pot.
I have blown through some research, and I will put up some details in a new thread.
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I use a Wellbrook ALA-1530S+ and its directivity is very noticable at 518 kHz. However, would like to have a dedicated directional antenna for 518 kHz.
The Wellbrook has pulled in stations from Greenland and Bermuda. Now that its the dark side of the year again, would like to see if I can get even further.