HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: MDK2 on August 07, 2016, 1418 UTC
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At the request of JCMaxwell, here are some photos of my homebrewed magnetic loop antenna. I followed the directions linked below (provided by DJ Dick Weed, tip o' the hat to him) but modified it by making it bigger because the lower bands are where I have trouble. (My random wire is 48', or half a wavelength of about 11.4 MHz.) It's a 15' circumference using 1/2" diameter copper water line rather than 10' of 1/4", and the PVC pipe used is accordingly bigger as well. I also used a variable capacitor bought from Amazon rather than salvaged from an old AM/FM radio. The feedline is RG58 with a BNC connector. The face plate on which it's mounted it broken from falling, and the tripod is wonky because even with clamps it's hard to use a hand drill and get those holes perfectly through the center of the pipe. Or maybe it's because I'm just not much of a craftsman. But hey, it works. Range is about 3.5-12 MHz.
Instructions:
http://www.kr1st.com/swlloop.htm
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More.
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Thank you MDK2!
It looks great!
Based on your posts lately, it looks like it works well.
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It does the trick. I wouldn't consider it a true DX antenna - there are things my 48' wire pulls in better, even at it's relatively short length. But I have copied a lot with it.
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Nice build!
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There are a couple of low-noise amp schematics for loops floating around the web, youtube and the various maker sites if you want a little more "oomph" without adding much noise from the thing. I've found rotating the things and re-tuning works just as well though.
Don't sweat it falling over, all of the things do that from time to time. It's the nature of the beast. I think you could mount one in reinforced concrete and it would still tip over and conk you in the head.
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I used to make and sell tabletop multiturn versions out of pvc, with a 365pf variable, tuned for 160m HAM use or ambc. A good loop is an amazing thing, peaking up desired signals and decreasing all others, also they can enhance in band selectivity. Pretty neat capability for some passive parts arranged in a magical order.
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I've been making minor improvements to the antenna recently, such as making the tripod more stable, remounting the variable capacitor, etc. I found a rubber covering from a banana plug to put on the capacitor where a knob goes (I never found a knob that really fit it). So, now that I'm able to tune without causing a short or ground or whatever makes the signal drop out from direct contact, I have discovered that the upper range is much higher than previously thought. I just logged Russian polytone station XPA2, which broadcast at 1500z today (9/16/16) on 16147 USB using the loop. And it came in much better than it did on the 48' wire, which at 14.4m is usually quite reasonable for higher band reception.
Up above I said I don't think this is a true DX antenna, but I'm reconsidering that conclusion.
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Awesome MDK2!
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I've been making minor improvements to the antenna recently, such as making the tripod more stable, remounting the variable capacitor, etc. I found a rubber covering from a banana plug to put on the capacitor where a knob goes (I never found a knob that really fit it). So, now that I'm able to tune without causing a short or ground or whatever makes the signal drop out from direct contact, I have discovered that the upper range is much higher than previously thought. I just logged Russian polytone station XPA2, which broadcast at 1500z today (9/16/16) on 16147 USB using the loop. And it came in much better than it did on the 48' wire, which at 14.4m is usually quite reasonable for higher band reception.
Up above I said I don't think this is a true DX antenna, but I'm reconsidering that conclusion.
Back in the late 80's I had a poor man's NSA setup, an Icom R70 and an infotec M600 with shielded crt fed by a cliff dweller antenna resting upon the curtain rod in the bedroom or a sloper out about 70ft or so from the same window. All of wich provided the means for a selectable horizontal and vertical polarisation rx setup. For example, when Rabat Morocco fsk signal was coming in around 19MHz (as I recall), I would switch between the two antennas and usually see a difference, however slight, between horizontal and vertical polarity of rx antennas. At fade in/out the difference was amazing, often one polarity would be perfectly readable and the other no sig at all. It's nice to have both.
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Back in the late 80's I had a poor man's NSA setup, an Icom R70 and an infotec M600 with shielded crt fed by a cliff dweller antenna resting upon the curtain rod in the bedroom or a sloper out about 70ft or so from the same window. All of wich provided the means for a selectable horizontal and vertical polarisation rx setup. For example, when Rabat Morocco fsk signal was coming in around 19MHz (as I recall), I would switch between the two antennas and usually see a difference, however slight, between horizontal and vertical polarity of rx antennas. At fade in/out the difference was amazing, often one polarity would be perfectly readable and the other no sig at all. It's nice to have both.
I do need more antennas... not sure I'll have an easy time convincing my wife of that. ;D
I have decided to do another one that follows KR1ST's dimensions in order to have fuller coverage of the HF spectrum between the two of them, and also to have one that's more portable that the current one, which at 4'9" in diameter is just too big to fit into my cars, neither of which are a pickup truck or full size van. (A 10' loop has a diameter of about 3'2", much easier to fit in the back of a car.) I wasn't able to take this loop with us on vacation last month, which was a real shame because it was pin-drop quiet as far as RF noise was concerned. However, I will be going back to that area in three weeks and thought I might try to hear the Saturday night pirate show from out there.
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Cool MDK2, take lots of pics! ;) ;D