HFU HF Underground
Technical Topics => Equipment => Topic started by: Shawn1967 on November 27, 2022, 2132 UTC
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Greetings first post... was very involved in SW DXing in the early 80's and would like to get back to DXing but now focused on LW (NDBs) and MW Dxing...with a smattering of the old Tropical Bands. I was lucky enough to be located in Mustang, OK with 5 acres and access to another five acres in winter. Would like to build a loop on ground. The loop would be 354 ft on each leg totalling 1455ft. I would be corner fed so that directionality would be NE/SW. I would go through a 9:1 balun. Do you guys think this would be sufficient for the frequencies desired? Thanks in advance
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I suspect you'll be fine.
I had a 1000 ft LoG for a few years. I found it worked quiet well on MW, and was still OK on HF, although it no doubt had some weird lobes/nulls in the pattern. Much like my 900 ft Horizontal Sky Loop. Think of it as a LoG that is not actually on the ground ;D
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I ran a 1000 ft LoG one winter season. It worked well (my interest was in LF at the time). I didn’t pay much attention to directionality effects. It worked on other bands too. I used a 9:1 balun.
When I went to pull it up in the Spring I found it was in pieces. Squirrels had chewed on it during the winter. I would recommend you take a continuity measurement of the loop after you install it. Later, if performance seems off you can recheck it to see if squirrels have had their way with your antenna. This is a lot quicker than walking the length of the antenna.
Considering that BoG antennas are shorter than their elevated counterparts, a 1000 ft on the ground is like a much larger elevated loop.
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When I went to pull it up in the Spring I found it was in pieces. Squirrels had chewed on it during the winter. I would recommend you take a continuity measurement of the loop after you install it. Later, if performance seems off you can recheck it to see if squirrels have had their way with your antenna. This is a lot quicker than walking the length of the antenna.
I was continuously playing whack-a-mole (or other varmints) with mine, repairing it more often than I was listening with it, it seemed. Which is why I eventually gave up on it. It's still out there, pieces of it anyway. The only way to make one last (here anyway) would be to use large gauge wire, which is cost prohibitive.
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Welcome to the community! :)
Do not put too much thought into impedance transformation IMO. An 1:1 should suffice if needing additional isolation for common mode, though admittedly I ran my LoG for many years without any balun or transformer. 1:1, 4:1, or whatever else tried when constructed did not affect it much. FWIW, there were just a few relatively useless ferrites on the feedline near the feedpoint.
Your LoG as described should be "resonant" somewhere below 700KHz. Being on ground is going to lower resonant frequency below the usual freespace calculation. LW through MW including 160m should be fine, but getting much above there the signal pattern is going to break into multiple lobes. There are also ground losses to consider.
My comparatively small 148' LoG over lossy ground was resonant somewhat below 7MHz with usable receive performance from LW up to about the 20m band. Above 15MHz usually required a preamp.
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I built my shielded LoG out of cheap RG-6, and it survived many years of use deployed in a moderately wooded area. It is up for repair or replacement now, but seeing as I have yet to examine it, the actually recent failure just as well be a feedline or feedpoint issue instead of the RG-6 loop being damaged. Anyway, here is what I did:
https://www.hfunderground.com/board/index.php/topic,29940.msg114696.html#msg114696
BTW, while we often use the term "shielded," most coaxial loop designs are more about potentially improving current balancing versus a single wire. It should not really matter either way assuming good loop symmetry. YMMV.
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The only way to make one last (here anyway) would be to use large gauge wire, which is cost prohibitive.
I’m going to try another loop this year….a bit smaller, maybe 500 ft. This time I will use #14 THHN electrical wire (I used cheap CCA CAT5 last time). I’m hopping this will be more rodent resistant. I’ve got no idea how sunlight tolerant THHN is, but I will know soon enough.
THHN has jumped in price - a few years ago a 500 ft spool was about $50. Today it is closer to $80.
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Do not put too much thought into impedance transformation IMO. An 1:1 should suffice if needing additional isolation for common mode, though admittedly I ran my LoG for many years without any balun or transformer. 1:1, 4:1, or whatever else tried when constructed did not affect it much. FWIW, there were just a few relatively useless ferrites on the feedline near the feedpoint.
I think that a significant part of a successful LoG is suppressing common mode pickup on the feedline. I would either use ferrite cores (type 77 or 75 if your interest is LF) or a CM filter (KD9SV and DXEngineering sell them). Place a filter at the antenna feedpoint and at the receiver. Burying the feedline helps a lot and may even negate the need for the common mode filter.
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I use KD9SV common mode chokes on some feedlines. I never bothered measuring them, but I suspect it might be a mix 7x ferrite considering much of his product line is biased towards the 160m and 80m amatuer bands.
If just receiving at HF and lower frequencies, even a small mix 73 binocular core with a 4:4 or whatever isolation winding often will suffice for basic common mode choking. The popular BN-73-202 is about $1 each or sometimes less in small quantities. For that price place one on each end of the coaxial feedline. For those not wanting to solder, even a terminal block will suffice for HF and below.
Another aspect to consider is differential noise mitigation, which can be helped by balancing the loop currents through good symmetry and/or design considerations. Many of the "shielded" magnetic loop designs are more about balancing loop currents than actual effective electrostatic "shielding."
I plan to soon replace my old LoG, and I will likely again use a "shielded" coaxial design. I have an 1000' spool of decent Belden RG-11 for feedline on the way, but I am leaning towards again using cheap RG-6 for the actual loop antenna. I probably have plenty of RG-6 on partial spools here to build the loop, but for those buying, an 1000' spool of flooded direct bury bulk RG-6 is under $100 shipped on eBay. That builds several typical LoGs or one quite large LoG.
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I am a huge fan and user of LoG antennas, but being realistic about LW to MW reception, a remotely-tuned FSL antenna on a rotator is likely the "better" solution for serious DX'ing of low frequencies.
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….but for those buying, an 1000' spool of flooded direct bury bulk RG-6 is under $100 shipped on eBay. That builds several typical LoGs or one quite large LoG.
Good point, but I will stick with the THHN wire for this season because I still think the squirrels will still be a
problem. Repairing a wire break will be much easier.
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Squirrels are rodents, rodents gnaw on anything that will sharpen their teeth including you, (try to pet one). You can shoot them, you can poison them(use rat poison, they're nothing but tree rats, info the rat poison folks are loathe to put it on the box.),you can bop them with a projectile from your antenna line slingshot,there are lot's of way's to get rid of them. Just kill the things. I suspect old fashioned mole traps would do the trick for ground squirrels. A couple of bloodthirsty 10 year olds w/ .410 shotguns should keep all of them in check nicely.
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Isn’t that the truth? Lord help you if they find a way into your attic. My Lab has gotten quite good at chasing them from the bird feeders while barking “Get off my lawn”. She caught one once and it had the audacity to bite her before she dispatched it. Since then it has changed from a game to serious Lab business.
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Interestingly enough, they do not seem to bother my various unburied feedlines. AFAIK, likewise for the LoG.
I intend to change out most of the existing antenna farm feedlines for RG-11 direct bury over the upcoming weeks, though it could awhile before getting around to burying them. I can not imagine coaxial flooding compound is tasty, but then again, I would not know personally. I do not plan to know, either. ;p