160 FT loop lying on the ground -tests and results

[Opcom]

Note this is about a loop being tried for listening in the house. If it can be made a little better, it can be spliced for entering the shack, which is in my "lab" (so-called lab, according to some).

After discussing with Dennis W5FRS and reading up here I found the remains of a spool of CAT-5 cable and stared laying it along the fence line. The wire is 40Ft on each leg, making a square. The fence has two runs, once from the house to the lab on each side of the yard. The house and the lab are front and back enclosures of the private section of the back yard, respectively. It's important to describe details, they could be affecting performance.

The fence is made of wood, with 10FT poles sunk in 2 FT holes having gravel at the bottom and filled with concrete. The boards are above the ground level, sitting on a 4" concrete beam that runs the length of the fence. There is no rebar in that, it was just molded in place. The wire was run in the little 1.5" wide, 3" tall space between the poles and the bottom of the fence boards. Surprisingly, it pulled through all that nicely as I went, but by the time I was finished, I was about done in by the 95 degree heat and everything else.

At the lab wall, the wire was run across on the ground right next to the foundation, and at the house it is the same. I mention this because I was concerned that the vertical fence poles would attract noise, or that the rebar of the lab foundation as well as the metal wall would degrade performance. I have no way to tell. Maybe y'all can comment.

The wires meet at the back door and go vertically, almost together, up the wall 8 FT and enter through a disused dryer vent.

Inside, each end of the CAT-5 cable has all of its conductors twisted together. This makes it act as a single conductor,  for the 160 FT loop, I hope.

From there, a 15 FT piece of CAT-5 is used to get to the back of the R-390A. That wire is set up so the orange twisted pair is combined to one conductor, and same for the green twisted pair.

I was still getting the terrible rough buzz and noise, but was able to hear a few stations, but no intelligibility on AM. This was extremely rare before as I hardly ever could pick out stations in the noise.

The testing was done on 5MHz using WWV since it was always transmitting.

Dennis Brady W5FRS told me to make an RF transformer and to rewire the loop so that each twisted pair was to be one conductor, and with the resulting 4 conductors, I was to wire it as a 4 turn loop. I found a red Amidon core, about 5/8" inside dia. On this I was to wind 40T for the loop side and 4T for the radio side. As it worked out, I made up 45T, 6T, and 12T for the sake of experimenting. The 45T winding was to go to the loop.

The 4-turn loop business and the 40T-4T setup worked well for Dennis with his loop maybe equivalent to 25 FT diameter, but did not work here. (I did not like the idea anyway because the turns would be interwoven). The transformer made it worse.

I went back and rewired the loop to a single turn using all 8 wires in parallel for one conductor, to get back to where i had been before. Then I played with the transformer and found that the 12T as the primary and the 6T as the secondary worked much better, as signals were as loud as direct from the loop, but the noise was greatly reduced. Grounding the 40T section to the R-390A chassis reduced the noise a little more. I note that arthritis and winding small toroids are not very compatible. There must be a machine for this.

@8:19 PM CST, I was able to hear these guys on 3887 (3885?), whereas before the noise was too bad to hear anyone on that band.
G5WL talking to  "Charlie" K?5SWL. A DX-100, 4x 813 HB amp, and a 150W carrier was mentioned. For some reason the letter after the "K" could not be distinguished here despite several IDs. The signal was loud-sounding and strong enough to be clear, but there was some kind of distortion in the overall circuit. Charlie's carrier moved the meter from 0 to about '5dB'. According to the calibration chart with the R-390A from its last repair and alignment, this would be -98dBm.

There was more discovered during this. The set-up is using the unbalanced connector on the R-390. There is a plug with a 12" clip lead.. 12" of wire external to the R-390 is enough to pick up all kinds of crud because this set is very sensitive.

1.) When the 12" pigtail is removed, the set is silent.
2.) When only the pigtail is there, there is buzzing and noises and strong SW BC stations can usually be heard.
3.) When the pigtail is hooked up to this new antenna, I get ham AM stations.
4.) When the pigtail is removed and I can (barely reach) touch the ant. lash-up leads to the input connector of the set, I get reception and less noise that with the unshielded pigtail.

Maybe I should relocate the transformer to where the loop ends come in, and run a shielded cable from that point to the receiver. I must be poisoning my already meager reception with these small bits of unshielded wire.

[K5IIA]

kg5wl charlie and
w5kgz perry.   didnt know if it would help knowing what stations you were hearing to see their location.

[Opcom]

definitely helps, thank you. See, the noise is confounding but I'm jazzed at being able to actually listen to the QSO.

226 miles and 241 miles. Not bad considering my QTH is in a 'bowl' topographically speaking. KG5WL was the louder one but both were almost full copy. It seemed to me like the noise was garbling or modulating/cutting out the signal in a way that didn't happen when both signals and noise are much stronger.

This is completely new territory for me. It's not like working on equipment. I hope I can improve the antenna some more, any advice or comments welcome esp. regarding proximity to fence poles and buildings. I'll report again after I put in the shielded cable and see if the immediate 'indoor' noise is gone.

[Ed/KB1HYS]

I think your in unexplored territory here.  Having the antenna wire directly on the ground sort of flies in the face of conventional wisdom.   Very interesting idea though, Given the issues I have here, small city lot, XYL, and electrical noise, that maybe a good way to go.  Certainly easy enough to experiment on, simply walk outside and you're at the antenna! 

Please keep posting with results etc.

[Steve - K4HX]

Think about how you feed the loop. A good coax feed with a balun could reduce the noise.  See some other threads here on how a good balun can reduce noise.

[KB3DKS]

A raspy buzz. Do you have any long tube florescent lights in your lab? Turn them off.
 In my temporary workshop there are a pair of dual 8ft fixtures. I have an old Halli S-40 B with about 10ft of wire ant for SWL and it is completely unusable when those lights are on.
 With just the Q Halogen workbench lamps on I only get the outside sodium lamp cycling constantly.
 Good AM copy if that isn't trying to fire up.
You might try tuning the loop also or use the center wire of some RG58 with the shield grounded at the receiver or balun and opened out at the middle of the loop for a shielded loop.
Bill

[W2PHL]

A loop antenna is a balanced antenna. Have you tried feeding the balanced antenna input on the R-390/A? Using the balanced input will probably yield the best possible common mode noise rejection. Impedance matching doesn't matter too much since this is a receive only system. Try it first without any matching transformer. Feed the balanced line directly into the receiver.You are trying to achieve the best possible signal to noise ratio not the strongest possible signal strength.The antenna trimmer in the R-390/A should  be sufficient in that regard. The balanced input works wonders for me and my R-390/A here in the big noisy city. Good luck!

Phil

BTW I'm not sure how good cat 5 line works as a balanced transmission line. To keep a balanced transmission line from acting like an antenna it should be symmetrical. You might want to try removing one twisted pair from the cat 5 and simply use that.

[W3NE]

A loop lying on the ground is an NVIS antenna, e.g., Near Vertical Incident Skywave antenna. This type antenna has a long history beginning with its use by the Germans in WW-II. It was subsequently widely employed by the U.S. military and other agencies, and recently has received a lot of comment in the amateur press. I believe it was used in some instances by MARS stations and CD operators who are interested in state-wide and other near-in communications.

Its operation is based on the principle that radiation directly over  the antenna will be reflected by the ionosphere straight down to the earth as long as the frequency is less than the current critical frequency, in which case the signal would continue into space. 75M is almost always below the critical frequency, although 40M might not be. It is a low noise antenna because most noise signals arrive at an oblique angle to the earth's surface and the antenna is so low it is insensitive to them. NVIS antennas are noted for their coverage within a radius of about 300 miles of the receiver; they are unsuitable for DX signals, which are the result of skip arriving from the ionosphere at an angle to the earth's surface. Contrary to usual antenna location, NVIS antennas perform best in a valley!

Your antenna on the ground is an extreme deployment of an NVIS antenna; optimum height is 2 to 10 feet above the ground (great for catching prowlers at night!). If you raise your antenna above ten feet (twenty at the most) you will find its low noise characteristics will vanish, your coverage will become wider, and QRM/QRN will increase.

The internet has numerous references to the NVIS antenna. I wrote an article about them for the Phil-Mont Mobile Radio Club's newsletter a few months ago but it is 38 kB. I'll send you copy in a PM if you are interested.

Bob, W3NE

[Opcom]

The raspy buzz is not anything here, lights or otherwise. It's partly grid noise and partly who knows what, city noise of 1000 homes and lights, atmosphere noise. The setup is in the house, I have had the power off and used a DC powered radio and the noise remains. I have not yet tried feeding the balanced input. I have a connector but no twinax to make a correctly shielded setup. I could partially assemble the connector using two shielded cables minimizing exposed conductors. I just bought a spool of RG58 so next I'll try eliminating bare wire at the unbalanced input. I was told the big loop like this picks up at low angles, but it's not from my knowledge. No matter how it works, it seems like the unshielded unbalanced feed is picking up noise so I better eliminate that first, or try to. The noise I am receiving at this time, it is of very low level and significant only because the desired signals are also much lower than what a regular antenna would produce.

[W2PHL]

Suggestion. Take the R390/A to the antenna. Although it's not fun lugging the radio outside, it will establish a best case reception scenario for the antenna you're testing. JS the antenna leads right into the balanced connector. If you like what you hear (or don't hear) then you can work on creating a proper transmission line to get the signal inside the building. If the antenna is still too noisy when jacked directly into the radio, then you should look at a different antenna configuration.

[W2VW]

Suggestion. Take the R390/A to the antenna. Although it's not fun lugging the radio outside, it will establish a best case reception scenario for the antenna you're testing. JS the antenna leads right into the balanced connector. If you like what you hear (or don't hear) then you can work on creating a proper transmission line to get the signal inside the building. If the antenna is still too noisy when jacked directly into the radio, then you should look at a different antenna configuration.

You can make that work with tv twinlead.
One time I was working overnights in an industrial park and had a huge loop on the ground. Broke one side of the loop midway and ran twinlead. Excellent SNR in a rotten location.

[WA3VJB]

... they are unsuitable for DX signals...

Bob, W3NE

Not that there's anything wrong with that...

[W3NE]

Amen to that, Paul.
NE

[Opcom]

This in the end did not work well for me but Dennis W5FRS got real good results with his. What I finally have done is this:
http://amfone.net/Amforum/index.php?topic=32860.msg254912#msg254912

[KM1H]

Instead of a balun which is usually wound as an autotransformer which is what I think you were describing, try it as a traditional transformer with a isolated primary and secondary. Keep the 2 windings seperated to minimize capacitive coupling.

The purpose is to isolate local common mode (buzz and crud) signals from the radio. It may not work but doesnt hurt to try.

Have you located and muffled all inhouse noise sources? 

I noticed when I added a couple of 500' BOGs pointed at neighbors the crud level went down compared to the elevated Beverages. Might work at half that length according to some.