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Zip-Cord rx loop in 15 minutes

Joined
May 28, 2009
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2,603
#21
Actually just a low cost MFJ 945E - pretty standard although the metering has no purpose for swl. I needed 160m coverage and the all-important bypass switch.

So here nothing specialized for loop-tuning - just a way to overcome a bit of overall feedline/system loss.

BEST DX SO FAR! - Just now S51YI in Slovenia on 40m cw.

Sure, stations copied is no way to judge an antenna. But it does prove that something like this is even possible with zipcord.

I don't know if it is multiple hops reaching me in LA over the pole, but for sure that signal is not coming in at any sort of "nvis" angle. I had reached out a few thousand miles or so for some interesting dx, but this makes this stupidly simple loop a keeper hearing Slovenia on it.

No preamp, and no major filtering - just ran across him calling cq dx with an Icom 718 in usb mode. When the cw filter was used, total armchair copy.

Maybe now I'll hunt down my original Dentron Super Tuner or maybe a Nye Viking... :)
 
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May 28, 2009
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2,603
#22
Half-waves and quarter-waves in this loop

My prior math was totally wrong for a 28 foot right-sided triangle vertical loop. Aka, the RSTVL. :)
Let's correct that. Using 1005 / f mhz as the full wave loop reference, this should be the calc:

Half-wave : 502.5 / F mhz
502.5 / 18.1 mhz (17 meters) = 27.76 feet. Close enough.

Yet I seem to be able to copy 17 meters as a half-wave loop ok. Ordinarily, a half-wave loop is unnatural and should basically counteract itself on all sides. MAYBE the current distribution in this triangle shape helps overcome that - despite the 45K complex impedance.

Quarter-wave: 251.25 / F mhz
251.25 / 10.1 mhz (30 meters) = 24.87 feet. Close.

Maybe I am having more of an issue at a quarter-wave loop than I am at a half-wave. It's not a deal-breaker issue (10db preamp or tuner fixes that), but I want to know why.

I'll let you know when I find out - but don't let this napkin number crunching put you off from making one of your own. While contemplating all this, 40m SSB was rocking with a ZF2 in the Cayman Islands causing a nice pileup in Japan. And I was in the middle basically. Awesome.
 
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May 28, 2009
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2,603
#23
Just making things louder, not improving the s/n

All things considered, unless it is on 160 meters, the tuner is really only making things louder, and not improving the s/n ratio. In other words, I have plenty of normal band noise, and while the tuner helps increase *overall output*, the ability to copy weak stations is the same, when I'm increasing the noise floor along with high s-meter readings.

So my concern about 30m being a bit quieter than the rest, is taken care of by merely adjusting the volume just a hair more than others. The resulting s/n is the same.

I've noted that the tuner improvement is more noticeable on the lower end Icom 718 general purpose rig, compared to something like the Kenwood 590s. Makes sense.

The temptation to make things louder and more system efficient makes sense from a transmit standpoint. But for rx-only, I think I reached a point where the antenna is doing all it can, and unless it is ridiculously quiet, even with the bad match and a bit of line loss, from an rx-only stanpoint, it is performing well. Amazingly, sometimes I actually prefer to listen without the tuner, because the s/n ratio is actually BETTER depending on band noise setting the pace.

The antenna has enough gain to push signals into agc, and I suppose anything more is just a waste. Most of the time the band-noise is just under S1 and that seems sufficient. Pushing it higher with a preamp doesn't really achieve anything but making it louder overall.

I'm still very happy. 25-30 feet total circumference for a triangle loop, inverted such that one side vertical, top side horizontal, and a diagonal run back down to the bottom feedpoint seems to be working for me. Kind of unique I guess.

It seems to be a nice safe option for kids or anyone who is limited in any other way (space, physical, legal etc etc) to get started listening.
 
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May 28, 2009
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2,603
#24
Got schooled by old posts from elsewhere by W8JI, N3OX, and K4SAV about why this rx-only antenna with a very high swr is actually working .....

Their 2007 posts in an elmer's forum about why SWR doesn't matter on receive finally sank in. And how this little inverted triangle rx-only antenna is working with swr's over 100 for the most part. And why that doesn't seem to hurt me in the feedline loss department, as it would with transmit. <smacks head>

On receive, the signal from the triangle, even though badly matched to the coax, is enough to make the trip down the coax, and into the receiver and absorbed by it in one trip. Why? Because the coax impedance matches the receiver's designed input impedance. Nothing is reflected by the receiver back to the antenna. ( I guess 50/75 ohm mismatch if you are using RG6 is not that big a deal even on rx too, since I've seen small differences with the tuner test between the Icom 718 and a Kenwood 590 - both are probably not *exactly* 50 ohms input impedance)

Transmit - if I were to do so, and the transmitter not immediately freak out, power would make it out to the coax, meet the super bad mismatch at the antenna, reflect back and forth a bunch of times, with each reflection being attenuated each time it would run back band forth, hence major feedline loss.

All this time I never truly understood. And here I was thinking of marketing some "receive-only" golden coax. :)
 
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May 28, 2009
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#25
DIY 1:1 zip-cord Isolation Transformer installed !

While it's no match for my PAR EF-SWL transformer, I decided to go old-school and see how badly an air-core DIY isolation transformer made from zip cord is. Long story short: for rx-only, not *that* bad.

Here's how:

1) Wind 20 bifilar turns of zip cord around a 1.5 inch piece of pvc. (Thats 40 wires total if counting by fingernail). Leave a foot or so of pigtail on each end which you bring the loose wires together and tie-wrap together to neaten things up. Cut any unnecessary lengths from pigtails, but leave enough to work with.

2) Attach red-wires to loop. Attach black wires to coax. Or in the case of my brown lamp-cord, smooth sided to coax and ribbed wires to loop. Or visca versca - just don't cross the wires.

Does the primary function of isolating the antenna from the coax work? You bet it does. Helps keep common-mode problems at bay. Still, that wasn't entirely sufficient for my location, so I still have the good inline ferrite choke right after the receiver.

Is it efficient from 160 to 10 meters? Erm, not really - I wouldn't transmit into it. BUT, it doesn't make the loop totally deaf. I turn up the volume just a liiiitle bit more, or activate the rig's 10db preamp more than I used to. Usable? Totally. Kid-friendly? Absolutely. A paragon of efficiency? Heck no. But not a total dummy load either.

I could tweak it I suppose, but am going to run for a few days leaving well enough alone. :)
 
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Joined
May 28, 2009
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2,603
#26
Reduced the turns to 15 bifilar wound (30 individual wires close wound to each other).

Without trying to over-engineer zip cord and pvc, I forgot that this isn't being wound with household thhn #14 wire, but lossy zipcord with some velocity factor added in.

Overall reducing the turns to just 15 is sufficient. The 10db rig preamp is likely to be on anyway, covering up a bunch of technical sins. This is the "15 minute" version. :)
 
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May 28, 2009
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#27
Meh, removed the diy air-core isolator.

Didn't really need it. Used the 18ga zip-cord as feedline back to the rig. Plenty of choking since I'm paranoid about common-mode.

But get this - about 15 feet of the 30 foot zip cord feedline run is sitting directly on cement, rather than being suspended. Perhaps the loss of the zipcord covering itself, is enough not to be too affected by the cement / brick it is running over. Seriously. This is part of another experiment inspired by W6NBC's own work with open wire recently. Zip cord's a really poor relative I suppose. :)

30 meters - I think it's just a bit quieter on that band than others in my rf location. Didn't stop me from hearing a european pileup on 10.105 cw or so at about 2am pt! Hearing THREE G3 / G4 UK stations at the same time calling (needed the preamp though) put my mind at ease.
 
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May 28, 2009
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2,603
#28
I don't need no stinking coax!

Can I get it down to a 10-minute loop - indoors even? Yes.

* Split 7.5 feet of 18/2 brown lamp cord apart and twisted the loose ends together.
* Spooled out another 20 feet or so to reach the radio gear. ON THE CARPETED FLOOR mostly.
* Connected the radio end with nothing more than a banana-jack to the center of the receivers S0-239 threaded uhf jack. Other wire connected to receivers ground lug. I'm not trying to "optimize" anything, as the use of the preamp mops up a lot of non-perfection.

Loop is shaped as an inverted triangle for convenience. Strips of blue painter's tape used to hang the two ends with extra X-braced strips on wall. Otherwise heater quickly takes it down. :) I wouldn't go larger than about 18 gauge if you are hanging things with blue tape.

The small loop seems inherently resistant to common-mode problems, but I did use a large MFJ snap-on and wrapped the zip cord a few times inside it at the loop feedpoint, and also used a binocular-core version of the same chokes near the rig. Also tried a little indoor black LDG 1:1 current balun and a jumper. Results are about the same as the simple chokes.

Heh, but what's powering the receiver gear? THAT was where my main problem lay with this and other antennas. I'm now running all the gear from a Tripp-Lite "Isobar", with the power supply attached to the furthest outlet on the Isobar. In addition, I'm using a linear power supply - so I picked up a *new* Tripp-Lite PR-7b to run the Kenwood and the Icom (for receive only) at the same time. Surprisingly, it looked good inside. As an Astron guy, I never used a Tripp-Lite and wanted to see what some of the horror stories are all about. Not horrible at all, unless you do stupid stuff, like overload it, not use insulated lugs, or pick up a hamfest reject that's been sitting in the rain for 20 years. Hence my new version.

PREAMP: compared to my outdoor loop where I normally don't run with the rig's preamp, with this indoor version I do run the preamp. That was expected.

Conclusion - one can get entirely lost in all the variables. Sure, go ahead and use an isolated transformer. Beef up the conductor diameter. Put the loop on a rotatable brace. Make the loop larger or smaller. Whatever floats your boat.

At least to me, the *minimum* version of using nothing more than zipcord for both loop and feedline works well enough. It sure beat my triple-tree of Hustler mobile whips and radials running all around the place. Yeah, I've done that. :)
 
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May 28, 2009
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#29
Resistant to common mode doesn't mean impervious!

Just so nobody gets the wrong idea .... just because I'm feeding a balanced loop with balanced feedline does NOT mean the line is impervious to common mode. The antenna might be resistant, but ingress from my own setup is still the issue.

The simple chokes and / or LDG 1:1 current balun suffice - if it weren't for the fact that my hard-core CMC-0510-R inline choke wasn't in use for another antenna, I'd be using that.
 
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#30
BIG chokes are in - I think I'm done - the objectives have been met.

Got some big mix-31 snap-ons with a 1-inch inside diameter. Plenty of room to wrap/thread about 20 turns of #18 true copper zipcord inside - nice an neat so that they are not such a jumble, and the turns are right next to each other, and cross-wrapped half-way so that the input and output aren't right next to each other. Works GREAT on my major common-mode.

Got these online from "RF Choke 311000". Self-explanatory, 31 mix for wide hf coverage and 1 inch inside diameter. Similar ones can be found. For me, the moral was to go-big, or go home. :)

So now using zip-cord for both feedline and loop, there are no mechanical breaks (other than the one I soldered together to make the loop elements meet each other), common-mode is under control, I can put the transmission line part of it on ground even, and during the recent heavy rains, no loss of signal that I could detect vs dry was heard. Moisture in the zip-cord channel doesn't seem to be an issue. Oh, and no coax connectors even - just a banana jack to one side of the cord, and the other to the ground terminal of the rig. Big choke follows that, and one at the loop feedpoint.

Other than the single soldered and protected connection for the loop itself, I have no moisture issues giving me problems, no exposed connectors, no moisture in coax, no goopy coax-seal and tons of electrical tape - all this help keep the resistance of the wire itself the same as when I put the thing up.

In about a year when the sun eats up the zipcord dielectric, I'll just cut a new one. 5 minutes and I'm done. I suppose I could extend the life by using an Armor-All or similar wipe on it. Mostly near ground level, this isn't a problem.

It's not supposed to be this easy. There's gotta' be a catch, but I can't seem to find one now. Thrilled.
 
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May 28, 2009
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2,603
#31
The no-solder option

What about no zip-cord at all, but just a long length of a single wire at hand? Perhaps something low resistance and highly UV resistant to withstand being outdoors better.

Just work backwards - hang your loop starting at the center in whatever shape and size you desire (although for a small - medium sized loop I'm not going to exceed about 25 - 30 feet in total circumference, and not much more than an additional 35 feet for the feedline), and then make your own twisted pair out of it for the feedline running back to the rig. No breaks at all.

The chokes for me were key - otherwise I was listening to a random-wire shaped like a lollipop - whether I fed it with coax or zipcord. If you move that feedline around, or drop it to the ground and notice a big change in reception, well then you are listening to common-mode coupling and not so much the loop. The chokes were convenient to use at the feedpoint and at the rig to keep the lines from zipping apart.

This is the most low-cost rx-antenna fun I've had in a loooong time.

Most importantly, hand your kid a long length of wire, two chokes, a banana jack, and a perhaps a small step-ladder. Its a relatively safe project with a lot of bang for buck that could spark a young mind's interest in the wonders of HF radio - even during this lull in the cycle.
 
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