HFU HF Underground
Technical Topics => Equipment => Topic started by: ultravista on September 02, 2017, 1625 UTC
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I am getting ready to string up another long/random wire, +100 feet. Receive only.
My wire is single pairs from a CAT5 spool (eight wires - four pairs). The individual pairs are twisted together.
If I understand, the potential for the electromagnetic fields to cancel out by the equal and opposite currents (in each wire) is probable as a result of the twist. The purpose of the twist is to keep it from radiating or receiving, correct?
For the antenna, would it be better to separate the pair - leaving a single wire?
Would connecting both ends, effectively making a twisted flat loop, provide a positive or negative result for receiving? Would this be connecting the pairs in parallel or series?
The cable is good and I want to get the best out of it.
To twist or untwist, that is the question ...
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I'd be tempted to feed them in series, each wire individually to see how it plays. A form of linear loading results that can have beneficial effects upon reception.
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I'd be tempted to feed them in series
How do I do that?
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Is the cable intact or are these individual pairs of wires without any covering? How are you feeding it to the rig?
To put the wires in series I'd solder and insulate each pair at one end, they're color coded so you know wich is wich, then solder the pair to the next in the sheath and continue. The beauty is you can undo it all (the soldering) and change it to compare performance.
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The fields will cancel out if you use the two wires in a pair in differential mode. If you use them as a single conductor (in effect) by shorting the two wires on each end, that will not happen. It will just appear to be a single wire. I guess you could imagine it as Litz wire with only two conductors. I'd suggest doing that, it will be (slightly) more mechanically sturdy vs a single wire. That's a pretty thin gauge wire typically, so I am not sure how well it will hold up over time.
Using them in series (that is connecting say wire A to your radio, then shorting wires A and B at the far end, then leaving the radio end of B open at your radio) would not be a good idea.
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Is the cable intact or are these individual pairs of wires without any covering? How are you feeding it to the rig?
To put the wires in series I'd solder and insulate each pair at one end, they're color coded so you know wich is wich, then solder the pair to the next in the sheath and continue. The beauty is you can undo it all (the soldering) and change it to compare performance.
Josh, the wires now are without the jacket and in individual pairs. I do have a lot of wire though so I can make a long run with a jacketed CAT5. I will feed it in the SDR via 9:1 unun, or direct to the SDRPlay RSP2 high Z port.
So ... solder the entire bundle @ one end and the individual pairs to 'the next pair' on the other end? Or pair one to pair two, pair two to pair three, pair three to pair four? I'm trying to picture it.
Or one wire to the 2nd, the 2nd to the 3rd, and so-on through the 8 wires - effectively creating one long wire out of all 8?
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You have a lot of options there. To create linear loading you need wires or conductors folded over each other, this is typically done to get more wire, inductance, and radiation resistance for a given amount of space the antenna can occupy. What chrismo said regarding canceling out rf due to the pairs being twisted together is correct to a degree, and if it hinders performance you can violate the pair and solder say a yellow white not to the other yellow white but say to brown white. You get the idea.
See here;
http://download.prgm.org/ham/ant/linearloaded7mhz.pdf
See how he's taking ladder line and folding it back to get more wire in the air? That's what you're doing. To eliminate any reduction in performance due to cancellation from twisted pairing, wich is what twisted pairs are intended to do, solder each color coded pair together as chrismo suggested, then solder each soldered pair to only one soldered pair at the other end, each soldered in series until you have no more pairs, then terminate and insulate the ends as desired. For vlf work you might go ahead and solder each color coded wire to its mate at one end only and feed the pairs in series, making for longest length of wire and ignoring any cancellation.
The resonance points will depend upon the length of the total wire, inductance, stray capacitance, and so on and so forth, but for ambc and lf/vlf it should work dandy, and be very good at hf. If you terminate it in a variable resistor at the far end from the shack you've made a Beveridge antenna and can fine tune it for directivity - it will send to ground signals coming from the end opposite to the resistor. I suspect 400 to 1k ohms variable range will do and this will change due to moisture content of the soil but not by a large extent, once you find the sweet spot in resistance ie the stations at the back end of the antenna (away from the resistor) are markedly lowered in power, swap the variable - after you measure its value and find the closest aproximate fixed value resistor - for a fixed resistor for outdoors durability, you might want a 2 watt resistor as some find their get popped by nearby lightning strikes often if a lower wattage resistor is used. You can get tricky and place a cds cell (in the same range as the variable resistor) in place of the resistor and it'll allow for omnidirectional coverage when sunlight hits it, rather than as a Beveridge. This Beveridge antenna system provides for very wide bandwidth of reception (technically this is an aperiodic antenna, meaning it should have no peaks or nulls in response to rf energy intercepted) and if the far end is directly away from the home it should also do much in the way of reducing or eliminating rf noise coming from the home. A note should be added here - linear loading may interfere with the classic Beveridge directivity as the antenna wires are run back and forth over the length of the installation rather than a single wire, you can break new ground in a very old antenna design!
A "long wire" antenna typically is at least 1 wavelength at the intended freq of operation and will be a high impedance antenna, pretty much anything other than quarter wavelength and multiples thereof will be higher z than our rigs want to see unless they have hi z inputs. Any halfwave antenna will be around 2k ohms at the ends but around 300 to 75 in the middle and of course everything is frequency dependant. So having a balun or other means to match the hi z of the antenna will provide for the most efficient transfer of energy, if the antenna sees a low z input it will swamp the energy.
Here's a loaded vertical;
http://www.qsl.net/pa3hbb/ll.htm
A friend runs this page and it's chock full of antennae and HAM radio hijinks with emphasis upon Beveridges;
http://www.w0btu.com/
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Josh - is this correct?
I realize the color combinations will differ.
Would this effectively lengthen the antenna electrically? 75 ft * 4 pairs = 300 feet?
(https://i.imgur.com/enA4zqb.jpg)
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Eggsactly!
You could also series solder each wire making for 600 feet of wire, better for ambc and lf/vlf. A friend had a 600ft dipole that was ossum on 160m and ambc but had the drawbacks of arcing over at the pl plug in wind, snow, dust, and storm activity within a few hundred miles, as well as the higher you went in hf, the more directive it became until at 10m if the stations weren't off the ends of the antenna you'd not hear them. This antenna could stand some static discharge protection.
I like and use these guys;
http://www.mfjenterprises.com/Product.php?productid=MFJ-270
As well as these;
http://www.polyphaser.com/products/rf-surge-protection/is-50ux-c0
(keep in mind the above device blocks dc from the rig end and grounds via an inductance the antenna side of the port, it also has a gas discharge capsule)
Experiment and see what you get, antennas are a fun thing to play with.
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Josh - thank you for helping - your antenna-FU is strong!
Do you have any idea how this the single wire in series would compare to the pair in series?
Does the fact that it is folded back upon itself many times have any effect?
Also, is there a length you suggest I use for the CAT 5?
A friend had a 600ft dipole - what was the dipole made from?
HF and below are my target frequencies.
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Effectively folding the pairs back and forth on each other, but keeping them bundled, is not going to work very well.
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Effectively folding the pairs back and forth on each other, but keeping them bundled, is not going to work very well.
Chris - do you recommend that I not connect the wires in series, just leave them in the bundle and connect the ends?
What do you recommend?
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Q: Do you have any idea how this the single wire in series would compare to the pair in series?
A: It will increase the total amount of energy intercepted and lower the resonating point over the twisted pairs soldered together as one conductor at each end.
Q: Does the fact that it is folded back upon itself many times have any effect?
A: Yes, linear loading, especially in the hobby market known as "cobra" dipoles are well known for their outstanding performance and durability. Comparing this true linear loading to a "bazooka" dipole - where coax is used as the lengths of the dipole and feeding the coax so the shield is in series with the center conductor- will show the linear loaded dipole outperforms the bazooka as the bazooka's entire center conductors are shielded from the impinging magnetic flux of the radio wave were the linear loaded cobra has all elements exposed.
Q: Also, is there a length you suggest I use for the CAT 5?
A: How much room do you have to work with? I will fit a wire or dipole to the landscape, I'm not a big fan of tuning to resonance and using complex methods of supporting the tuned wire, I rely on transmatches to tune a wire to resonance if needed.
Q: A friend had a 600ft dipole - what was the dipole made from?
A: It was copperweld, wich is steel wire witth a thin copper coating, rf pretty much travels only on the skin of a conductor, the coppwer is more conductive than the steel but has nowhere near the strength.
As I recall the tower holding the center feed to the dipole was about 70ft up and the ends were in trees up maybe 25ft or so. The cows enjoyed the antenna too.
Comment: HF and below are my target frequencies.
Response: It will perform well here whether a linear loaded antenna or simple single wire on hf and below.
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Effectively folding the pairs back and forth on each other, but keeping them bundled, is not going to work very well.
Chris - do you recommend that I not connect the wires in series, just leave them in the bundle and connect the ends?
What do you recommend?
What are you trying to do?
It sounds like you have 100 ft of ethernet wire, and you want to use it to make an antenna, rather than buying some antenna wire. It's not inherently wrong to use "non antenna wire" for an antenna, most of my antennas are made with insulated #16 hookup wire, because you can often find spools of it cheap on eBay. I routinely buy 500 ft spools for $20 shipped. But that is stranded wire, and it will hold up. Ethernet wire is very small gauge solid copper. It is very weak. So bear that in mind.
How much room do you have? How long of an antenna can you run?
If you have 100 ft, you could short all 8 wires at each end. You now have a 100 ft long wire, you can make a 100 ft long random wire antenna with it. It will have some additional mechanical strength vs just using one wire, or one pair. Which, for normal solid copper ethernet wire, is very low, and it would probably fail with the first breeze. I'm somewhat dubious of this cable, actually. If it was still in the outer jacket, you might get some additional support from that, but it sounds like that was removed? Without the jacket, it may last for a while, but eventually it is going to fail, much earlier than stranded wire.
Trying to short out alternate ends of the wire, to make it electrically longer by effectively making the electrical path run back and forth along the cable, is not going to work.
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Yes, I will build an antenna from CAT5 cable. I have some already de-cased and split into pairs and have access to 1000's of feet of cable if needed.
The split pairs still have the casing - the vinyl casee was removed to expose the individual wires.
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I would think that the individual wires from a Cat 5 cable would be too thin to really hold their own (mechanically or physically speaking) against wind, squirrels, etc (when used as a random wire or long wire antenna).
Chris is likely right here, the outer jacket would probably be more helpful than hurtful. You can get speaker hookup wire or "zip wire" for super cheap on Internet sites like eBay and Amazon and not have to worry about the wire breaking the first time a storm comes through.
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Agreed, cat5 although made of the right stuff, is likely too weak to hold itself under tension for long. The jacket will also break down from the sun's UV and all the insulation will come off in a few years. I would think you'd be better off with #14 house wire for tension type service.
FWIW, i recently made a small AM receiving loop out of some scraps of cat5 for a radio station so they could monitor their off air feed. shorting the pair wires together and alternating pairs made for basically a 4 turn loop. Slap a ferrite on each end of another piece of cat5 using one pair and it worked surprisingly well.
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Thanks for the feedback. I don't plan this as a permanent solution, just something I can string-up and get on the air. Once I find a working solution, it will be built better & stronger.
Can you share the full design for the receiving loop? Each pair was sorted @ each end and all four pairs were used?
>> Slap a ferrite on each end of another piece of cat5 using one pair and it worked surprisingly well ???
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http://www.mile204.us/PotsdamDan/RubeGoldberg/Ultimate_AM.html (http://www.mile204.us/PotsdamDan/RubeGoldberg/Ultimate_AM.html)
This is the basic idea, except mine was made out of a 3' piece of cat5 with each pair's conductors shorted together and then wired alternatingly to produce a four turn loop. I just hung mine on the wall with push pins and used another 6' scrap piece of cat 5 with the white/blue pair tied to the output of the loop. The receiver I was feeding it with had an unbalanced input, so I placed a high permiability ferrite snap-on core at the receive end to make a crude balun. The transmitter was only a few miles from the building so just about anything would have worked, but I needed to keep the electrical noise out of the AM audio, and a loop is a good way to do that.
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If you have a length of twisted pair cable, just untwist an end to get the two arms of a dipole (horizontal, far from house), then run the twisted part to your shack window (at exact right angle to the dipole for as long as possible, to get the best balancing), then use a differential ATU to tune the whole thing to any SW frequency. Typical length of wire (including the dipole and both wires of the line) should be about 3 half wavelengths (for exemple), but the coils in the ATU can change this widely).
In fact, this is the doublet antenna, tunable to almost any frequency by the ATU at your hand near the receiver. Instead, the total length maybe about 5 or 7 half wavelengths, or only 1 wavelength (for example on 75m and close to the house). The twisted line can be coiled near the house entrance and/or near the central pole to better fight the common mode.
Wrong questions: the complex impedance of the doublet, the characteristic impedance of the line. The right question is to tune the full assembly of the doublet + the bifilar line + the ATU with an odd number of half wavelengths (approx.).
I tried the doublet years ago, when my large horizontal loop broke down, just using the remnants, and got immediately much better DX results. Thinking back, maybe a part of the not so good "sky hook" results was due to the line not in the symetry plane of the loop (it was easier with the remnants to place the line just at equal distances from both arms), but the doublet has also a better differential behaviour (the ends are nodes!), and is easier to place far from the house.
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Thank you Ray