Cat5/6 use as antenna Feed Line (Receive in my case) - Page 1
Forum section: electronics / rf-microwave

author: Qmavam
author_rank: Regular Contributor
timestamp: April 15, 2020, 03:29:13 pm
content: I noted a thread started in 2016 about Cat5 used as feed line, it started with "not much info","nobody has done it so it must not work".In 2020, I think there is more information and people are using it.Let me state my use is receive for MW and HF up to 4MHz.Two big advantages at least for directional antennas is your pattern is not altered by signal ingress on the feed line. The twisted pair rejects ingress very well. The second advantage is the extra wires for control and or power.I did find this usable tidbit."If the pairs are unused then they must either be terminated in 100 ohms at each end or shorted out. If any of the pairs are used for feeding power (two pairs are used for 12V feed in the active antenna) then a decoupling capacitor must be fitted at either end to short the pair to RF.If the spare pairs are left floating then it really upsets the crosstalk performance and adds losses.We buy the CAT 5 outdoor grade cable we supply with the antenna on drums of 305m or 1000 ft. Measuring the losses when correctly terminated it's around 15 dB at 28 MHz with the full length. We usually recommend a maximum length of 100m to customers.If you want to make a quick balun to match 50 to 100 ohms wing 5 turns of a FT50-43 for the 50 ohm port and 7 turns on the same toroid for the 100 ohm balanced port."Now, my question, There is shielded and un-shielded Cat5/6, if I were to use a shielded Cat5/6,How do I handle the shield, Ground at one end, ground at both ends, or leave it floating?It seems to me the shield could actually be detrimental if not handled properly.I'm wondering if I should just get un-shielded Cat5/6?

author: Qmavam
author_rank: Regular Contributor
timestamp: April 15, 2020, 11:32:24 pm
content: Anybody?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2020, 02:19:28 am
content: Wat. I can see why it might matter if it's open vs. terminated, but how is shorted just as okay as terminated, yet open is somehow unacceptable?At least such advice tells you up front how valuable it is...More particulars --Quote from: Qmavam on April 15, 2020, 03:29:13 pmSo, first of all, if you're using say 10m or less, it's basically just wire.. not a lot of transmission line going on here (electrically short, small phase angle). What you do with the other pairs, doesn't really matter. There's not enough to drive a lot of coupling between them, even if they weren't twisted at different rates.And, if you need lower feedline losses say, you can happily connect pairs in parallel -- mind the impedance goes down of course, but the power capacity goes up. (Uh, hm, power capacity and attenuation aren't the same thing, I'm not sure if that would actually help with overall losses.)A power combiner would be ideal, of course. You can connect pairs in series-parallel to keep the same 100 ohm total, but this does put fully half the signal, in common mode, on each pair-of-pairs. So the cable is balanced overall, but individual pairs aren't. Not ideal, but would probably still be fine. Note that because there's a TL impedance between pairs in the cable, the impedance of this configuration will be lower than the 100 ohms of ideal TLs in series-parallel.Anyway, anywhere you're using multiple pairs in sync, mind that their velocities won't match -- a consequence of the differing twist rate. A few wavelengths' total length should still be fine. At some point (10s of wavelengths?), you'll find anomalous VSWR in the cable itself, due to circulating currents.QuoteI mean... it rejects as well as anything properly balanced does, and it rejects as well as anything unbalanced does when properly grounded and shielded. You aren't granted any magical powers either way. Still need a good balun, and if you expect [common mode] feedline voltages/currents, you'll still need good CMRR in that balun, or the antenna itself, or the tuner, or transmitter, or anywhere along the signal chain that those common mode currents can go.The consequence in terms of directional pattern would be, your feedline acting as a monopole or "long wire" or what have you, coupling in via poor CMRR somewhere, and summing with whatever the antenna's normal radiation pattern is. For example, a dipole mixed with a monopole would be... some kind of tilted or twisted torus, I guess?QuoteAlready gave my thoughts on this, but I will note that the source and load for that 12V DC feed will probably appreciate the bypass caps (and the RF won't care, because, that's why you're using twisted pair in the first place, innit?). Even more so: if there is CM coupling in the system, then adding a CM choke would be even more welcome! And maybe some caps to ground, ground early and often, y'know? For the RF line, that can be done using a CT'd choke, the CT providing a ground point. (A balun, that doesn't go anywhere. But it's also balanced only. So, a balbal. But, just a bal?)QuoteWhy introduce another conductor you need to worry about? Wasn't that the point of getting UTP in the first place?A shield probably doesn't do all that much good in a feedline context. It can provide a ground of sorts, but it is by itself a wire through space, so it's not going to earth anything over a distance. If it's run in a (metal) tower, you can ground it to the structure, but you can also use the CT choke with the unshielded pair to provide local CM grounding. Or use coax and ground the shield without using a choke at all (but, still needing a balun for the  antenna itself, of course). Likewise, where it enters the shack, you can ground it, but if you've not taken measures to also terminate the pair-to-shield (common mode) path at the antenna end, that same mode is piped straight through right up to your hardware.The UTP/STP dilemma seems to fit into the "more is better" anti-pattern. If you can't reason out all the ways to use the additional features, and all the ways they might go wrong, it's probably better to just not use them in the first place.Tim

author: cdev
author_rank: Super Contributor
timestamp: April 16, 2020, 02:47:43 am
content: Read the article by Chris Trask where he puts forward a fairly easy to build design for a remote, varactor tuned loop with a preamp. So, its not a broadband loop its a very nice magnetic loop that uses varactor diodes in lieu of a variable capacitor. Ive built several of them (make sure you use a 10 turn vernier pot to tune it). His design comes with a preamp but you dont even need it because it develops a lot of signal. This is for HF, not anything higher. If you just want a quick and rirty magnetic loop you can put a bit away from your station this is very easy to do in just a few minutes. You just need a big varactor diode like an MVAM 109 (I think that is it) Or you can use several of them. See his design, its very nice. Especially if you have a very quiet QTH. Yes, it works well. Also, if you have a powered low noise amplifier on the antenna side, that amplifier determines the noise figure of the system. That also applies to any non-tunable active antenna. The beauty of using a loop however is the ability to use something like a balanced or even a balanced shielded loop.To see a good balun implementation look at Chris Trask's two stage balun in his "A Varactor-Tuned Indoor Loop Antenna". make sure you use thin, enameled wire in a tightly twisted pair for the bifilar windings.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2020, 04:12:00 am
content: Ground the shield at both ends. Like any other shielded twisted pair, that is how it is intended to be used.

author: Roger Need
author_rank: Regular Contributor
timestamp: April 16, 2020, 07:20:26 pm
content: Quote from: Qmavam on April 15, 2020, 03:29:13 pmThere is a very popular wideband active loop amplifier sold by Chavdar Levkov LZ1AQ. It uses shielded CAT5/6 cable as transmission line. He states "The FTP shield must be connected to RX ground (chassis), but at the far (antenna) end should be left floating." I assume this is done to prevent ground loops and noise on the shield.https://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htmRoger

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2020, 03:38:50 am
content: A shield with one end left open is as good as no shield at all. In some cases it's worse. In a few cases it's better.Tim

author: Qmavam
author_rank: Regular Contributor
timestamp: April 18, 2020, 10:48:29 am
content: Some additional info added for your scrutiny, in case I have any stupid mistakes.First I'm replacing a BOG I had, it had an argument with a bulldozer and lost.I'm adding an experiment to the build, I'm comparing a High input impedance low output impedance amp to a matching transformer. When 15V is applied, the amp is energized and relays route the signal in and out of the amp.I tried to add attachments, didn't work. Here's a link.https://www.dropbox.com/s/o0nlwlvrqtr5jto/Bog%20Switching%20Box%20two%20relays%20S.jpg?dl=0This is the power supply, just to show the routing of DC and RF.Note: I have no shield connection in the PS, If I ground this end it will beoutside to a at least two ground rods. It's then 4 ft into the house and the radio.https://www.dropbox.com/s/ty8ojt3p0bgpzuo/bog%20ps%20wiring%20diagram%20with%20notes.jpg?dl=0Mikek

author: tkamiya
author_rank: Super Contributor
timestamp: April 19, 2020, 03:34:02 am
content: The twisted pairs are supposed to have shielding quality. It is just implemented in different way. Instead of confining electrical field between inner and outer conductor, twisted pair cancels each others field by having current follow in opposite way, thus field going in a direction that is exact opposite.Outer shield is to reject common mode noise. It'll depend on your environment. I'd say try both. You initially stated you are basically trying to do if other's conventional wisdom says you cannot. So try both ways.As to unused pairs, cat 5 and 6 cables have different rate of twisting to avoid co-interferance. It shouldn't require anything special. At least that's the design principle.Don't forget to take care of balanced and unbalanced issue.I'd say it'll be a very interesting experiment.

author: cdev
author_rank: Super Contributor
timestamp: May 04, 2020, 12:54:06 am
content: If there is an LNA at the loop end there should be absolutely no difference between the two. Because the noise figure of the system will be determined at the input of the front end amplifier outside.You can use varactors to make a tuned resonant magnetic loop too.

author: Qmavam
author_rank: Regular Contributor
timestamp: May 29, 2020, 02:02:26 am
content: I finally got far enough along to work on the Feed Line ingress problem, which I think is just Common Mode signal. I went through about 10 iterations before I eliminated all of the signalcoming from my Feed Line from the radio. I have a 235ft Feed Line. I terminated the Feed Line in a grounded metal box at the antenna with a 100Ω resistor to match the CAT5/6 impedance.At the radio end I have a box with a CMC, a 1 to 1 transformer and a 100Ω to 50Ω matching transformer. The 1 to 1 transformer is Center Tapped on both the Primary and the Secondary.I don't know why the 100Ω to 50Ω transformer alone was not the isolation needed to quiet the Feed Line, but adding the 1 to 1 transformer made a huge difference.Here's a drawing of the circuit.

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 04:40:43 pm
content: Quote from: Qmavam on April 15, 2020, 03:29:13 pmyou can use symmetric line, but it will not give you advantage, on the contrary - it will be a part of antenna, because your antenna will work against feed line. Also symmetric line has radiation loss, so you will listen all noise sources which is near to your feed line.Just put balun on coax cable and it will works ok.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2020, 06:47:04 pm
content: How much radiation loss does twisted pair have, do you know?Tim

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 07:02:05 pm
content: Quote from: T3sl4co1l on May 29, 2020, 06:47:04 pmIt depends on many factors, such as frequency, transmission line geometry, distance, material and geometry of nearby objects, etc...Radiation loss of twisted pair is quite difficult to calculate, but can be measured if necessary

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2020, 09:35:56 pm
content: Oh dang, I suppose it could be anything, I better avoid using twisted pair in the future if it's that complicated to use. Thanks!Tim

author: cdev
author_rank: Super Contributor
timestamp: June 01, 2020, 12:57:29 am
content: "Galvanic isolation" I have a very noisy QTH and that does wonders for me too.