Cat 5/6 instead of coax for Amateur Radio feedline ??? - Page 1
Forum section: electronics / rf-microwave

author: KM4FER
author_rank: Regular Contributor
timestamp: February 02, 2016, 07:14:19 pm
content: In my "Shack" (implies higher quality than what I actually have) there is an unused Cat 5 jack that used to be for AT&T Uverse high speed internet feed. Outside there is enough cable that I could use to feed a dipole antenna. So rather than make a new hole in the wall and buying coax feedline I wondered if I could use this Cat 5 cable for low power HF (30MHz max) QRP work. Having the extra pairs in the cable could be handy for power and control of remote amplifiers, antenna tuners or such.Belkin specs for Cat 5e:Electrical CharacteristicsImpedance: 100+15 OhmsMutual Capacitance, Max. nf/1000 ft.: 17.1DC Resistance, Max. Ohms/1000 ft.: 28.6DC Resistance Unbalance of a pair: 5% Max.Capacitance Unbalance (Pair to Ground): 330pf/100m Max.Propagation Delay Skew: 25 nS/100MNormal Velocity of Propagation: Plenum FEP insulated 69%,Non-Plenum insulated 66%MHz IMPEDANCE ATTN. NEXT ACR ELFEXT PSNEXT PSACR PSELFEXT R.L.1 100+15 2 68.3 66.3 68.8 66.3 64.3 66.8 224 100+15 4.1 59.3 55.2 56.7 57.3 53.2 54.7 258 100+15 5.8 54.8 49 50.7 52.8 47 48.7 26.510 100+15 6.5 53.3 46.8 48.8 51.3 44.8 46.8 2716 100+15 8.2 50.3 42.1 44.7 48.3 40.1 42.7 2720 100+15 9.3 48.8 39.5 42.8 46.8 37.5 40.8 2725 100+15 10.4 47.3 36.9 40.8 45.3 34.9 38.8 26.331.25 100+15 11.7 45.9 34.2 38.9 43.9 32.2 36.9 25.662.5 100+15 17 41.4 24.4 32.8 39.4 22.4 30.8 23.5100 100+15 22 38.3 16.3 28.8 36.3 14.3 26.8 22.1I'm not sure I understand transmission lines and antennas yet but it looks to me like with impedance matching at each end this ought to work.Am I way off in my thinking?thanks...earl...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2016, 08:11:38 pm
content: Losses are a bit higher. For HF, probably not a concern.You can't really connect pairs in parallel or series, because their velocities aren't matched. (Again, may not be a problem for HF, but beware.) Obviously, it's not coax and it's not 50 ohm, so you'll need a matching balun, or else your transmitter and antenna grounds will fry everything, even for QRP.The additional wires probably need to be well filtered. Theoretically they are all balanced, but if your balun isn't ideal, for example, expect coupling. If the control signals are slow anyway (provide filtering at both ends if they are driven by switches), using them in an unbalanced manner would be okay.Tim

author: TheSteve
author_rank: Unknown
timestamp: February 02, 2016, 09:26:59 pm
content: You can certainly use it for control of other devices but should really stick with proper coax for your feed line. All kinds of nasty things will likely happen if you try to use it for RF.

author: hendorog
author_rank: Super Contributor
timestamp: February 02, 2016, 09:59:58 pm
content: Quote from: TheSteve on February 02, 2016, 09:26:59 pmI'd like to know more about why it wouldn't work. Electrically its just a balanced feed line with an impedance different to 50 ohms and there are many HF installations that fit that description.I don't know what the effect of the other pairs in the cable would be though.

author: Unknown
author_rank: Unknown
timestamp: February 03, 2016, 03:08:12 pm
content: In theory I don't see why you couldn't do it at very low power levels. I've seen video sent over cat5 using small adapters which are just a toroidial balun in a box. The problem is though if you try and put any kind of power down it and you get any mismatching you could get arcing inside the cable. Coax is rated for high voltages, cat5 isn't.

author: cdev
author_rank: Super Contributor
timestamp: February 06, 2016, 01:15:29 am
content: Look up the Chris Trask *receiving loop* with varactor tuning and balanced feed line. I would do that and use the feedline to control things like some additional caps to switch in and out and possibly switching the loop so you can have 1,2 or 4 loops (series or parallel or all combined into one). You can also put a balanced matching network and a low noise preamp in it so then the length of the feedline wouldn't matter at all. But because it was balanced you also might have superior noise immunity.

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 01:26:09 am
content: HiThe lan cable is not as well shielded as good coax, but it's better than some of the cheap stuff out there. It most certainly does not have the power handling capability of coax so QRP needs to mean just that -- very low power. The limitation comes when you heat up the dielectric in the twisted pairs. At best, only run one antenna per cable. The cross talk isolation that is fine for a LAN (all signals are pretty strong) is not going to be that good for RF.Bob

author: vk6zgo
author_rank: Super Contributor
timestamp: February 06, 2016, 01:47:25 am
content: It will be a lot of messing around for a fairly poor result.Use the CAT cable as a "draw wire" & pull plain old RG58 through.You now have a real feeder,& you have salvaged a useful length of CAT cable.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 06, 2016, 02:23:17 am
content: If you wanted to make a DIY remote-controlled transceiver mounted aloft with the antenna, that is one thing. And not terribly difficult with modern technology.However, using UTP for RF communication seems pretty dubious. Mostly, I think because of significant line-losses in the cable. If you are running QRP you can't afford such significant losses either transmitting or receiving.BangGood have a couple of QRP transceiver kits for $10 or less. They could easily be adapted for remote-control via Cat5 UTP.

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 02:37:05 am
content: HiOne thing to note, your Belden table of attenuation versus frequency is based on a fairly short run of wire. You already have lost 6db at 10 MHz based on that table. If your transmitter puts out 1W, only 1/4W is getting to the antenna. The rest either goes up as heat (losses) or is radiated to who knows where. By the time you get to 30 MHz, it's really pretty crazy.Assuming you are running 10 to 30 feet, all that may not be an issue. If the antenna location involves a run of over 100 feet ... you likely are stuck at 3 MHz.Bob

author: cdev
author_rank: Super Contributor
timestamp: February 06, 2016, 02:54:48 am
content: Two conductor balanced feedline for receive might be far superior to coax in terms of noise immunity, for the same reasons that pro microphones like to use balanced XLR audio connections. Also, you can send DC up that feedline and put a receive preamp at the far end so then no amount of loss is going to matter because the first receive gain stage is going to be the important one and that is outside.That solves a great many receive problems.here is a two varactor remotely tuned loop design..You can put two more in there in parallel to get it to tune lower.http://home.earthlink.net/~christrask/Varactor%20Tuned%20Loop%20Antenna.pdf

author: hendorog
author_rank: Super Contributor
timestamp: February 06, 2016, 03:23:46 am
content: Quote from: uncle_bob on February 06, 2016, 02:37:05 amAccording to the spec sheet the table is based on a 100m run - thats 300 feet for the American chapsAt 30MHz its 1.17 dB per 10m (30ft).At 10MHz its 0.65 dB per 10m.At 10MHz RG58 is 0.42 dB per 10m.Other considerations:The cable is rated for 300V RMS Max according to the spec sheet.Its only rated for use indoors

author: DimitriP
author_rank: Super Contributor
timestamp: February 06, 2016, 04:13:19 am
content: It's like this:You can try it.Worse case you'll blow your antenna tuner or the radios' output stage or the wire will heat up and either smell funny or start a fire, or you'll be left wondering "where is everyboady" because the receiver can't hear anything.Best case none of these things happen, you saved 50 bucks, they hear you just fine in JA land and you are left scratching your head "why do we need coaxial cable anyway"?Necessity is, after all, the mother of invention...

author: vk6zgo
author_rank: Super Contributor
timestamp: February 06, 2016, 04:52:10 am
content: Quote from: hendorog on February 06, 2016, 03:23:46 amThe "only rated for indoors" bit is what worried me.Perhaps if is a special "outdoors" version,seeing it went outside in its original use.Of course,if it was just cut off when the AT & T guys decommissioned the original service,it has been sitting out in the weather deteriorating.I'm also a bit concerned about the thickness of the conductors------from memory,they are quite thin.Googling,some cat5 is shielded,some isn't.( there is some out in the back room I could check,but there's too much junk in the way)The main argument (& a very valid one),as far as I can see,is "nobody else uses it!"I guess KM4FER could just try it & "see what happens".

author: cdev
author_rank: Super Contributor
timestamp: February 07, 2016, 12:39:04 am
content: Its my understanding that shielded twisted pair is quite finicky and often results in more noise problems (radiating noise from Ethernet) than UTP.Also, all of the Cat 5/6 cable I have ever seen uses wire that's fairly thin so I would not expect it would handle power well at all. It likely would melt. OTOH, if used for receive only, with a low noise preamp at the actual antenna, neither that nor any losses (within the range that would be made up for by the LNA) wouldn't matter and the balanced nature of the system would have noise advantages.

author: crun
author_rank: Newbie
timestamp: January 07, 2019, 02:53:33 am
content: An old topic, but I recently had reason to consider it, and try to calculate power handling and loss based on skin resistance..TL:DR: P.max 0.8W@10MHz, loss 1.42db/30m@10MHz ~=RG58Here is thelive smath calculation(updated 8Jan18)While you might be able to parallel two pairs to make 50ohms, the pairs deliberately have different twists (to reduce crosstalk), so might have poor delay matching. Stripping back some length and matching two pairs with closest twist would be a start.This suggests that crosstalk and return loss are pretty good:http://www.ieee802.org/3/10GBT/public/mar03/cohen_1_0303.pdfI plan to use CAT5 to separately feed each dipole of a multiband fan dipole from a separate qrp beacon transmitter.http://www.hamuniverse.com/multidipole.htmlI have black sheathed for outside use, and also some gel filled underground I could use too.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2019, 03:15:43 am
content: The problem is the loss. Unless both ends of the cable are impedance matched, the high SWR leads to loss greater than the one way loss because the signal is bouncing back and forth. Twin lead or ladder line has such low loss that it can work with impedance matching at only one end.The SWR and matching problem also lowers the power handing capability. It is like running a poor power factor on a power line resulting in circulating current losses.

author: cdev
author_rank: Super Contributor
timestamp: January 07, 2019, 03:31:00 am
content: This works really well I gather for a very low noise active receiving antenna.Chris Trask on his web site has a design for a remotely tuned magnetic loop (using MV-109 varactor diodes) and a preamp (which is unnecessary because the signal is super strong even without it) that uses cat-5 as its feedline.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2019, 03:39:38 am
content: If the signal to noise ratio is high and the power levels are low, then it is not a problem at all.

author: crun
author_rank: Newbie
timestamp: January 07, 2019, 05:08:43 am
content: Quote from: David Hess on January 07, 2019, 03:15:43 amThe problem of loss is overstated. The forward loss is much like RG58, and much better than RG174.A thin wire dipole is 70ohms , basically as mismatched to 100ohms as it is to 50ohmsVSWR is 1.4 - considered good, and mismatch loss is 0.14In any case 50ohms is not obligatorySince someone using a 50ohm unbalanced transmitter would be using a balun, it could obviously be a 2:1 balunuse a 4:1 Ruthroff balun (wrap the twisted pair around a core) and make the transmitter for 25ohmanyone using an L or pi match can match to 100ohms as easy as 50.QuoteIn the case of 1dB feedline loss, power handling is only reduced by 12% (less for greater feedline loss, as the return power is attenuated before it reaches the source).

author: crun
author_rank: Newbie
timestamp: January 07, 2019, 05:22:12 am
content: An interesting thought is that you could use the R.dc to tell you the actual temperature rise of the conductors, and thus find what DC power -> MaxTemp, or let you measure the temperature live when running voice to see when it gets too hot. The cable is rated 60C, but that probably allows POE. RG174 is rated to 85C, so probably the hard limit is 85C in the conductors.With a single pair a choke could join the far end, or using a double pair 50ohm setup, the whole lot could be DC looped.Of course as T rises, loss rises. Might add T to the calculations

author: deBug
author_rank: Contributor
timestamp: January 08, 2019, 01:57:04 am
content: Yes, it will work at low power.You need to impedance match both sides. That is pretty easy to do with a couple of ferrite cores.The turn relationship needs to be 1:1.41 (square root of 100/50) and the reactance of the winding should be at least 5 times larger than the impedance. As this is dependent on frequency there will be a lowest usable frequency for a certain number of turns. If you want to go lower in frequency it generally means you need more turns.So if you for example use an FT50-61 core make the 50ohm side 10 turns and the 100ohms side 14 turns. 10 turns on FT50-61 gives about 7uHfor 7MHz the reactance is about 300ohm so that is fine down to 7MHz. Use these at both ends.//Harry

author: crun
author_rank: Newbie
timestamp: January 08, 2019, 10:07:44 am
content: My previous calculations were based on ~0.6A current limit, leading to ~1W limit.So actually I measured the max continuous DC current capacity at 5A for two pairs, 50C temp rise of the actual conductors, using the 4 port resistance and copper temperature coefficient methodFrom this I calculate an RF power limit, from heating, of >60W RMS per pair @10MHz. Using two pairs thats >120WRMS.The Vbreakdown rating leads to a limit of 78W Peak per pair or 160W total.So an SSB transmitter of 100W PEP is probably OK, even allowing for some SWR increasing the voltage.These are theoretical calculations from DC measurements -see calculation worksheet hereSo my statement that its only good for low power looks likely to be bollocks. Next up: actually trying it with RF

author: tkamiya
author_rank: Super Contributor
timestamp: January 08, 2019, 05:57:50 pm
content: I would try it. Ethernet signal is really in VHF region. On 1Gbit network, 4 pairs of VHF signals are sent and received simultaneously and it is working fine.In old days, I've used things like TV ribbon cables just fine for HF. Some of us also experimented with zip cord....Just make sure your antenna coupler can handle this though. Twisted pair is a balanced line. Coax output on your radio is unbalanced. So a balun or specially made antenna tuner would be necessary. Also, sharp bend and touching metal would cause issues.I'm not sure if I'd trust it for 100 watts. Dipole being 75ohm is in free space. In reality, it's much lower in realistic height. So SWR will be high. You could have breakdown in insulation.Hum.... Now I want to try this. Muahahaha..... (evil scientist style ham radio'ing)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2019, 06:20:46 pm
content: FWIW, CAT6 is good for almost 100W at DC. The voltage limit there is driven by safety, but the ultimate limit for the cable isn't a whole lot much more than that (300V?).I would think a 1-to-4 splitter balun would do fine, maybe not up to 100W at modest frequencies (say ~100MHz), but certainly more than 10s of W.Attenuation is in the datasheet; it's not as good as twin-lead though.Tim

author: tkamiya
author_rank: Super Contributor
timestamp: January 08, 2019, 06:36:16 pm
content: Well, you are assuming VSWR will be 1 or near 1. I imagine it will be much higher, if not for its entire length but in places. Then voltage will rise beyond specifications. If you use 1:4 balun, the voltage will only double, assuming it's a perfect match to it.If I have time tonight, I'm going to test this out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2019, 10:44:06 pm
content: VSWR isn't defined at a point...I didn't mean a 1:4 balun, I meant 4 port splitter -- to use all the pairs at once. Apologies if that wasn't clear.Hmm, speaking of which, the velocity varies ~10% between pairs, so don't expect a splitter/combiner to work very well above some frequency-length margin! A phaser would be able to resolve that, but then you need to check three out of four lines for power sharing. Yuck.Tim

author: Xnke
author_rank: Regular Contributor
timestamp: January 27, 2019, 07:27:36 am
content: So on paper this looks good-and I'm sure someone can make this work.But as I read this this morning, and today happened to be ARRL Winter Field Day, so we tried it today. It doesn't work worth a crap compared to RG58, which is what some in this thread have been comparing it to. We used a 100ft length.A matching balun on each end, to convert from 50R to 100R impedance, feeding a dipole antenna at 72 ohms feed point impedance-exactly the same configuration as if the dipole was being fed from a 50R coaxial cable.With the RG58, signals were coming in fairly strong-there was no shortage of contacts coming in! We were a bit weak on transmit due to feedline loss, but not terrible. Normally this run is fed by RG8U.With the CAT5e, using a single pair, it was as if we were hooked up to a dummy load. I think we heard 1 or two very weak calls come in, and we could not get any kind of power out. Couldn't be heard even across town.As said-I'm sure someone can make this work. But spending a bit for even some RG8X will be a significant improvement in radiated emissions, power handling, and feedline loss.Assuming the holes are big enough outside, You could use your cable, tie on a pull string, and pull the cable back till the string is accessible outside, then tie on your coax, and pull both the coax and the CAT5 back in. That way you do not have to drill any more holes. If the hole isn't big enough for both cables, you could just pull the coax back in, and leave the CAT5 pulled out-just make sure you pull in a new pull string with the coax. This way, in the future you can put it all back the way it was with ease.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 27, 2019, 07:31:58 am
content: I would think that the line loss (from parallel capacitance) would be pretty bad at RF frequencies.