Putting up a "random" wire antenna for HF bands - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: August 06, 2020, 01:02:30 pm
content: Let me start by saying that, what follows, doesn't pretend to be "the definitive guide" or something like that, but just some notes about putting up a well performing "random" wire antenna covering the HF bands; the below is just some kind of "digest" coming out from informations I've collected from several sources (internet ones are reported) and from personal experience.* Note about antenna wire selectionIn what follows I'm referring to stranded, insulated copper wire (for the antenna and counterpoise wires), but if you prefer using solid and/or bare wire, it will be ok too; things will change in case you'll use a different material (e.g. steel) since, due to the velocity factor of the chosen material, the suggested lengths may change and the resulting antenna may not work as expected, also notice that when you'll choose your wire, it will be a good idea avoiding too thin wire, since it may break more easily, a good pick would be a wire diameter around 2 or 2.5 mm (AWG #10 to #12) since it's thick enough but not too heavy, then, the choice is up to you* Antenna wire lengthFor a full discussion about best wire lengths to use for a random wire antenna, seehttp://www.hamuniverse.com/randomwireantennalengths.htmlin short, to be efficient on a given band, the wire must be longer than 1/4 lambda on that band and must NOT be a multiple of 1/2 on ANY band which we want to use the antenna on, so for ham bands here's a table summing up the best lengths up to 423ft, the "band" column shows the "lower" (longer wavelength) band on which a given length of wire will operate in an efficient way (longer than 1/4 lambda), when possible pick from the table the longest length which fits your available spaceband mt wire meters wire feet-------- ----------- -----------20 8.84 2940 10.82 35.540 12.50 4140 17.68 5880 21.64 7180 25.60 8480 32.61 10780 36.27 119160 45.11 148160 61.87 203160 105.77 347160 124.05 407160 128.93 423-------- ----------- -----------the antenna wire should, if possible, be laid out in a straight line, and even if the horizontal placement would be the best one, it may also be laid out in a sloping configuration; if this isn't possible, and/or if you want a longer wire to cover a lower band, you may lay it out in a "zig-zag" fashion, taking care that it never describes an angle of less than 100°; in this latter case the radiation pattern will be almost unpredictable and the SWR may be a bit higher, so if possible, try to keep is straight or at least try to keep most of the antenna straight* Counterpoise/groundingThe "random" needs a counterpoise to work properly, such a counterpoise may just be a run of wire connected to the "ground" of the UnUn,according to the ARRL informations, a good counterpoise for a random wire antenna should have a length of 1/4 lambda at the lowest operating frequency (which for the 160m band means 40m/131ft - but don't worry and read on), such a counterpoise may be just laid down on ground in a straight line and/or looped around trying to keep the loop as large as possible; alternatively it's possible to connect a run of wire from the counterpoise connection to the ground and, from there, run whatever number of radials of various lengths and in various directions, another alternative could be using some metallic wire net (the so called "chiken fence") laid down on ground and connected to the wire going to the UnUn ground binding post; if your feedpoint is high enough here's an idea for a simple, yet effective, counterpoise systemin the above sketch, the black vertical lines are the counterpoise wires which should possibly be cut around 1/4 wave at the various bands, the longer ones may then even lay on ground (use insulated wire), the red lines are the top junction going to the UnUn "gnd" post and the blue horizontal bars are spacers built usin some insulating material (small diameter PVC pipe will do)* FeedingWhile a "random" may be fed directly to a tuner, to avoid the (very bad) "RF in the shack" you'd need to use a remote tuner, otherwise an easier yet effective way of feeding the "random" is using an UnUn (since it is an Unbalanced antenna), now if we build the antenna using one of the above lengths, its feedpoint impedance will be around 450 Ohm on the Ham bands, so to bring it back to 50 Ohm and feed it using a standard coax cable we will need a 9:1 UnUn, the informations to build such an UnUn can be found at this pagehttps://vk6ysf.com/unun_9-1.htmbut, instead of using a "T" series toroid, which has linearity issues if used as in a wideband transformer (it's an high "Q" mix toroid), it's better to pick an "FT" series one, a good pick could be a #43 material, for example an FT240-43http://toroids.info/FT240-43.phpusing an FT240-43 toroid your UnUn will be able to support at least 400W continuous operation, if you use less power you can go for a 140 which will allow around 150W, if you need more power, pick two 240 and stack them one over the other before winding the wire; in any case, put your UnUn inside a good water/weather proof enclosure, the enclosure will then host a hook/eyelet to hold the unit up, two binding posts for the antenna and the ground/counterpoise and a coax connector, here's an example showing a good way to put things together (note the image is NOT referred to the 9:1 UnUn seen above, I use it just to show an idea showing how to correctly host it inside a box and how to setup the various external connections)just ensure to seal all the connector/post/hook holes to avoid problems with water/mist, if you want, you may use some tape to stick, to the inner side of the cover (not shown in the above image) a bag of dessiccant (e.g. "silica gel") which will help keeping the box content dry* ChokingBeing an unbalanced antenna, even if we are using a counterpoise, the "random" will need a choke on the coax feedline to stop common mode currents from causing issues, the simplest way of building such a choke will be using a #31 material toroid to wind a "guanella" choke over the coax cable, the instructions to build such a choke can be found herehttps://www.dj0ip.de/rf-cmc-chokes/different-kinds-of-chokes/d2-guanella-choke/notice that it would be a good idea adding a second choke at the point where your coax feedline enter the shack to further help removing CMC, be careful to "hang" the choke(s) and not to lay them on ground; if you don't have toroids for the choke, a less efficient alternative could be winding about 10 turns of coax to form a 30cm (12in) coil, ensuring that the two ends of the coil come out to opposite directions and that the loops are properly wound, possibly use a former (e.g. a piece of PVC pipe or the like), the result could be something like theseand, as you can see, such chokes would be quite bigger than the "guanella" type one (and the bandwidth at which they'll be efficient will be narrower), but if you'll decide to go for such a design, check this site for additional informationshttp://www.hamuniverse.com/balun.html* Assembling the antennaStart by selecting the desired length (usually the longest one which fits your available space) from the table we've already seen, then measure the wire to obtain a length sligtly (say 20cm / 8in) longer, measure the wire again leaving half of the excess at both ends and mark the points with some tape, then just to stay on the safe side, measure the length between the tape markers againInsert one piece of shrink tubing at one end of the wire (this will be our remote end), fold the excess wire over the antenna near the tape marker, lock it with two spaced nylon cable tiers, now fold the end of the excess wire back again toward the end and place two more nylon tiers at the spacing points of the first two and use them to lock the three wires (antenna, first fold and second one) together, add a third tier passing through the eyelet formed by this last folding (the one which is over the antenna, not at end); cut the excess from the nylon tiers, slide the shrink tubing over the tiers/folded wire and warm it as needed, cover the tubing with UV resistant duct tapeRepeat the above for the other end of the wire (feedpoint) but this time slide THREE shrink tubing over the wire, use one for the "eyelet" as we did above and leave the other two alone along the wire (do not shrink them - we'll use them later on)Now you should have a wire with two eyelets at its ends, it's time to add your feedpoint; gently remove a short run of insulation from the end of the wire (near the point where you placed the marking tape), to expose the copper and slide one of the two shrink tubings toward the end (the eyelet) of the antenna wire so that the two tubings will now sit at both ends of the exposed portion, pick another piece of wire of appropriate length (say 20cm / 8in at MAX), remove the insulator for a bit from one end, wrap that end of the wire around the exposed point of the antenna wire and solder it, try avoiding to melt the surrounding insulator; done so slide the two shrink tubings over the junction point and shrink them, slide another tubing over the soldered wire up to the junction point and shrink it too, now wrap the whole area with teflon tape and then wrap the teflon with amalgamating tape, once done cover the amalgamating tape with UV resistant duct tape and the antenna wire will be almost complete; remove the insulator from the end of the "feed wire" (the one we soldered), slide another shring tubing over the wire and add a fork spade "U" or ring wire crimpable connector, solder and crimp the connector, slide the tubing over it and shrink the tubing, cover the tubing with teflon tape, amalgamating tape and UV resistant duct tape, that will be the connection between the "random" and our UnUnPrepare the counterpoise, as I wrote above, use whatever method you prefer (or fits your available space), just ensure to add a fork spade or ring connector to the wire end of the counterpoise which will go to the UnUn "ground" binding post to ensure you'll have a good connection to it* Installing the antennaPlace two steel pulleys to the points where the antenna ends will be placed, the pulleys may be directly tied to the supports or you may use some rope to tie it, if possible try to keep the feedpoint end at at least 9mt (29ft) and the far end at at least 3m (9ft), if not, do your best to keep the antenna as high and free from nearby buildings/wires/poles as possibleCut two pieces of rope, the length of each will be the double of the distance between the pulley and ground (or the terrace floor or in any case, the point where you can comfortably service that end of the antenna)Tie each piece of rope to the corresponding "eyelet" present at the end of the antenna wire, to tie them, I suggest using abowline knotand then locking the knot end with a pair of nylon tiers, pass the other end of the ropes inside the pulleys and down to ground and tie a simple knot to keep them from sliding (such a twin pulleys setup will allow you to easily lower the antenna to inspect or service it)Time to raise the antenna; start from the feedpoint, hang the UnUn to the rope near the antenna end, to do that, wrap some tape over the rope and use some nylon tiers to hang the UnUn, don't pull the tiers, leave them forming some loop so that the rope won't be shrinked, then use two or four additional tiers to lock the "loop" at the desired position, such a setup wont put strain on the rope reducing its resistance; done the above, connect the coax (which should already have the choke) to the UnUn and pull the rope until the distance from the antenna and the pulley will be around (or possibly more than) 1mt (3ft), tie the rope with a simple knot, just to avoid it slides out and move to the other end of the antenna, raise it as needed and again tie with a simple knot check both ends and adjust spacing and tension as desired by adjusting the rope at both endsNow, pick a pair of strong, empty plastic bottles, the ones used for chlorine or home detergents should fit, just ensure the bottles have a handle, a bottle like this (just an example) will fit wellhttps://hyclor.com.au/wp-content/uploads/LiquidChlorine.5L.jpguntie the rope and pass it through the handle of one of the bottles, pull the rope to put the antenna in tension and tie the rope to the handle with a good knot (ahangman knotwill do) ensuring that the bottle is sitting on ground when the antenna is in tension; now the "funny" part, pick up the (empty) tied bottle and pick a recipient (bottle...) filled with a mix of water and chlorine (the latter will be useful to avoid growth or organisms or algae), start filling the empty bottle (the one tied to the rope) a bit at a time until the bottle, due to weight, will sit on ground and the antenna wire will be in tension again, cap the bottle, seal it and repeat the operation for the rope at the other end, such a trick will keep the antenna in position BUT, if there's some load from wind/ice... it will also allow the antenna to lower (and raise back when the load disappears) so that the wire won't be subject to excess strain and break* Testing and adjustingnow it's time to test the antenna, connect the coax to your TRX, set it to lowest TX power and test the SWR on the various bands, if things are ok, you should see an SWR at around 1.5 or even lower; notice that the random isn't a RESONANT antenna, so the only thing you can do lower the SWR (assuming you'll need to) is improving the counterpoise system, by making the counterpoise longer/better, that is adding radials and so on; but in general if you have a good counterpoise (as discussed above) your SWR will be quite low on all the Ham bands which the antenna can cover, the only bands which may, in some cases, need the use of a tuner are some of the WARC ones, but even then, the SWR would be low enough to be easily dealt with by a regular ATUOnce your SWR test and possibly some listening test will be complete, you may go on and check how the signal from your new "random wire" antenna is received at various spots, to do so, you may just use one of the (many) available public Web SDR receivers, here's a list containing some of themhttp://websdr.org/(also see them on a map at bottom of page), start by picking a receiver near to your spot, find a free spot on the desired band, and try transmitting, that will tell you if and how your signal is received at that spot and could be a first, coarse test, of your antenna performanceThat's all, hope the instructions are clear and correct (if not... just tell me so, I'll edit the post); I just hope that the above could be of help to any new Ham willing to put up his first antenna !

author: A.Z.
author_rank: Super Contributor
timestamp: November 13, 2020, 02:42:21 pm
content: Revamping this post since I think it may be worth adding some notes about the latest buildI've been helping a friend putting up a random for his (limited space) QTH, we selected the longest length fitting his available space, which resulted to be 84ft (25.60mt), the antenna had to be installed in a sloping configuration, with the top at about 10m (roof top) and the far end at about 6m (tree), so we bought some 2.5mm (AWG #14) insulated wire and the other needed stuff (toroids, connectors, insulators and misc hardware), first of all we built the UnUn, the latter was built followingthis schematicbut using an FT240-43 core, at the output of the UnUn we then placed an 1:1 guanella balun (choke) built using this schematichttps://vk6ysf.com/balun_guanella_current_1-1.htmthe two cores were hosted inside the same box but placed perpendicular (each core respect to the other), so the 9:1 was sitting horizontally on the bottom while the 1:1 was sitting on a side; the cores were locked using some silicon and we added two bags of dessicant (silica-gel) sticked to the box cover before closing the box, the outside of the balun box had a hook to the upper side to hang it, a hook and a wire clamp to the left side to connect the antenna wire and the bottom side hosted a hook+clamp for the ground/counterpoise and the coax connectorThen we cut the wire to the needed length (ok, a bit longer to have enough to create eyelets to connect to the far end insulator and to the UnUn hook), we passed the far end through a dogbone insulator and then wrapped it back over the antenna wire and locked it with cable ties and shrink tubing, then we covered everything with self-amalgamating tape; the "feed end" of the wire was then tied to the UnUn box hook and connected to the wire clamp, the other wire clamp was connected to a 7m (about) piece of wire used as the counterpoise (not optimal but we were going to run a quick test and come over it later on), then we connected the coax feedline, the latter, just to stay on the safe side, carried a choke built "ugly balun" style by winding 10 turns of coax to form an 11 inches coil, it wasn't probably needed, but... better going for "belt and suspenders"All the above done, we used a pair of pulleys and ropes to raise both ends of the antenna wire keeping them at some distance from both supports and, once the antenna was up we ran the first tests on the various bands; to our surprise, the antenna offered an SWR lower than 1.5:1 on all the bands, to the point that we started wondering if we made some mistakes and just built some kind of dummy load, but after making contacts with the antenna, it turned out NOT to be the case, the antenna was radiating and radiating decently well !After some time, the antenna was improved by adding multiple counterpoise wires (as in the original post) and the resulting antenna was working well, the only problem was that, as for all "randoms", the antenna wasn't exactly a quiet one, that is, noise floor was pretty high, so, after fiddling with chokes and other countermeasures and lowering the noise as much as possible, since the noise level was still high, we decided to go for a separate "RX only" antenna, the choice felt on the KK5JY "Small Receiving Loop" described herehttps://www.kk5jy.net/rx-loop/it's a small, passive loop antenna which can be built in minutes; in our case we used the same 2.5mm (AWG #14) wire and a cross-shaped structure built using PVC pipes to put together the loop, the transformer was built using the original design (6+2 turns of enameled wire on a #73 binocular core) and the combo "random + loop" is now serving my friend quite wellConsidering that both antennas are easy and cheap to put up, that the setup allows to cover all ham bands from 80 to 10 meters and that it may fit restricted spaces (and low-budgets as well), I believe that the combo may be of interest to hams seeking for some multiband setup or willing to put up their first antenna; sure, it won't be a hexbeam, but will still allow to make interesting (and often surprising) contacts

author: bob91343
author_rank: Super Contributor
timestamp: November 13, 2020, 07:31:38 pm
content: I didn't take the time and trouble to read through all the details but my feelings are:1. If it works and is a practical build, go for it.2. My own experience has always been best with a self resonant simple antenna. Yes you can get contacts with almost anything but to have the most fun on the ham bands I feel you need to put up a resonant antenna like a dipole or yagi etc.The oddball ideas can be made to work and can give decent results but of course there is no substitute for a big antenna that is high above the ground. This isn't always possible but it's a worthy goal in my opinion.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: November 14, 2020, 12:06:09 am
content: I keep it simple. An end-fed wire about one-half (or at least 3/8 wavelength) on your lowest frequency and a good L-match coupler.More here:https://vk3ye.com/gateway/nojun98.htmSome demonstrations/comparisons/results achieved with low power:

author: A.Z.
author_rank: Super Contributor
timestamp: November 23, 2020, 09:55:37 am
content: Quote from: bob91343 on November 13, 2020, 07:31:38 pmBob, let's say you have limited space and want a cheap, easy to build antenna which will get you on air on as many bands as possible; in such a case, the humble "random" will fit pretty well; I'm not comparing it to a resonant dipole, nor to a hexbeam or yagi or whatever, but the antenna, possibly paired with a receiving loop would allow even a newbie to get on air and making contacts, then, by the way, there's always room for improvement, but that's part of the fun, I believe

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 23, 2020, 01:55:07 pm
content: obviously there really seems to be no way around the so magic 9:1 balun in any contemporary antenna setup. I'm wondering, if either people don't know about the cheap vector antenna analyzers being available today for almost everybody or they just don't know how to use them.otherwise nearly all hams, after hooking up their VNA, would realize that their low-band antennas are far, far away from 450 ohms real impedance, but rather go in direction of a few 1-10 ohms real with several -100 ohms complex impedancebut sure, urban legends always die last...

author: A.Z.
author_rank: Super Contributor
timestamp: November 23, 2020, 02:04:12 pm
content: Quote from: HB9EVI on November 23, 2020, 01:55:07 pmThere's no "magic" in 9:1, if you want you can use a 1:1, but before doing so, try simulating an endfed with one of the indicated lengths (say 25.65mt - 107ft) using a 450Ohm feeding impedance, check the ham bands and then decide it by yourself, again, no magic, just a matter of using a wire length which keeps the antenna impedance around 450Ohm at the frequencies we're interested in; then if you prefer, you can bring the wire directly in the shack (and have RF there) or spend some bucks and buy/build a remote tuner, but not everyone can or wants to do that.

author: Co6aka
author_rank: Unknown
timestamp: November 23, 2020, 10:34:47 pm
content: FWIW, my experience has been that 73 material is "better" than 43 material for HF transformers and baluns. Another turn or two (above what's specified for 43 material) could be necessary though. The 43 material might be "better" for 160-meters, AM-BCB, etc. YMMV, of course. I've made two identical of whatever, connected them back-to-back, and then taken measurements.

author: profdc9
author_rank: Frequent Contributor
timestamp: November 24, 2020, 12:47:35 am
content: The random wire with 9:1 unun works pretty well in general as long as you have a decent counterpoise or ground system. It doesn't require a really good ground, just a passable one, and can be adapted for many installations easily. However, usually it needs a tuner. If it doesn't it means the system probably has significant losses. I did some write ups on ithttps://drive.google.com/file/d/0BwFicJLV0O4jbVBHQkg2c2w1aFk/view?usp=sharinghttps://drive.google.com/file/d/0BwFicJLV0O4jOHpobkJYa0UxYXM/view?usp=sharingincluding calculations modeling the unun and common wire lengths.

author: A.Z.
author_rank: Super Contributor
timestamp: November 24, 2020, 08:36:56 am
content: Quote from: profdc9 on November 24, 2020, 12:47:35 amTotally agreed; at least if one doesn't want to use the coax feedline as the antenna counterpoise(which isn't exactly a good idea)Quoteright, and that's also why I was surprised by that particular "random", the SWR was so low (maxed at about 1.7/1.8 ) that I suspected the antenna was just a "dummy load", but instead it seemed (and seems) to work as it should (and there was no unun/coax heating); but yes, in general you'll need a tuner, the idea behind using a 9:1 UnUn is to allow bringing the antenna feed impedance into the "ballpark" of coax impedance, so that we could then use an in-shack tuner, otherwise we'd need a remote tuner connected directly to the antenna wire, but that isn't always possible and would add quite some cost and complexity to such a simple antenna; on the other hand, a simple "L" match in the shack is easy and relatively cheap to build (or buy) if the rig one uses hasn't an internal ATUQuoteInteresting stuff, thanks for posting it (and thanks for your reply); as for the UnUn, I found theVK6SF designto work pretty well, and then I believe it's a good idea placing a 1:1 guanella choke (like the one described at thebottom of this page) immediately after the 9:1 UnUn and enclosed inside the same box; I found that it helps a lot minimizing CMC and improving the overall antenna performance.Another design which I find interesting (but never tried) is the one found at the second page ofthis document, again, didn't try it, but judging from the measurements the author performed, it seems to offer pretty good performance.Also, looking at your setup, sounds like your UnUn is sitting on ground; did you try raising it a bit (say 3ft or the like) to see if it makes some difference (thinking to radiation pattern and ground losses) ? Then, by the way, I became curious, so if I'll find a bit of time, I'm going to try modeling your antenna using 4NEC2 and then fiddle a bit with the parameters; looking at your setup, the antenna is more or less an "inverted L", am I correct ?