mashoo
mashoo

skywire loop antenna

The skywire loop antenna is a horizontal loop antenna. This one is approximately 50 metres in perimeter (roughly one wavelength for the 60 metres band), between 4 and 8 metres above ground, corner-fed with 450-ohm ladder line and matched using a link-coupled transmatch. Above a certain frequency (10 MHz in this case), the radiation pattern becomes quite low and narrow, just like a centre-fed horizontal doublet, but in only one direction (opposite the feed point).

(entries from blog follow)



two bamboo rods, for passing the skywire loop antenna over the television aerial mast; the lowest piece on the left-hand rod is 15mm copper tube

000039fb.jpg | 2007-04-04T16:14:22Z | photo | view in blog



having thrust the first rod past the television mast from the side of the house, i took the ladder to the front of the house, to collect the end of the skywire loop antenna with the other rod; when it eventually found its mark, i was holding onto the top rung of the ladder, with my feet just a couple of rungs beneath; my other hand was holding my 3-cane rod, right at the end, and it was sagging badly at the other end; it was scary

000039fc.jpg | 2007-04-04T16:16:14Z | photo | view in blog



the recently-pruned cherry tree, the next port of call for the skywire loop antenna; blue sky

000039fd.jpg | 2007-04-04T16:16:48Z | photo | view in blog



the ladder up the willow tree for another scary climb to raise the skywire loop antenna; blue sky

00003a03.jpg | 2007-04-05T14:29:46Z | photo | view in blog



link-coupled atu, feeding the skywire loop antenna

000039ff.jpg | 2007-04-06T07:27:48Z | photo | view in blog



Skywire loop tour

Tour of the skywire loop antenna, taken from the back garden. From the rear of the house over the roof to the television mast; then to the cherry tree; a long stretch to the willow tree; and then back to the rear of the house.

00003a00.webm | 2007-04-06T08:00:28Z | video | alt | view in blog



where the feeder meets the skywire loop antenna; gutter; drainpipe; blue sky

00003a01.jpg | 2007-04-06T08:02:04Z | photo | view in blog



link-coupled atu for the higher hf frequencies, feeding the skywire loop antenna

00003a07.jpg | 2007-04-12T23:13:58Z | photo | view in blog

having established a capacitance range of 11 to 190 pf and made the motion reasonably smooth, i took the skeleton of the old loop to make a new one; there is more foil in this main loop, using 300 mm kitchen foil, folded twice, for a 75mm effective (?) width; the previous loop used the same foil cut in half, before folding just once, for the same final width; the motion is generally good enough to make tuning simple; however, tuning on some bands is a little fiddly, due to the capacitor ’sticking’; (it’s really friction brought about by warped epoxy board, rather than jam or something); at 10.1 mhz, where the capacitance is highest, the sticking is quite pronounced, but tuning can be achieved in time; tuning, yes, but not a low swr; not with significant power used; not on any frequency; the old loop required a much larger coupling loop than was forecast; 90 cm perimeter, rather than the 60 cm predicted; however, having adjusted the coupling loop of this ’thicker’ loop four times, i perceive that the coupling loop would have to be larger still; perhaps quite a lot larger; a motor instead of the servo would prevent the sticking; the only other matter i can think of now is the foil tags which connect the static plates to each other; they are not very big and are currently only taped to the plate surfaces; it wouldn’t take much (hopefully) to dismantle the capacitor a bit to attach the tags to the threaded rod for a better connection; but the tags would still be small; but that should be alright; and there is another thing; i was able to compare some 14 mhz signals on the magloop and the skywire; the magloop (for whatever reasons) removed the strange noises, which seemed to be local computery/electronic stuff; but the signals were way down; 9 and below compared to 9+20 (all with pre-amp); perhaps the magloop was suffering from proximity to other conductors

00004d42 | 2007-05-24T22:12:00Z | view in blog



Magloop-skywire comparison

Signal-to-noise ratios of first the magnetic loop and then the skywire loop. F4ECJ and ON7USB.

00007fda.opus | 2007-05-25T13:08:51Z | audio | alt | view in blog

when g4crt called out earlier, i was testing the router; i used the skywire loop; ma had returned from town with a couple of ferrite rings from maplin; i tried the rings on both patch cables and the phone cable; at one point, noticing no difference between the noise level with the cables and that without them, i concluded that the noise pertained to the dsl connection, rather than the intranet; later, using the largest capacitor available, i tuned the 10-30 mhz magnetic loop to the same frequency; it was hung on my shower room door; turning the loop, so that the null was in the direction of the router (door open 90 degrees), the speakers up and opening (from the other side) the other door, i returned to the router to test it again; this time, i noticed that emissions were significantly lower with the dsl connection running and no intranet cables attached; the addition of the cable to my room returned the noise to a high level; and then the removal of the phone cable resulted in nearly no reduction in noise, if any; i therefore conclude that the intranet is manageable, provided that the antenna is situated at a distance from it; that’s not a good conclusion, because the intranet cable ’antenna’ might be more potent than the dsl connection; one more thing; i did not think to try the ’dsl connection only’ configuration with the antenna turned through 90 degrees; that might have affected the noise level

00004de4 | 2007-05-29T19:23:00Z | view in blog

the noise on this channel seems too much; perhaps the larger loop is more susceptible to electrical signals; perhaps that’s because it’s slightly closer to the router circuit; perhaps because there are metal objects nearby; in any case, this does not feel like a good project; (also, that loose servo control box connection is playing up again, making this experiment more of a pain;) granted, the noise of the router on the skywire is greater; oh, and one of the top bamboos broke; probably because i gambled that the string connecting top and bottom would not be required

00004e67 | 2007-06-23T10:43:10Z | view in blog



the roof from the top of the bay tree, featuring the skywire loop antenna

00004634.jpg | 2007-10-08T12:51:30Z | photo | view in blog



the roof from the top of the bay tree, featuring the skywire loop antenna

0000463a.jpg | 2007-10-08T12:55:58Z | photo | view in blog

using pre-amp, which is a much less noisy experience than when using the skywire

0000567c | 2007-10-08T13:12:38Z | view in blog



willow tree and skywire loop in front of a sunrise of distant red clouds

00005dd2.jpg | 2007-12-11T07:57:42Z | photo | view in blog

just repaired some wind damage; the copper at one end of the skywire loop had snapped under strain from the tree; i replaced that end-piece with a slightly longer one and redesigned the safety ties so that the backup tie would no longer share the load; it all seems quite healthy now;

it’s funny that this should happen the day after i was contemplating overhauling that safety system;

before getting out of bed this morning, i heard a loud crash nearby, like a shed collapsing or something; nothing seen though

000071fb | 2008-06-22T09:45:09Z | view in blog



fig, bay, skywire loop and collinear antennas, tall trees and blue sky; the fig had just been watered (hose out of shot left, stuck in one of mum’s many pots)

000075e8.jpg | 2008-07-26T14:29:02Z | photo | view in blog

i might have mentioned detuning my skywire matching network on 60m to reduce the router noise; i found a similar effect on 17m today; but there was a nice surprise on 15m, where the tuned state was also the quietest; i suppose the loop feedpoint has a high impedance or something at that frequency

00007d11 | 2008-11-11T17:39:04Z | view in blog



this l-match + choke balun solution is intended for portable use, but i thought i’d try it with the skywire loop antenna; clothes pegs on standby for securing the capacitor; i later added a spark gap between the antenna end of the coil and earth

00008e18.jpg | 2009-06-21T16:24:44Z | photo | view in blog