Not bad for 90mW out of a Japanese radio made in 1971!
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Code: Select all
CM File: DirLoop.nec
CM Small Receiving Loop antenna
CM with unidirectional pattern
CE
' symbols definition
SY freq=7.100 ' test frequenct
SY hght=3 ' height of base corner from ground
SY side=0.762 ' length of one side
SY diag=(sqr(2)*(side/2)) ' half diagonal
SY wire=0.00125 ' wire radius
SY vres=530 ' loading resistor value
SY segm=13 ' number of segment in wires
SY segs=5 ' short wires segments
SY wfed=1 ' feedpoint wire
SY sfed=segm ' feedpoint segment
SY wres=4 ' wire hosting the resistor
SY sres=1 ' segment hosting the resistor
SY spac=(wire*4) ' spacing to simulate coax
' wires geometry
' ID seg x0 y0 z0 x1 y1 z1 wire rad
GW 1 segm 0 0 hght -diag 0 hght+diag wire
GW 2 segm 0 0 hght diag 0 hght+diag wire
GW 3 segm -diag 0 hght+diag 0 0 hght+(diag*2) wire
GW 4 segm diag 0 hght+diag 0 0 hght+(diag*2) wire
' coax feeder
'GW 10 segs -diag spac hght+diag 0 spac hght+diag wire*2
'GW 11 segm 0 spac hght+diag 0 spac wire*4 wire*2
GW 20 segs -diag spac hght+diag -diag spac hght-diag wire*2
GW 21 segs -diag spac hght-diag 0 spac hght-diag wire*2
GW 22 segs 0 spac hght-diag 0 spac wire*4 wire*2
' ground parameters
GE 1
GN 2 0 0 0 13 0.005
' wires loading
LD 5 0 0 0 58000000
LD 1 wres sres sres vres 0
' enable extended kernel for calc
EK
' feedpoint
EX 0 wfed sfed 0 1 0 0
' initial test frequency
FR 0 1 0 0 freq 0
' end
EN
Code: Select all
CM ---------------------------------------------
CM File: LoG.nec
CM Loop on Ground - http://www.kk5jy.net/LoG/
CM antenna sideth changed using optimizer
CM feedpoint impedance set to 450 Ohms
CM ---------------------------------------------
CE
' symbols
SY freq=1.840 ' test frequency (MHz)
SY leng=18.288 ' loop perimeter (mt) :: 21.336
SY side=(leng/4) ' square loop side (mt) :: 4.572
SY wire=0.00075 ' antenna wire radius (mt)
SY hght=(wire*3.2) ' height from ground (mt)
SY segs=25 ' total segments for each wire
SY fnum=1 ' feedpoint wire #
SY fseg=1 ' feedpoint wire segment #
SY vres=530 ' loading resistor value
SY wres=2 ' loading resistor wire
SY sres=segs ' loading resistor segment
' -------------------- antenna wires ---------------
' # segs x0 y0 z0 x1 y1 z1 rad
' -- ---- ---- ---- ---- ---- ---- ---- ----
GW 1 segs 0 0 hght side 0 hght wire
GW 2 segs side 0 hght side side hght wire
GW 3 segs side side hght 0 side hght wire
GW 4 segs 0 side hght 0 0 hght wire
' ground parameters
GE -1
GN 2 0 0 0 13 0.005
' wires loading
LD 7 0 0 0 2.1 wire ' insulator
LD 5 0 0 0 58000000 ' copper
' loading resistor
'LD 1 wres sres sres vres 0
' antenna feedpoint params
EK
EX 0 fnum fseg 0 1 0 0
' test params
FR 0 1 0 0 freq 0
' end of model
EN
Guilty Your Honor !! The idea of routing the cable through the center of the loop was mine and mainly related to an attempt to avoid loading the support structure too much, but according to NEC that isn't a good idea <sigh>13dka wrote: ↑Mon Jul 25, 2022 3:34 pm Wow, that does seem to make a difference! BTW, that's how I initially intended to arrange the feedline, that's why I was asking "what horizontal stretch?" when we discussed this first.But I wasn't fond of some coax permanently dangling around in the wind either, adding the pull of the coax to the weight and longer arm of balun and preamp, causing some really bad weight distribution causing the fiberglass pole tending to bow etc.
another way may be using a short run of thinner coax describing a curve (in NEC i modeled that with corner just for simplicity) from the feedpoint down to the vertical support and from there start with "fat" coax; notice that judging from currents shown in NEC simulation, with such a routing the chokes may just be fitted on the "fat" coax running down the support and additional load shouldn't be a problem thereOne easy way to address that would be finding the center of gravity of the entire antenna plus 1m of coax hanging off the balun/preamp, then screwing an additional vertical mounting support to the horizontal boom at that place. That will also shorten the length of coax required to reach the pole a bit and avoiding a lot of ferrite beads tied to one side of the boom would certainly help too.
Maxwell chokes are easy, but the pitfall is that they're efficient only on a relatively narrow range of frequencies, a better approach would be winding a choke on a toroid core and possibly following the infos found here http://www.karinya.net/g3txq/chokes/ in such a case the choke may be hosted in a box hung on the vertical support at the point where the thin and the fat coax connect, but then, it also depends from IF we'll need chokes; if the preamp sits at the feedpoint, that alone should give decent isolation from CMCAfter reading the interesting VE7CA article I tried to read up more on "W2DU"/"Maxwell" style current baluns and found interesting bits in this document:
https://citeseerx.ist.psu.edu/viewdoc/d ... 1&type=pdf
According to that, it's doesn't look that easy to create a CMC choke/current balun, there's more to it than clipping a high amount of random ferrite material beads onto some coax. I'd have yet to try the maximum available (like 12) clip-on ferrites tho, maybe that would work sufficiently well. However that the 6 beads I have on there now are not sufficient may explain why I had the impression that the beam seemed to flip around on some frequencies at home, and a bit hard to predict at the dike.
removing the resistor from the circuit, the result will be an antenna with a pattern similar to KK5JY SRL http://www.kk5jy.net/rx-loop while opening the loop will result in a donut pattern similar to the one from a vertical antenna, the idea isn't bad at all and would give 3 antennas in one !The only thing that looks like a worthwhile variation (to me) is opening the loop circuit (R=infinity) to create a nice round and flat omnidirectional donut shaped pattern around the antenna, with much increased gain. This could be done by integrating a small SPDT into the support structure or small box, so the resistor can be switched in and out without much wiring hassle and disturbing the shape. A third switch position connecting the loop wire ends directly would change the omni pattern to a somewhat more oblong shape including the near-vertical incidence signals.
Well, sounds like we found a way to use those 90° adaptersOh well, time to get the antenna out of the trunk to change the feedline position. Looks like I didn't buy these 90° SMA bits in vain!![]()
yes, gain seems to be a bit higher, and front-to-back much better, as for bands, it may be possible to optimize the antenna for lower frequencies by increasing the size and, probably, adjusting the height of the antenna from ground, just tried raising the side to 2m (ok, probably too much) and raising the antenna to 5m and obtained a "gain" of -48dB on the 160m band, while the 76cm side version has a "gain" of -64dB down there and -42dB on 40 meters, but before going for such a mod, I think exploring the performances with the modified feedline and doing that even down in frequency could be useful to decide "where to go next"; also, regarding gain, an alternative may be trying a higher gain preamp for the lower bands, there are some 30dB or more LNAs around, but then I'm not sure they may be worthEdit: Playing with the new .nec file and comparing that to the old one it looks like this even has 3dB more gain on 17m and maybe more above. The pattern really looks better and it retains more of a minimum (to avoid the term "null") on the upper bands (which I think is where this design is likely to shine most, also considering the upcoming solar maximum!).