Re: The small SWL beam (1...30 MHz)
Posted: Thu Aug 04, 2022 7:38 am
Just for the sake of discussion, let's take the 40m band, now a Yagi-Uda antenna for that band would have two elements with a total length of about 20m and a boom of about 5m, such an antenna installed at 10m from ground will have the following radiation pattern
now let's look at the radiation pattern of the small loop, installed at an height of just 3m on the same band
if we let aside the gain, which for a receive only antenna isn't the main factor, as field tests of the loop (and tests with the LoG and other antennas) demonstrated, I believe that the very small loop offers some remarkable features, even if compared to a big and high Yagi beam, as an example, look at the lower angle of the beam or at the front to back ratio, the latter is about 9dB for the Yagi and a whooping 29dB for the small loop, while the lower radiation angle is 20° from the horizon for the Yagi and 10° for the loop, if one considers that the Yagi is up at 10m from ground while the loop is just at 3m the thing is pretty remarkable, also considering the fact that to lower the Yagi angle down to 10° we should raise it at 1/2 wave, which, on the 40m band means about 20m from ground
forgot, for the curious ones, the NEC model for the Yagi-Uda antenna is this one
the model is for a 3 elements antenna, willing to test a 2 elements one, just comment out the director line (or the reflector one, if you prefer), notice that the ideal height of the antenna should be about 1/2 wavelength (0.60 being a good pick), I used 1/4 lambda but that can be changed too in the NEC model, just consider that it would mean raising the antenna at around 20m from ground
now let's look at the radiation pattern of the small loop, installed at an height of just 3m on the same band
if we let aside the gain, which for a receive only antenna isn't the main factor, as field tests of the loop (and tests with the LoG and other antennas) demonstrated, I believe that the very small loop offers some remarkable features, even if compared to a big and high Yagi beam, as an example, look at the lower angle of the beam or at the front to back ratio, the latter is about 9dB for the Yagi and a whooping 29dB for the small loop, while the lower radiation angle is 20° from the horizon for the Yagi and 10° for the loop, if one considers that the Yagi is up at 10m from ground while the loop is just at 3m the thing is pretty remarkable, also considering the fact that to lower the Yagi angle down to 10° we should raise it at 1/2 wave, which, on the 40m band means about 20m from ground
forgot, for the curious ones, the NEC model for the Yagi-Uda antenna is this one
Code: Select all
CM ----------------
CM Yagi-Uda antenna
CM ----------------
CE
' symbols
SY freq=7.100 ' calc frequency
SY wave=(300/freq) ' wavelength
SY hght=(wave*0.25) ' height from ground
SY wire=0.0050 ' elements radius
SY lref=(wave*0.495)/2 ' reflector length
SY ldip=(wave*0.473)/2 ' dipole length
SY ldir=(wave*0.440)/2 ' director length
SY sref=(wave*0.114) ' reflector spacing (from 0.1 to 0.25)
SY sdir=(wave*0.206) ' director spacing (from 0.1 to 0.5)
SY segm=51 ' segments in model
' antenna geometry
GW 1 segm -ldip 0 hght ldip 0 hght wire ' dipole
GW 2 segm -lref -sref hght lref -sref hght wire ' reflector
GW 3 segm -ldir sdir hght ldir sdir hght wire ' director
' terrain (standard)
GE -1
GN 2 0 0 0 13 0.005
' loading (aluminium)
LD 5 0 0 0 37700000
' enable extended kernel for calc
EK
' feedpoint
EX 0 1 (segm/2) 0 1. 0 0
' test frequency
FR 0 0 0 0 freq 0
' end of model
EN