https://qrper.com/2024/09/jeffs-backcou ... ctivation/
looking at the contacts map and the bands used I started thinking and, to verify that my thoughts were correct I then fired up the free 4NEC2 modeler
https://www.qsl.net/4nec2/
and went on creating a simple model for a vertical EFHW for the 20m band, after running the model on 20, 17, 15 and 10 meters bands the results, which can be viewed in the attached image confirmed that my thoughts were correct, in short...
When the top of the antenna starts being near to 1 Lambda, the radiation pattern raises toward the sky and while the resulting radiation may be ok for lower frequencies, for example the 30 or 40 meters band, due to the ionosphere reflection and the so-called NVIS radiation, this isn't true when it comes to higher frequencies which will just be absorbed or pass through the ionosphere, and that explains why the contacts on 17/15/10 meters, as reported in the QRPer blog post above, weren't so much and were mostly at short distances
[edit]
Notice that the graphs below won't apply to "trapped" or "collinears" but only to vanilla EFHW radiators laid out as vertical antennas, I briely explained the reason for such a behavior in the above text, but in case someone wants to fathom it, I suggest reading this
https://practicalantennas.com/designs/v ... s/5eights/
while it focus on comparing heights (apex ones), the base concept remains the same, whenever a vertical antenna apex gets near to 1 Lambda the takeoff angle, that is the angle of max radiation will raise toward the sky
[edit #2]
forgot, if someone wants to play with 4NEC2, the model I used to run my quick check is below, just save it as "EFHW20_VERTICAL.nec" and load it into 4NEC2, set the impedance to 2450 and run the model, and you'll see the result and be able to fiddle with the model or change the frequency to set the fundamental resonation and size to the proper value (in this latter case you may want to adjust the shortening factor "dfac") but keeping the size as is and changing frequency will show what reported below
Code: Select all
CM vertical EFHW 20m band
CE
SY freq=14.000
SY wave=(300/freq)
SY dfac=0.9652
SY lfac=0.50
SY leng=(wave*dfac)*lfac
SY cfac=0.0625
SY cpse=(wave*cfac)
SY base=(cpse*2)
SY htop=leng+base
SY hbot=base-cpse
SY wire=0.00075
SY segl=31
SY segm=11
SY segs=3
GW 1 segl 0 0 base 0 0 htop wire
GW 2 segs 0 0 base 0 0 hbot wire
GE 1
GN 2 0 0 0 13 0.005
LD 7 0 0 0 2.1 wire
LD 5 0 0 0 58000000
EK
EX 0 1 1 0 1.0 0 0
FR 0 0 0 0 freq 0
EN