Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2ZDE, May 24, 2011.
Sounds interesting , posting so learn more .
"Tom, I know about the Fresnel region and that it's been there since Before Christ, so that's fine What I was hoping you could clarify is why, when modelling a 7m tall vertical on 0.25 wavelength radials on 160m, the low angle gain is 6 or 7 dB LOWER than the same 7m tall vertical over radials just 4m (0.025 wavelength) long? "
Any answers anyone? Please.
Ho hum. I guess nobody knows!
I guess the only way to know for sure is by further practical tests and modelling. Looks like we may have to wait a while for both!
It's probably because you are using Software that uses NEC2 engine which does not accurately model radials low to the ground. I am guessing since you did not provide much detail...
Its likely to be due to shortcoming in the model.
One way to test is to move the entire antenna and radials up a few feet from the ground say in 1 ft increments to 10 feet and see what the results say.
You seem to want this to be like a telephone!!! Sorry I left the room.
It's pretty well known the models are most unreliable and fussy when dealing with wires very close to earth. Maybe you did something wrong with height, segments, or even number of radials. Using copper losses and no loading losses over average ground.......
If I model 60 radials 100 feet long on a 7 meter vertical it shows:
Gain at 5 degrees -5.68 dBi
Gain maximum -.82 dBi @ 25 deg
Changing it to 14 foot long radials I have:
Gain at 5 degrees -10.53 dBi
Max gain -5.65 dBi @ 25 degrees.
This is reasonably in line with RCA's measurements.
There are a million ways though to screw up a model when a wire is near earth.
Hi Harry. Thanks for the thought. I've just given that a go - elevating the antenna a metre at a time to 4 metres AGL, and still, with 0.25 wavelength radials the gain at low angles is still 3 to 4dB below that with 0.025 wavelength radials at low take off angles.
Hi Tom. Nah, I know it's not a telephone
I've only done the model with 4 radials - reason being, I don't think the average ham over here, on a small plot, who decides to buy a short vertical is going to have the inclination to lay out heaps and heaps of radials - maybe some will, but I best most won't. Is this likely to be the reason why I'm seeing what I'm seeing? Even when the antenna is well above ground, I'm getting similar results.
Any chance you could upload a copy of your model for me to have a look at?
It's got to be some problem with the model. Post it here and folks can opine...
Looks like you made a pretty good go of it Tom!