Rather than get lost in the loop on ground thread, I thought I'd try to keep it short and under control here. Thanks to KK5JY for sending me on this crazy journey in the first place.
A dipole-on-ground, or "dog", will have a lower resonant frequency than up in the air. Both EZnec and my own ground conditions have revealed this easy to remember formula - which is ballpark, since ground conditions differ at various locations:
365 / f Mhz = half-wave length in feet. Do NOT confuse this with the typical "in the air" formula using 468 !
Matching: For single band use, 50/75 ohm coax is fine, although 75-100 ohms is preferable.
This seems counterintuitive as we know when we bring a horizontal antenna closer to ground, the feedpoint impedance drops. It does but not to zero, -- once you get *below* 1/10th wavelength above ground, the feedpoint impedance starts to rise again, mostly dominated by the ground-loss resistance. Very handy. (Thanks to Les Moxon, G6XN's book HF Antennas For All Locations". (P115, fig 6.17)
As always, try to prevent any pattern distortion, by using a physical / galvanically isolated 1:1 balun, and a nice common-mode-choke at the feedpoint to keep the common mode of the transmission line from distorting it. Or for multiband use, a 9:1 ratio. Preamp? Up to you. Just know that when you start to go higher in frequency than what the dipole is cut for, the pattern distortion generated by the antenna itself is usually undesirable, never mind the problem of keeping the common-mode of the transmission line in check! In my world, I need BOTH galvanic isolation and rf-choking.
(More advanced users will want to try EZnec to model the pattern - maybe they *would* like what they see, like at 4x the frequency the dipole is cut for etc etc)
When I discovered the easy to remember mnemonic value of 365/f mhz = dipole length in feet works well for the on-ground dipole, thought I'd mention it. Insulated wires and connections of course.
A dipole-on-ground, or "dog", will have a lower resonant frequency than up in the air. Both EZnec and my own ground conditions have revealed this easy to remember formula - which is ballpark, since ground conditions differ at various locations:
365 / f Mhz = half-wave length in feet. Do NOT confuse this with the typical "in the air" formula using 468 !
Matching: For single band use, 50/75 ohm coax is fine, although 75-100 ohms is preferable.
This seems counterintuitive as we know when we bring a horizontal antenna closer to ground, the feedpoint impedance drops. It does but not to zero, -- once you get *below* 1/10th wavelength above ground, the feedpoint impedance starts to rise again, mostly dominated by the ground-loss resistance. Very handy. (Thanks to Les Moxon, G6XN's book HF Antennas For All Locations". (P115, fig 6.17)
As always, try to prevent any pattern distortion, by using a physical / galvanically isolated 1:1 balun, and a nice common-mode-choke at the feedpoint to keep the common mode of the transmission line from distorting it. Or for multiband use, a 9:1 ratio. Preamp? Up to you. Just know that when you start to go higher in frequency than what the dipole is cut for, the pattern distortion generated by the antenna itself is usually undesirable, never mind the problem of keeping the common-mode of the transmission line in check! In my world, I need BOTH galvanic isolation and rf-choking.
(More advanced users will want to try EZnec to model the pattern - maybe they *would* like what they see, like at 4x the frequency the dipole is cut for etc etc)
When I discovered the easy to remember mnemonic value of 365/f mhz = dipole length in feet works well for the on-ground dipole, thought I'd mention it. Insulated wires and connections of course.
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