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OCF: Proprer type of balun (transformer)
On 15 f v, 19:22, Roy Lewallen wrote:
Sometimes the radiation from the feedline, mains wiring, and stuff in the shack can be beneficial -- it might, for instance, fill in deep nulls in the main antenna's pattern and result in a spectacular signal strength improvement in particular directions. I am slightly OT here, but the same argumentation could be used about non-resonant antennas like the G5RV. Yes, ATU needed, but L-type autotuners for example have very small insertion loss. I am wondering about the real advantage of a razor cut dipole (resonant anyway on a small region around a freq) versus a multi-band dipole using a length of feed line matching section like the G5RV or ZS6BKW/G0GSF Antenna System... 73 de Pierre VE2PID |
#12
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OCF: Proprer type of balun (transformer)
On Feb 16, 4:48*am, ve2pid wrote:
On 15 f v, 19:22, Roy Lewallen wrote: Sometimes the radiation from the feedline, mains wiring, and stuff in the shack can be beneficial -- it might, for instance, fill in deep nulls in the main antenna's pattern and result in a spectacular signal strength improvement in particular directions. I am slightly OT here, but the same argumentation could be used about non-resonant antennas like the G5RV. Yes, ATU needed, but L-type autotuners for example have very small insertion loss. I am wondering about the real advantage of a razor cut dipole (resonant anyway on a small region around a freq) versus a multi-band dipole using a length of feed line matching section like the G5RV or ZS6BKW/G0GSF Antenna System... 73 de Pierre VE2PID As far as I know, the key advantage of a resonant half-wave dipole is that its feedpoint impedance is a pure resistance, and generally low enough to provide a decent match to a coaxial feedline. The radiation pattern of a dipole changes very little with fairly large changes in frequency, until it becomes somewhat longer than one wavelength. Even then, it doesn't become inefficient; it just develops lots of lobes and nulls as its length in wavelengths increases (which may or may not be a disadvantage). Reading between the lines of what Roy wrote, a bit of feedline radiation will at least change where those lobes and nulls are. But even a relatively short antenna (e.g. a dipole that's only 0.1 wavelength long) can be quite efficient--the loss is typically mainly in the network (including the feedline) used to feed power to it. Another factor to consider for electrically short antenna is that you have to tune out a reactance that's large compared with the radiation resistance, and that makes them narrow-band if the matching network is efficient. You have to re-tune for even small changes in frequency. Cheers, Tom |
#13
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OCF: Proprer type of balun (transformer)
Something that hasn't been mentioned is that common-mode energy forms
into standing waves, e.g. on the outside braid of the coax. An isolator (choke), as exists on a Carolina Windom, may only change that point to a current minimum while simply moving the current maximum point to a different location. Has anyone actually measured the common- mode current on a Carolina Windom on all bands up and down the coax on the shack side of the isolator? -- 73, Cecil, w5dxp.com |
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