Originally Posted by
Jofm5
Are the dimensions your checking the electical dimensions or the physical dimensions ? The optimum aerial will be equal to the wavelength electrically or a dimenion thereof - e.g. a quarter wavelength will work much better than a misc length - some aerial manufacturers will "load" the antenna by adding a coil to make up the electrical length. In a permanent fixture you will adjust by physically cutting the antenna to length based upon SWR value.
'Tis the electrical dimension. It's a half-wave dipole (two x ¼-wave lengths of wire). All the dipoles on there are the same. I did the calculations very carefully when I cut the wire and installed it. All the others matched spot on, first time.
I'm wondering if there's some strange harmonic relationship between the 18MHz dipole and one of the others, but I can't work out what that would be. I know that 21MHz signals react "interestingly" with 7 MHz antennas, but I can't see any such relationship between the ones I have on that feeder.
If I recall correctly a SWR of 5:1 is totally unacceptable and my meter only ever went up to 5....
5:1 is certainly not good - certainly for a 50 ohm dipole that should be resonant. However, my meter goes all the way to infinity.
Typically if your SWR is incorrect you have an insufficient ground plane (your signal is being reflected back to itself) - try placing some sheet metal under the dipole and grounding the outer brade of the coax from 18mhz dipole to that - for best results again you will want a factor of the wave length on your ground plane for what your trying to balance.
I don't think so. There shouldn't be a ground plane under a dipole. In theory the dipole should be in "free space" with nothing else around it.
Edited to add an inline amplifier after the SWR meter will distort your readings so balance it before adding an amplifier.
Not much point in putting an SWR bridge between the exciter and the amplifier, surely? If you designed and built them correctly, then the matching should be OK. Otherwise, it will show the mismatch at 50 ohms between the output of the exciter and the input circuit of the amplifier.
This reading is the SWR on the coax feedline to the dipoles. After the SWR bridge comes coax, then the balun, then the dipoles. All bands except 18MHz show 1:1 or close.
Meanwhile, I just don't operate on 18MHz.