Don Wilhelm <w3fpr@...>
Dale,toggle quoted messageShow quoted text
The best impedance for the 1/4 wave transmission line section should be 300 ohms.ï¿½ If you can find the definitive work of Gary O'Neil N3GO related to the J-pole, you can see the reason for that choice.ï¿½ Sorry, but I cannot locate that reference at the moment.
There are charts and formulas in the ARRL Antenna Book and Handbook for determining the spacing of the conductors in a transmission line given a particular diameter.
I don't know about perturbations using a 5/8 wavelength radiator, but I suspect they will buy you little and because the impedance at the base of the 5/8 wavelength will not be the highest impedance possible from the radiator, the transmission line section would have to be changed from a 1/4 wavelength to something else and another element (capacitor) would likely have to be used to cancel out the reactance.ï¿½ I just do not think it worth the effort for that extra 1 or 2 dB.
I would stick with a 1/2 wavelength radiator and 1/4 wavelength transmission line section to keep the reactive element in check.
If you want to increase the gain, put an insulator at the 1/2 wavelength point of the radiator and insert a folded 1/2 wavelength of wire, then stack another 1/2 wavelength on top of that.ï¿½ You then have a co-linear J-pole.ï¿½ Practical for 2 meters, but may become physically too large on 6 meters.
I do have a co-linear J-pole on 2 meters constructed of 1/2 inch copper pipe. A co-linear should give you 3 dB of gain and bring your radiation pattern down to a stronger (but more flat) pattern in the horizontal direction.
When tuning a J-Pole, the method I use is to first adjust the 1/2 wavelength section to resonance (reactance equals zero on the antenna analyzer), and then adjust the tap to produce a 50 ohm match to the coax.ï¿½ If you do not have an antenna analyzer, borrow one.ï¿½ Trying to adjust both the radiator and the coax tap position using only lowest SWR can lead to confusion between the two and a poorly performing J-Pole.
Gary, N3GO is a local friend (as well as a superb engineer), and I will ask him where his paper on the J-pole can be found.ï¿½ If it is still available on the web, I will post the link here.
On 8/11/2015 7:16 PM, Dale Putnam daleputnam@... [4sqrp] wrote: