01-06-2024, 09:37 AM
Colin
Unless you have a fairly significant coupling factor between one leg (or both) to the coaxial feeder then the nanoVNA will not really show any significant difference between the no-balun or a balun fed dipole. It is showing the effective load impedance (R/L/C) at the test end of the cable, not how that impedance is achieved at the far end.
The style of balun is unimportant in your case.
It could be an approx 1.5 metre sleeve of conductor (eg 1/4 wave of RG213 braid) over the outside of your existing coax feeder, finishing just short of the feed point and insulated from the braid from that point and connected to the feeder braid outer that 1/4 wavelength away. Messy to achieve but would do what you need.
A 1/4 wavelength of the main feeder coiled up as an air-cored coil just before the feed point is just as effective and easier to achieve. Use a bit of 40-50mm PVC plumbing pipe as a former if you need to get the shape and hot glue to its adjacent turn then heatshrink the whole lot to maintain shape and the PVC former can be removed - or left in place.. Your choice.
The same criteria applies though - minimise the coupling between either arm of the dipole to the coax feeder by perpendicular positioning of the feeder, balun or no balun.
Coupling between the feeder and the dipole will tend to skew the polar pattern more than anything else, but with a dipole that is not a major concern.
73 Doug
Unless you have a fairly significant coupling factor between one leg (or both) to the coaxial feeder then the nanoVNA will not really show any significant difference between the no-balun or a balun fed dipole. It is showing the effective load impedance (R/L/C) at the test end of the cable, not how that impedance is achieved at the far end.
The style of balun is unimportant in your case.
It could be an approx 1.5 metre sleeve of conductor (eg 1/4 wave of RG213 braid) over the outside of your existing coax feeder, finishing just short of the feed point and insulated from the braid from that point and connected to the feeder braid outer that 1/4 wavelength away. Messy to achieve but would do what you need.
A 1/4 wavelength of the main feeder coiled up as an air-cored coil just before the feed point is just as effective and easier to achieve. Use a bit of 40-50mm PVC plumbing pipe as a former if you need to get the shape and hot glue to its adjacent turn then heatshrink the whole lot to maintain shape and the PVC former can be removed - or left in place.. Your choice.
The same criteria applies though - minimise the coupling between either arm of the dipole to the coax feeder by perpendicular positioning of the feeder, balun or no balun.
Coupling between the feeder and the dipole will tend to skew the polar pattern more than anything else, but with a dipole that is not a major concern.
73 Doug
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.