08-03-2022, 06:41 PM
Russ
The coax matching length is usually a quarter of a wavelength though sometimes they use odd multiples. Single quarter wave give wide bandwidth but multiples results is narrow bandwidth. I had a rather fat SMD made for 450 MHz that looked promising for 70cm but it was useless on anything other than the specific frequency it was made for. On dismantling it I found it was using a coax length something like seven quarter waves. This made manufacture easy as no inline coax joints were required. Reducing the matching length to a quarter wave made it extremely broad band.
I was told by a ham that worked at an major commercial antenna manufacturer that with a certain brand of RSG213 coax (not all work) it was possible to join a quarter wave section of desired coax impedance (125 for SMDs (RG63), 75 ohm for beams (RG11)) and pull this into the RG213 sheath and braid so as to take place of the existing RG213 inner. The result is that the coax still looks like RG213 but for a section of it is actually 100 or 75 ohm. You can only tell by carefully feeling down the coax until you find the connection between the two cable inners. You can be easily led to believe into thinking it is just RG213.
The coax inside the dipole acts as a balun.
73 Igor
The coax matching length is usually a quarter of a wavelength though sometimes they use odd multiples. Single quarter wave give wide bandwidth but multiples results is narrow bandwidth. I had a rather fat SMD made for 450 MHz that looked promising for 70cm but it was useless on anything other than the specific frequency it was made for. On dismantling it I found it was using a coax length something like seven quarter waves. This made manufacture easy as no inline coax joints were required. Reducing the matching length to a quarter wave made it extremely broad band.
I was told by a ham that worked at an major commercial antenna manufacturer that with a certain brand of RSG213 coax (not all work) it was possible to join a quarter wave section of desired coax impedance (125 for SMDs (RG63), 75 ohm for beams (RG11)) and pull this into the RG213 sheath and braid so as to take place of the existing RG213 inner. The result is that the coax still looks like RG213 but for a section of it is actually 100 or 75 ohm. You can only tell by carefully feeling down the coax until you find the connection between the two cable inners. You can be easily led to believe into thinking it is just RG213.
The coax inside the dipole acts as a balun.
73 Igor