24-08-2024, 07:27 AM
I had a thought about why the OCF legs are appearing to be far too long.
The original overall length (OAL) was to be 20.8m and was cut at a bit over 21m for headroom. Currently the OAL is less than 20m and it will be closer to 18m if I fold or cut it according to the M0MCX calculation. Which seems far too short for the OAL for a 40m dipole.
Below is a rough sketch of the layout. Forgive the diagram, you wouldn't think I am an illustrator by trade, but I don't have any real tools here...
It has a 4:1 balun at the dipole's division, coax down to the cable entry where there is a choke (coax on a toroid) some more coax to another choke at where it would connect to the tuner, but in this case it connects to the NanoVNA.
In this arrangement I suspect it is actually behaving like a Windom style antenna rather than a dipole. I think the coax between balun and entry choke is forming part of the antenna.
The next test will be to move the entry choke to next to the balun and do a sweep and see where the lowest VSWR/Resonance ends up. Happily, to do that doesn't require a ladder, the design using pulleys and halyards allows me to lower everything to ground level)
If the lowest VSWR point moves higher in frequency then, as Adam Savage was wont to say "Well there's your problem...".
(Forgive me if my thinking out loud here is annoying).
The original overall length (OAL) was to be 20.8m and was cut at a bit over 21m for headroom. Currently the OAL is less than 20m and it will be closer to 18m if I fold or cut it according to the M0MCX calculation. Which seems far too short for the OAL for a 40m dipole.
Below is a rough sketch of the layout. Forgive the diagram, you wouldn't think I am an illustrator by trade, but I don't have any real tools here...
It has a 4:1 balun at the dipole's division, coax down to the cable entry where there is a choke (coax on a toroid) some more coax to another choke at where it would connect to the tuner, but in this case it connects to the NanoVNA.
In this arrangement I suspect it is actually behaving like a Windom style antenna rather than a dipole. I think the coax between balun and entry choke is forming part of the antenna.
The next test will be to move the entry choke to next to the balun and do a sweep and see where the lowest VSWR/Resonance ends up. Happily, to do that doesn't require a ladder, the design using pulleys and halyards allows me to lower everything to ground level)
If the lowest VSWR point moves higher in frequency then, as Adam Savage was wont to say "Well there's your problem...".
(Forgive me if my thinking out loud here is annoying).
Colin
Barossa Valley, SAP. PF95ln
(aka VK5CSW)
Barossa Valley, SAP. PF95ln
(aka VK5CSW)