Good morning,
I'm sorry to disturb you, but I need your skill.
I’m french, sorry for my bad English
I have a TH3 Junior beam with BN86 balun.
It is 3.5m above my roof.
It was set to PHONE mode, with acceptable SWR on all three bands.
I had to modify the recommended dimensions because the beam is not very high.
In addition, the steel rail ceiling support create an artificial floor !
After more than twenty years of good service, I dismantled it for repair and replacement of all the screws...
Assembly in CW mode with the recommended dimensions.
I get the following results.
On 28,100 Mhz: SWR 2.5 - On 21,100 Mhz: SWR >3 - On 14,100 Mhz: SWR 2.7
The NanoVNA antenna analyzer indicates resonant frequencies: 27,500 - 20,500 - 13,750
The antenna is physically too long.
I made a rough calculation, taking into account the wavelength and physical length of the trap elements and windings.
I found a ratio of 4.8 for 28 MHz, 4.02 for 21 MHz and 3.32 for 14 MHz.
I must remove 4.6 cm for 28 Mhz, 4.48 cm for 21 Mhz and add 3.31 cm for 14 Mhz.
This is of course theoretical, but it gives an indication…
Finally, here is my question:
Since the reflector and the director do not have the same physical lengths as the dipole, should I apply new ratios specific to these two elements?
Attached is the documentation.
I thank you in advance for your appreciated help.
Sincerely. 73 Gilles F6GYF
I'm sorry to disturb you, but I need your skill.
I’m french, sorry for my bad English
I have a TH3 Junior beam with BN86 balun.
It is 3.5m above my roof.
It was set to PHONE mode, with acceptable SWR on all three bands.
I had to modify the recommended dimensions because the beam is not very high.
In addition, the steel rail ceiling support create an artificial floor !
After more than twenty years of good service, I dismantled it for repair and replacement of all the screws...
Assembly in CW mode with the recommended dimensions.
I get the following results.
On 28,100 Mhz: SWR 2.5 - On 21,100 Mhz: SWR >3 - On 14,100 Mhz: SWR 2.7
The NanoVNA antenna analyzer indicates resonant frequencies: 27,500 - 20,500 - 13,750
The antenna is physically too long.
I made a rough calculation, taking into account the wavelength and physical length of the trap elements and windings.
I found a ratio of 4.8 for 28 MHz, 4.02 for 21 MHz and 3.32 for 14 MHz.
I must remove 4.6 cm for 28 Mhz, 4.48 cm for 21 Mhz and add 3.31 cm for 14 Mhz.
This is of course theoretical, but it gives an indication…
Finally, here is my question:
Since the reflector and the director do not have the same physical lengths as the dipole, should I apply new ratios specific to these two elements?
Attached is the documentation.
I thank you in advance for your appreciated help.
Sincerely. 73 Gilles F6GYF
