Dipole Length Weirdness

[ultravista]

Building a 6m dipole using fiberglass rod for stability - running wire on the rod. To start, cutting it for 50 MHz which is 4' 8''. At 4' 8'' per element, the lowest SWR/center frequency is < 47 Mhz.

Folding the wire back to 3' 11'' gives me resonance at 50.400. For VHF, 9 inches per element, or 18 inches today, is quite a lot.

Trying to figure out why 3' 11'' is resonant for 50 MHz and not the 'standard' length.

The rod is solid fiberglass, no metal, nothing conductive that I am aware of ...

 

[N1EXA]

Is the coax going directly to the elements or are you using a capacitor or a balun?
Sounds like a feed point impedance issue!

Id start off with 234/51.00 mhz x 12 = 55 inches on each side of a dipole

A grid dip meter with an impedance bridge would help seeing where this is 50 ohms and resonate than just hooking
it up to a radio and hoping for a low SWR. Heathkit makes a great dip meter and the AM-1 Bridge works great with it.

But feeding it direct is throwing unknowns into the formula.


Pete N1EXA

 

[ultravista]

Hi Pete, I am using a Rig Expert AA-55. I do not have an unun/balun @ the feedpoint. I would use an LDG 1:1 but they are only rated up to 30 MHz.

Ray W4RXK

 

[prcguy]

How long are the pig tails going from the coax to the elements? They will become part of the overall length of the dipole.

 

[ultravista]

prcguy, the pigtails are about 3 inches. I also accounted for this after the length was determined to be too long. As-is, at less than 4 feet, it is resonant below 50.x Mhz. With the wires folded back approximately 8-9 inches, it resonates, or shows lowest SWR at 50.3x which is where I want it for FT8. The fiberglass rods are having material impact on length. 6m is no popping here so I can't see how well it works. Waiting for the magic.

 

[N1EXA]

Hi Pete, I am using a Rig Expert AA-55. I do not have an unun/balun @ the feedpoint. I would use an LDG 1:1 but they are only rated up to 30 MHz.

Ray W4RXK

The AA-55 Will be just fine for your job.
Me personally I’d cut it to calculated length and then use an antenna tuner to equal out the impedance issue. See if it gets out that way.

Pete N1EXA

 

[prcguy]

prcguy, the pigtails are about 3 inches. I also accounted for this after the length was determined to be too long. As-is, at less than 4 feet, it is resonant below 50.x Mhz. With the wires folded back approximately 8-9 inches, it resonates, or shows lowest SWR at 50.3x which is where I want it for FT8. The fiberglass rods are having material impact on length. 6m is no popping here so I can't see how well it works. Waiting for the magic.

The fiberglass will affect the length slightly causing the resonant frequency to be lower, more so using bare wire than insulated but it should not be as severe as you’re seeing. If you can remove the wire from the fiberglass intact and measure the resonant frequency it can tell you if the fiberglass is the major problem or if there is something else going on. See if you can hang the dipole wires vertical with some string and in the clear to measure it.