Quick question about wire dipoles

[Snoman314]

I've got a wire dipole for VHF that comes with insulated ends to which a line is tied to, and then hoisted up a tree. The wire elements are wrapped around the insulators to pack the thing away. What's more, the wire is marked out every foot, and it has a table printed out with the number of feet to unroll for a given frequency to make it a half wavelength dipole.
My question is this: Does that really make the antenna resonant? Or does the length of the wire need to be cut to the appropriate length?

[G6NHU]

Providing the wires are bare, what they suggest will be fine.

[Snoman314]

Because then they would short out to be the correct length? Is that what you mean?
As it is right now the wires are plastic-coated insulated wires along their whole length.

[KM3F]

Your asking for a simple answer to a complex question.
First, the length should be unrolled to 1/2 wave length at the operating frequency.
Next is the feedline coax, open wire or ladderline?
These different feedlines have different impedences.
Next, the height above ground affects the antenna to feedline impedence match.
Bottom line is there is only one set of conditions that results in a 1 to 1 match making the 1/2 wave antenna truely resonant at the desires frequency.
It is best to look at the 1/2 wave antenna in terms of current and voltage distributuon.
The current is max at the feedpoint and min at the ends. This is why the feed point impedence is low.
The voltage distribution is just the reverse. The voltage is highest at the ends so makes the impedence very high on the ends and very sensitive to length changes and other effects.
To go further, the difference in bare or insulation is a matter of convience.
1. it affects the velocity of rf propagation on the wires such that it will affect the length.
2. it really makes 'little' dfference if the wire is bare and rolled up or insulated and rolled up since you can adjust the length to account.
As a last note, you don't even need an insulator on the ends if using rope or some other non conducting support. But it does affect length a small amount because of the high impedence at those points as mentioned above.
For those who doubt this, I have my Fan dipole suspended by rope only and work 75 meter dx into western Europe on a nightly basis in the US DX window with 10 over 9 reports.
Good luck.

[Snoman314]

Thanks for that info KM3F. I had developed the suspicion that insulated rolled up wire would be the same electrically as unrolled insulated wire.
In case it makes a difference, this antenna uses standard 50 ohm coaxial cable and connectors, and the feed point is right in the middle. I'm 90% sure that the transmitter is expecting 50ohms as well.

The reason I was wondering was because this antenna gets used for short periods, all over the place, for a range of frequencies (one at a time) from 30 - 54 MHz, and rolled out to length every time it's deployed. I was concerned that using f/2c to get the length to roll it out to might not be making much difference, if rolling and unrolling wasn't going to affect the resonant frequency, due to the insulation.

I take your point that the voltage would be highest at the ends, although if I remember correctly, inductance would be the dominating effect from rolling up the (insulated) wire, but inductance is a product of current right?

I'm aware it's a complex issue. I was in a hurry when I first asked the question, but now that the situation I was concerned about optimizing has passed, I'd still like to be able to know if I can make improvements for the future. So feel free to lay it on.
You mentioned that there would be some degree of length adjustment to account for insulated/non-insulated rolled up wire. Is this difference significant?

[K7KBN]

Get the antenna opened to ~ the desired frequency and somewhere close to its intended height.
Check the SWR at that frequency and a few KHz lower and higher.
Adjust length for best results at the desired frequency.

What bands are you using between 30 and 54 MHz, just out of curiosity...?