Richard Chism 2022/07/04 11:27
Ed,
That is VERY good to hear. Now all I have to do is take a couple of
feet off of the leg that I suspect is longer in order to optimize it for
the frequency range I use most often, and if I'm wrong it sounds like it
won't matter. Yes, it is a center fed (approximately) half wave dipole
cut for 75 meters, so the percentage difference won't be much. Maybe 4%
at most. Thank you. That simplifies everything.
73 de Rick AI7JN
On 7/4/2022 11:08 AM, djed1@aol.com wrote:
> Hi Richard-
>
> here's some advice from an old antenna engineer- you may already know
> all this but I hope it may prove useful.
>
> I assume you are discussing a half-wave dipole fed at the middle. As
> has been discussed, it is difficult to make a good measurement of an
> end-fed quarter wave because you don't know what the VNA sees as the
> counterpoise for the wire. I suggest you not worry about the dipole
> balance, and I assume you cut the two halves to be about the same. If
> not, read on.
>
> The impedance match of any antenna is not very important if it is
> below 2:1 SWR, because the power lost is about 1/2 dB, which is
> virtually undetectable in communications. My only exception to this
> is that some modern transceivers want to see 1.5:1 SWR for full power
> out. If you are getting 1.5 or less, you really don't need to do better.
>
> If you need to do better, here's how the wire affects the impedance.
> For the half-wave dipole, the resistance will be around 50 to 100 ohms
> at the center. If you move the feedpoint off center, the resistance
> will increase slowly until you get near the ends, and at the end you
> will have a resistance of thousands of ohms. So if your feedpoint is
> off by a few percent, it will make no practical difference.
>
> The reactance is largely determined by the overall length of the
> dipole, so trimming either end to get the reactance to near zero will
> work, even if the two halves are unequal.
>
> An example of this is the off-center fed dipole, where the two halves
> are unbalanced by 50% or so. The resistance is higher, but the
> resonant length is about the same.
>
> We engineers often have to decide what is good enough.
>
> 73 Ed W2EMN
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