Jim Lux 2023/04/11 13:29
On 4/11/23 8:23 AM, Anne Ranch wrote:
> In my opinion - "not to worry about (whatever)..." should not be the
> objective of amateur radio enthusiast - quite a contrary. Being
> satisfied / impressed by minimal knowledge or just plain "here say"
> about the subject is what is dumming this fascinating hobby. It is not
> the internet or cheap HT -
> "we have met the enemy and it is us ".
That's hardly what I wrote. It is the observation from decades of
experience designing, modeling, building, and using antennas in a
variety environments, both terrestrial and in space.
For antennas that are low gain (doublets and the like), the physical
configuration of the antenna, in particular, the feed point impedance,
does not change the *efficiency* of the antenna unless it's extreme.
Whether the antenna is resonant at the operating frequency doesn't make
much difference *as a radiator* - it might make a difference for match
in a particular system, it might not.
Sure, the pattern might be perturbed a small amount. Or you might have
untoward interactions with the environment or feedlines, and the
measured Z might give an indication of that, but then, it might not.
Sometimes people get wrapped up in minutia that have small effect on the
performance (however measured) for simple antennas: dipoles, loops,
monopoles.
Where it gets trickier is when you have things like a Yagi - where
there's significant interaction among elements, and the mutual Z and
physical configuration has a large influence on the pattern
(particularly nulls and back/side lobes). And even there, you can spend
a lot of time on unnecessary precision in some aspect, when other
aspects dominate. How many modelers model the droop in a yagi element?
What's the effect of mis alignment? Or bending of the elements in the wind?
Now, it is fair that since amateur radio is by definition unpaid, it
doesn't hurt you if you want to dive down multiple rabbit holes. One
might learn something unexpected, or at least, unanticipated by
conventional models. And there's a whole world of mechanical engineering
aspects to antennas - finding inexpensive compromises is the name of the
game.
The OP was asking about performance - I interpreted that as "antenna
efficiency" - They could just have well be asking about withstanding
wind or ice, about which I am substantially less well qualified to comment.
In particular, the OP was talking about matching to 50 ohms. As it
happens, equal leg lengths would be the best match - The feed point
impedance is around 70 ohms. In all other positions for the feed point
impedance is higher.
However, there was another comment about angle - and that is probably a
really effective way to adjust the impedance lower. But then, does it
make a difference? And those are all for "ideal doublet in free space".
Again, one shouldn't be optimizing the fine details when the gross
things are dominant. And more to the point, one should learn how to
tell the difference. It's not always easy... that's why I love modern
modeling tools that are fast - you can try a bunch of weird things
quickly, without having to "commit to a build", and you can tell what
makes a big difference, and what makes a small difference.
>
> On Tue, Apr 11, 2023 at 9:59 AM Jim Lux <jimlux@earthlink.net
> <mailto:jimlux@earthlink.net>> wrote:
>
> On 4/10/23 7:49 PM, W4JDY wrote:
> > And depends on the legs angle apart too.
> >
> > *From:* NanoVNAV2@groups.io <mailto:NanoVNAV2@groups.io>
> <NanoVNAV2@groups.io <mailto:NanoVNAV2@groups.io>> *On Behalf Of
> *Richard
> > Chism
> > *Sent:* Monday, July 4, 2022 10:16 AM
> > *To:* NanoVNAV2@groups.io <mailto:NanoVNAV2@groups.io>
> > *Subject:* Re: [nanovnav2] Balancing dipole leg lengths
> >
> > I just want the optimum performance from the antenna, and since I am
> > using a 50 ohm balun I assume the best match would be when the
> legs are
> > equal length.
> >
> >
>
> Everything affects the feedpoint impedance. But a lot of that has very
> small effect on the antenna performance/efficiency, in either a
> "effective aperture vs power presented at the feedpoint" for receive or
> "radiated power for power supplied by transmitter" sense.
>
> What does change is things like sensitivity to nearby (<1 wavelength)
> noise sources, coupling to other structures, and to a lesser extent,
> the
> radiation pattern.
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