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Diane Bruce VA3DB

What saddens me is so many hams think a dipole is best because it presents a "perfect match" of 50ohms. Wrong. A dipole is roughly 72 ohms if you are lucky and wildly different depending on height and structures nearby.
Yagi antennas are even worse depending on the design and often need an antenna tuner hidden at the antenna. Get the lowest loss feedline you can use and don't feel guilty about the tuner in the shack.

@va3db Well, if you have 72 ohm coax and a tube radio it's perfect.. works great for me! 🤪

@ai6yr tube radios typically, since TVI became a problem at least, have used a pi-network. A pi-network is a tuner.

@va3db

As with almost everything in radio, it depends. See this chart from the RCA Radiotron Designer's Handbook. The impedance is dependent on height, among other things. I could see in many installations dipoles of certain bands being put up at the 50 ohm height by pure dumb luck.

@va3db this is one of the reasons I love the fact that you can often get large lengths of stuff like RG11 for free from spool ends from cable installers. It’s almost as low loss as LMR400, it matches the dipole better, and the tuner in the rig can easily handle the 1.5:1 match

@N3VEM We used to be able to get hardline from cable companies here in Ottawa! But yes.

@va3db

I suspect that misconception comes from the center feed point being hung from something convenient, e.g. a tree, and the ends of the elements also being strung to something convenient, e.g. ropes/strings to stakes in the ground.

That is, an Inverted Vee with approximately 45-degree slopes on both ends.

Depending on height and surrounding flotsam, that's likely to be pretty close to 50 ohms impedance.

It's the simplest dipole, with only one high thing.

@kelvin0mql @va3db My 160m dipole was at 50ft and was about 22 ohms. Ground proximity really matters a lot at lower freqs. Once you get above a few wavelengths the feedpoint R does start to approach 70 ohms, but only above 180ft ish on 14 MHz. Add in losses and clutter absorption, and 50 ohms is about right at "normal" heights, at least on the high HF bands. I use open-wire feeder to a link-coupled balanced tuner at ground level below my multiband "dipole" and don't care about the impedance!

@kelvin0mql @va3db Of course, the design of the coupling networks is important to keep the loaded Q under control and the efficiency as high as possible so I'm not burning RF power heating up the coax run from the shack to the matching unit. It's all built for 10 kilowatts+ even though I only operate at 5 watts on HF these days. I want every one of those watts to reach the far field unimpeded!

@g4dbn @kelvin0mql It's fascinating to see the large capacitors on early spark transmitters for that reason.

@g4dbn @kelvin0mql I'm a great believer in open wire so we are furiously agreeing.

@va3db @kelvin0mql It's a bit impractical to use LDF6-50 to feed a dipole, although it might be possible to guy it and use it instead of a mast. Open wire is SO easy with 3D printable insulators these days