Need clarification on RF related concepts

Could some electronics guru please help ? I am a bit rusty on some RF concepts so I request some clarifications from the gurus here.

By definition, a 4:1 balun has 4 times the impedance on the balamced side as the unbalanced side. For any inductor, the impedance is based on the reactance, which in turn is based on the frequency of operation.

So, what frequency is being considered here ? The reason I ask is that a balun could be used as broadband impedance matching device. Thanks in advance for your help.

Reply to
dakupoto
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It is my understanding that a balun is a transformer. In order for a transformer to transform properly it needs to have at least 4 times the inductive reactance of the impedance it is transforming. Example: The primary of a 50 ohm transformer must have at least 200 ohms of reactance at the lowest frequency of operation. As you go lower in frequency. at some point the reactance becomes less than 200 ohms and the transformer starts to become less ideal in transforming the impedances. So build your balun with enough impeadance to cover your lowest frequency and it will be broadband, (up to a point). Mikek

Reply to
amdx

A balun is a matching element between a balanced and an unbalanced transmission line. The 4:1 balun is in its simplest form an auto-transformer (2 identical windings connected in series). The center tap is connected to the unbalanced side ground, one of the ends is connected to both the unbalanced non-grounded wire and one of the balanced wires. The remaining end of the transformer is connected to the remaining balanced line wire.

There can be an impedance transformation of 4:1 using a similar transformer, but connecting the center tap to the unbalanced non-grounded line and connecting the common end of the transformer to ground wires of both low-impedance and high-impedance line. The non-grounded high-impedance line it taken from the top end of the auto-transformer.

A similar effect can be achieved using transmission lines. If we have a feed of impedance Z0, we can connect it to a pair of lines of impedance 2*Z0 in parallel. If the other ends of the lines are connected in series, there will be a connection of impedance 4*Z0, but there is a requirement to prevent current flow outside the transmission lines, to prevent part of the circuit being short-circuited. This can be achieved by coiling the lines on a suitable magnetic core, creating an auto-transformer on low frequencies and a transmission-line transformer on high frequencies.

Google gave pretty good responses to 'trnasmission-line transformer'.

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Reply to
Tauno Voipio

As mentioned, it's a transformer. If it's in it's normal operating band of frequencies, then the leakage inductance of the transformer is negligible, and what the driver "sees" is largely 4x the impedance of the load.

Note, too, that "negligible" covers a much wider impedance variation than most other sub-disciplines can stand, unless you're trying to make a power combiner or some such.

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Reply to
Tim Wescott

Thanks to each of you for the insightful comments and jogging my memory. I am sorry I must have posted on the day when Phil's constipation was at its worst.

Reply to
dakupoto

The impedance of a transformer is not the inductive reactance of the coils in the transformer. A transformer reflects the circuit connected to the secondary to the primary according to the turns ratio. The voltage and current scales by the turns ratio and the impedance scales by the turns ratio squared.

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Rick C
Reply to
rickman

A balun is a transformer, a 4:1 balun is a 2:1 ratio transformer.

(this suprising relation can be derived from P=I squared R)

Useful frequency depends on the design of the transformer. 4:1 baluns good* for 45 to 900 MHZ operations were common in the days of analogue broadcast television.

(*) good-enough anyway.

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Reply to
Jasen Betts

Of an ideal transformer, yes, but those aren't made with coils!

All real transformers are transmission line transformers, and obey impedance matching rules, whether you like it or not. Some are even constructed with transmission line theory in mind, and attain stupendous bandwidths as a result. The best part is they're so easy to design.

A conventional (lumped equivalent) transformer cannot do flat bandwidth out beyond, say, 90 degrees phase shift. An ideal TLTF (again not physical, but a real one can get much closer) keeps on working, despite thousands of degrees phase shift! :-D

Tim

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Reply to
Tim Williams

BTW, if you want a book that covers Baluns, Ununs and Transmision Line transformers with dozens of built examples including frequency response and loss curves, I recommend, 'Transmision Line transformers' by Jerry Sevick. Also includes many different ratio transformers. Mikek

Reply to
amdx

I'm always on the look-out for stupendous-bandwidth transformers. How wide is that? I'm aware of Sevick's work: Roughly 3 decades of bandwidth. I want more!

I've managed to make inverting transformers, 1:4 transformers and hybrids with 5 decades of bandwidth. These are made of bits of straight UT-47 coax strung with Vacuumschmelze's tiny amorphous tape-wound ISDN transformer toroid cores. Amazing things, these little cores, though not without quirks.

I'd love to find a way to make similar performance 1:2 transformers, but my attempts have been disappointing up to present. The limiting factors are the unwanted capacitance to coax screens and the residual inductance of junctions. The end of the manufacture of my favourite Vacuumschmelze transformer cores has set me back by at least an octave, alas. I now wish I'd bought more of them...

Jeroen Belleman

Reply to
Jeroen Belleman

+1. Great book.

Cheers

Phil Hobbs

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Phil Hobbs

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Reply to
Michael A. Terrell

Ahh.. thanks. I found this from Amidon.

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(rather large.)

I think I'll ask Santa for one of those book reader things.

George H.

Reply to
George Herold

I've been looking for that for a long time, since I lost a lot of hardcopies of ARRL stuff during a move. Thanks.

Mark L. Fergerson

Reply to
Alien8752

You're welcome.

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Reply to
Michael A. Terrell

You're welcome.

I bought an Irulu X10, 10" tablet w/octal processor (UltraOcta-A83)for $100 a year ago for e-books. it has 16 GB of flash, and Bluetooth. It runs Android 4.4.4.

This looks like it's the current version:

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(iRULU-X2-Plus-10-1-Google-Android-5-1-Lollipop-Tablet-PC-Octa-Core-16GB-BT-4-0-)

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Reply to
Michael A. Terrell

The description says Android 5.1. Good price and nice CPU, but I'm concerned that 1GB working RAM is not enough for Android 5.1. I have several lesser phones and tablets floating around the office with 1GB RAM. They tend to be rather slow and jerky. My collection is the result of customer upgrades to tablets with 2GB RAM. Does the 8 core CPU compensate adequately for the lack of RAM or is it slow and jerky?

Also, the 1024x600 TFT screen is rather coarse, but probably adequate for ebooks, PDF, and document reading. The low res screen is probably why the price is so low. A higher resolution screen would be better, but also much more expensive.

My current favorite and what I use is a Google Nexus 7 2013 (2nd generation) tablet. About $130-$175 used on eBay:

1920x1200 IPS (not TFT) screen and 2GB RAM with Android 6.0. Make sure you get a 2amp, not 1amp charger.

Incidentally, in anticipation of Android and application bloat, I bought the 32GB internal storage version instead of the 16GB version. I have not regretted the decision. My 8GB devices are currently a constant battle to clear out storage, with my 16GB devices rapidly approaching where they will have the same problem. The Google Nexus 7 tablet does NOT have a Micro-USB slot and requires a USB-OTG adapter to read an external flash drive. This has turned out to be a rather clumsy way to expanding storage. For my collection of eBooks and PDF's, it's not a problem, but I also like to view movies, which average 500MB/each and which rapidly deplete available storage.

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150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I have used it to watch TV for hours at a time, with no problems. It sits on a stand, on top of a desktop computer, and under one of my 24" monitors so I can look over the top to use the computer, or down a few inches to see the tablet. I have a 32 GB micro SD card in it, full of old science fiction books.

I bought mine in August of last year. It replaced a similar quad core that I bought in 2013. It was Kocaso M9100 9", and it only has 8GB of flash with 1 GB of RAM. It would freeze on video, quite often, so I only use it to check Email, or to use with the Magic Jack app, over my wireless router.

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Reply to
Michael A. Terrell

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