Hello all,

This is my first high-precision RF design, and I am having real troubles with it. It would be really great if someone could lend a helping hand.

First of all, I am not able to understand the purpose of 50-ohm line feed and the matching circuit. We have plenty of antennae available these days, in whatever form factor one wants, and the good thing is that manufacturers design these antennae with standardized characteristic impedance (50-ohm is the most common, although 75-ohm isn't that rare). All the major manufacturers of such antennae emphasize the design of a proper line feed as well as a matching circuit, and that is what I am not able to digest. When we design a line feed with a characteristic impedance equivalent to that of the antenna, why do we need the matching circuit? Making transmission line characteristic impedance equal to the load impedance is sufficient to prevent any wave reflections, so what role does the matching circuit play?


Thanking in advance,
Abbas.

 

Hello,

Have a look at this page:
http://cablesondemandblog.com/wordp...ence-between-50-ohm-and-75-ohm-coaxial-cable/

Bertus

 

Hello,

Have a look at this page:
http://cablesondemandblog.com/wordp...ence-between-50-ohm-and-75-ohm-coaxial-cable/

Bertus

Thank you, bertus, for the response.

I didn't read the entire article, but by just reading the title question I get an impression that this isn't what I am asking.

I know the difference between the 50-ohm and 75-ohm probes, have a hint of their constructional differences, and already know which of the two is suitable for what type of signal. My question isn't about the difference between the 50-ohm and 75-ohm (or any other value) system.

I want to know the significance of the matching circuit, especially when the feed-line is appropriately designed.

If there is something I am missing, you are always welcome to point that out, or if you still want me to read it.

With best regards,
Abbas.

 

Hello,

Here is a page that is more dedicated on matching:
https://www.smeter.net/feeding/feed-lines-and-impedance-matching.php
There are a lot of links on that page on the matter.

Bertus

 

If the source, line, same Z as load I would agree, why the matching
network.

Regards, Dana.

 

Maybe the antenna is not 50 Ohms. None of mine are, that's why the matching network.
One is a Gamma match and the other is a 'T' match.

 

Usually the transmitter and antenna are not in the exact same physical space so a method of moving RF power from point A to point B while minimizing losses is needed. For this a properly terminated transmission line is used. While it's possible to design a line feed with a characteristic impedance equivalent to that of the antenna (that might have a complex impedance and/or higher/lower value) that would create a large number of needed transmission lines and require custom design work. Using standard values we can just peek at lookup tables or use simple equations for what we need most of the time (easier). One general rule of RF transmission is that 50 ohm lines permit greater power and 75 ohm lines have lower losses for equal lengths of X sized conductor transmission line. So, to maximize the signal for C type of antenna it's better to match the transmission line impedance from A (transmitter/receiver) to B (antenna match) and then match the possible complex impedance of the antenna C to the correct termination value of the transmission line at B.

This is not to say you need to match transmission line to transmit power, it just needs to handle the possible very high RF voltages that might present in a unmatched condition while being mated to a really good grounding system. We used a long wire rope antenna connected to the bare center connector of a high power ~100 foot helix cable at one end while the other end of the helix was connected to the antenna output of the 2-30 MHz match box.
http://kvantefir.com/assets/files/pdfdownloads/Andrew_cable.pdf

 

They have to match for best efficiency, the 50Ohm is just decided(an engineering decision). There is no physical explanation behind it.

 

... the 50Ohm is just decided(an engineering decision). There is no physical explanation behind it.

This is incorrect. There is a very good physical explanation behind it.
https://www.microwaves101.com/encyclopedias/coax-absolute-maximum-power-handling

 

Officially the RF engineers state there is no physical reason behind the 50Ohms, its picked for synchronization purposes only! Thats why now a days they have create 75 ohms and etc. because more is required.

 

Officially the RF engineers state there is no physical reason behind the 50Ohms, its picked for synchronization purposes only! Thats why now a days they have create 75 ohms and etc. because more is required.

Those RF 'engineers' officially don't know what they're talking about.

 

Thats the accepted in the engineering fields.

 

There a a practical reason for 50 ohms.

"A half wave center fed dipole antenna in free space has an impedance of 73 Ω, making it ideal to feed with 75 ohm feeder."

AND

"As can be seen from the impedance variation plot, the largest swings of impedance are seen when the dipole antenna is closest to the ground. It then closes in on the free space value. This means that the actual value for many HF dipoles will be relatively low as it is not possible to raise them very high in many cases. Feeding with 50Ω coaxial feeder is often a good compromise."

So that's why it's 50 ohms.

Matching is a different story. Matching is NOT necessary IF.........transmission line is low loss. and can take high voltages and high currents. An open feed line.

If you don't believe me, you can prove it to yourself with experiments with an open feed line. But the academics will argue about it.

There has always been a controversy over the importance of a matched feed line. And if you ever transmit much.....you, like others, will find it does not matter much on receiving end.

I use to do commercial radio work. That's different......look at the loss with coax cable. Now we match the feed line to the antenna.....because of line loss.

A folded dipole has 300 ohms impedance. A tv antenna using 300 ohm twin lead has very low loss. I have used hundreds of feet of twin feed line from hilltop to house with low loss. The whole signal would be lost using coax.

Check out the loss vs frequency with coax.

 

Some ref material -

http://www.arrl.org/feed-lines

https://ham.stackexchange.com/quest...e-vs-450-ohm-or-600-ohm-ladder-or-window-line


Regards, Dana.

 

Matching is a different story. Matching is NOT necessary IF.........transmission line is low loss. and can take high voltages and high currents. An open feed line.

If you don't believe me, you can prove it to yourself with experiments with an open feed line. But the academics will argue about it.

There has always been a controversy over the importance of a matched feed line. And if you ever transmit much.....you, like others, will find it does not matter much on receiving end.

I use to do commercial radio work. That's different......look at the loss with coax cable. Now we match the feed line to the antenna.....because of line loss.

A folded dipole has 300 ohms impedance. A tv antenna using 300 ohm twin lead has very low loss. I have used hundreds of feet of twin feed line from hilltop to house with low loss. The whole signal would be lost using coax.

Check out the loss vs frequency with coax.

There is no controversy. The academics do understand, it's the amateurs that argue.

This is not to say you need to match transmission line to transmit power, it just needs to handle the possible very high RF voltages that might present in a unmatched condition while being mated to a really good grounding system. We used a long wire rope antenna connected to the bare center connector of a high power ~100 foot helix cable at one end while the other end of the helix was connected to the antenna output of the 2-30 MHz match box.
http://kvantefir.com/assets/files/pdfdownloads/Andrew_cable.pdf

https://forum.allaboutcircuits.com/...and-the-matching-circuit.154975/#post-1335201

 

Hello all,

This is my first high-precision RF design, and I am having real troubles with it. It would be really great if someone could lend a helping hand.

First of all, I am not able to understand the purpose of 50-ohm line feed and the matching circuit. We have plenty of antennae available these days, in whatever form factor one wants, and the good thing is that manufacturers design these antennae with standardized characteristic impedance (50-ohm is the most common, although 75-ohm isn't that rare). All the major manufacturers of such antennae emphasize the design of a proper line feed as well as a matching circuit, and that is what I am not able to digest. When we design a line feed with a characteristic impedance equivalent to that of the antenna, why do we need the matching circuit? Making transmission line characteristic impedance equal to the load impedance is sufficient to prevent any wave reflections, so what role does the matching circuit play?


Thanking in advance,
Abbas.

The reason to use matching network is to maximize the output power and to reduce the VSWR. The benefit of maximizing output power is obvious. For high power amplifier, VSWR control is also very important since too high VSWR will destroy the expensive high-power amplifiers.

The antenna may have nominal 50 Ohm impedance, but in reality the impedance varies with frequency, PCB layout, temperature...The actually impedance may be very far away from the nominal 50 Ohm. The matching network will help to convert the impedance to be closer to 50 Ohm and thus maximize the output power.