Preferable antenna impedance?

[Zolton]

I understand that a tuner has an internal balun (vintage ones anyways). Does this change a 75 ohm antenna signal to 300 ohms or vise versa? In otherwords, if you had the option, would you be better off with a 75 ohm or 300 ohm line feeding your tuner in order to avoid the internal balun losses?

 

[catman]

G'day mate, go by whatever is marked on the antenna terminals on the tuner. This whole subject is actually rather involved, in fact I'd rather use an FM antenna with a folded dipole driven element (300 ohm balanced), as the feedline losses with 300 ohm balanced feedline are much lower that 75 ohm coaxial cable for the same physical length.

However, good quality 300 ohm balanced line has now mostly disappeared from use, so we're now struck with 75 ohm (unbalanced) coaxial cable and most tuners and antennas are designed for use with 75 ohm coaxial cable as a direct consequence. Yes a balun is used to transform impedances (in both directions) and all baluns have 'some' losses, but well designed ones have very little loss. Regards, Felix (vk4fuq) aka catman.

 

[Zolton]

Well...all of my gear is vintage and as such they seem to have terminals for both 300 ohm and 75 ohm. So I have the option of going with either one. I've been making my own antennas and they are all dipoles of some sort at this point. As such the line to the tuner / receiver is 300 ohms. I've got 4 of them in various locations in my house feeding individual systems.
I guess my question is; does this 300 ohm line get changed to a 75 ohm impedance in the tuner or does it get used as is? In other words, what does the internal balun do. Or...what would the tuner prefer as a source?

 

[AK 47]

A standard (split) diapole terminates to the feed in wire at 75 ohms. If you want to connect it to your receiver with 300 ohm lead in you should (as Felix stated) fold it (which will then terminate at 300 ohms).......

 

[Zolton]

I get that.

I've used folded dipoles so far so I'm dealing with 300 ohms.
My question boils down to this...
What does a vintage tuner / receiver with an internal balun convert the incoming signal to prior to introducing it into the internal circuitry?

I would think the internal balun would be used on only one of the two incoming signals to convert it to an impedance that the circuitry is made to work with.
The other input would be introduced to the circuitry unchanged.
Therefore the tuner would work internally with either a 75 ohm OR 300 ohm signal.
Which one?

Or am I out to lunch on this one?

 

[Bob@FM]

Well...all of my gear is vintage and as such they seem to have terminals for both 300 ohm and 75 ohm. So I have the option of going with either one. I've been making my own antennas and they are all dipoles of some sort at this point. As such the line to the tuner / receiver is 300 ohms. I've got 4 of them in various locations in my house feeding individual systems.
I guess my question is; does this 300 ohm line get changed to a 75 ohm impedance in the tuner or does it get used as is? In other words, what does the internal balun do. Or...what would the tuner prefer as a source?

Yes, most vintage tuners have an internal balun to go from 300 to 75 ohms. A common mod for better sensitivity is to remove the internal balun, as on most tuners removing it increases the 75 ohm sensitivity, as the 2 inputs are tied together internally.

This applies to 90% of the tuners I've seen, as every once in a while removing the balun decreases sensitivity.

I would say experiment on your own and see what works best. I think these sort of things are not nearly as important as getting the antenna higher, and pointed in the right direction.

 

[Zolton]

It's more to satisfy my own curiosity than anything else.:scratch2:
So the internal circuitry will take the 300 ohm input signal and convert it to 75. And it will not alter the 75 ohm input. And the internal circuitry processes the signal at 75 ohms?
Sound right?
So the signal from source with the least losses due to baluns would be from a non folded dipole to the 75 ohm input via a coax? No baluns involved. But best to keep the run short to avoid losses from the use of the coax (as opposed to a 300 ohm balanced feed line as per Catman's post?)

Maybe it would be simpler to stick with a coat hanger stuck in the antenna terminals.

 

[PioneerMike]

Zolton,

Putting my engineer/ham radio hat on for a minute, most vintage receivers, at least looking at the back of mine have 300 Ohms input, a few can handle 75 Ohms input. It is harder to find 300 Ohms line but it's still around. The goal with any antenna-to-receiver is to match the antenna impedance to the receiver input impedance so you get all the signal received to the tuner. Mismatched impedances cause losses in this signal and poorer reception.

The other goal with an antenna is what do you want it to do, be omnidirectional or directional?

If the receiver has 300 Ohms input and an internal 300-75 Ohms balun it's doing the work for you as long as you hook up 300 Ohms antenna wire and the antenna itself has 300 Ohms output impedance. If not then use a 75-300 Ohms balun and let the receiver reverse the process internally if all you have is a 300 Ohms input. You may lose a little bit of signal in the double translation but not enough to worry about.

I use a inline RF amp and get almost full strength with rabbit ears on stations 100-140 miles away.

It all sounds good.

Enjoy

Mike

 

[Syncopator]

Yes a balun is used to transform impedances

No.

A balun (it's a contraction of balanced to unbalanced) is to convert between balanced and unbalanced feeds.

A balun can be designed to transform impedances at the same time, but that function is not implicit in its definition.

 

[catman]

G'day mate, yes they do! In fact baluns do serve both purposes (depending). I don't wish to confuse all with theory but it's a fact.

Yes baluns do allow from a balanced to an unbalanced temination and vice versa and yes that is their main 'job' but they also provide impedance transformation eg. 1:1, 4:1, 9:1, 75 ohm to 300 ohm nominal, (an example of a 4:1 balun) etc etc.

The type of balun called the 'current' mode or 'choke' balun which I am very familiar with for transmitting applications can also function as an 'unun' (unbalanced to unbalanced), an excellent example of feedline 'decoupling', and also as a conventional 'balun'. So in actuality baluns can and do both things depending on the configuration. Regards, Felix (vk4fuq) aka catman.

 

[Syncopator]

Thank you Felix.

Your rather verbose response confirms what I said.

Yes, some baluns can transform impedances, but that is not implicit in its basic definition.

 

[Shackman55]

Maybe I read wrong, but the OP was asking what actually feeds a tuner inside, the final destination inside the unit, 75 or 300? Therefore needing the least manipulation before being used.

 

[Bob@FM]

It's more to satisfy my own curiosity than anything else.:scratch2:
So the internal circuitry will take the 300 ohm input signal and convert it to 75. And it will not alter the 75 ohm input. And the internal circuitry processes the signal at 75 ohms?
Sound right?
So the signal from source with the least losses due to baluns would be from a non folded dipole to the 75 ohm input via a coax? No baluns involved. But best to keep the run short to avoid losses from the use of the coax (as opposed to a 300 ohm balanced feed line as per Catman's post?)

D

First a little bounding on the question. Most early FM tuners, 50's-60's, only had twinlead connections. There was no 75 ohm input. This is almost always 300 ohm.
The 70's FM tuners had both, 75 and 300ohm inputs. Later tuners from the mid 80's on mostly only had 75 ohm FM antenna inputs.

The first tuned stage on the RF path is usually unbalanced. On almost all tuners I've seen that had both 300 and 75 ohm inputs, the first RF stage input was 75 ohms unbalanced. The internal balun converts 300 balanced to 75 unbalanced.
So yes, you got it right. You don't want to go from 75 to 300 external to the tuner, as it will only go from 300 to 75 inside the tuner.

BUT
The output of the internal 300 to 75 balun is also connected to the 75 ohm direct input.
When you connect direct to the 75 ohm input(on FM tuners that have BOTH inputs), you are also driving the 75 ohm side of the internal balun. There is no switch that separates them. Although you have nothing hooked to the 300 ohm side, there is still some minor energy loss in the balun when coming in 75 ohm direct. The solution is to unhook the internal connection between the 75 ohm input, and the balun.