RF impedance matching: 50 ohms vs 75 ohms

Factual Questions
1

Practical question about RF impedance matching:

I have a receiver with an SMA female connector on it. I would like to connect a cheap AM antenna to it.

SMA, like many other connectors, has a 50 ohm impedance, and AM loop antennas tend to have twin-lead outputs at 300 ohms.

All right–I know I need a balun transformer for good efficiency. 300 to 75 ohm transformers are extremely common, but 300 to 50 seems nonexistent. The common 300-75 transformers convert to an F connector, which makes sense since F is typically 75 ohm. I can get F to SMA converters, but they don’t seem to typically include a 75-50 ohm transformer.

So what’s the deal? It seems that if this were a valid thing, it would be easy to find converters. Maybe the difference between 50 and 75 is negligible enough not to matter? Is there something else I’m missing? Thanks.

2

Pretty much this for short runs of coax. And for a receiving antenna, don’t worry about it.

3

Gary’s got it. The impedance mismatch isn’t significant for receive-only radios.
When you are transmitting without a close match, you get energy reflected back into the transmitter which can quickly heat to the point of failure.

ETA: What kind of receiver are you playing with?

4

Thanks, guys! My knowledge in this area is very hit or miss due to my haphazard learning style…

For my current purposes, I’m using one of these guys as an SDR receiver. However, it only goes down to 25 MHz, and I want to see stuff on the AM bands, so I have a 100 MHz HF upconverter to put in front of it.

For more serious stuff I use a bladeRF, but that only goes down to 300 MHz and so I can’t even get FM.

5

That’s funny because I was wondering what you could have that was SMA and needed an AM broadcast loop antenna. And I started thinking about my SDR TV dongle and edited my post to ask about the receiver. I have a different one from NooElec and have wanted an upconvertor for HF for a while.
Let me know when you get tired of it!

How are you adapting the MCX connector? I just stick a piece of wire into my female PAL.
And what are you using for software? I was using SDR# but haven’t played with it in a while. Now, the dongle is on my desk at work and I can never remember to bring it home.

6

You could try making one:

http://www.spirat.com.au/vk5zvs/pic32.htm

You can also buy an AM antenna w/ 50 ohm impedance, but they’re pretty pricey:

http://www.godarusa.com/id68.html

7

Looks like fun stuff. I’m a ham and don’t currently have any SDR stuff, though several friends are running Flex Radios. I run an Icom 756 Pro III and a Yasue 857D.

I once bought my Dad a Win Radio which was receive only and he had fun with it.

8

I just use a cheap jumper cable.

I write my own software :). My program is pretty similar to SDR#, but I have somewhat divergent goals and I needed my own codebase to start from. I use it to receive AX.25 signals, among other things. But I wanted to play with AM and possibly HF stuff for fun.

I forget the dongles too, but have about a dozen of them at this point and have reached dongle saturation :). They’re really amazing units for the price.

9

That might just be worth a try! I think I have some old ferrites in my stash…

That GoDar antenna isn’t too bad. I was hoping to keep it under $50 and that fits the bill.

10

I’ve purchased little transformers to convert 50 to 75. Pretty little tubular things. I’ve seen baluns for 50 ohm coax too.

11

Using a 75-ohm antenna with a 50-ohm input will result in about a 2 dB loss of signal strength. Probably not enough to worry about.

12

run the antenna wires straight into the connector, don’t use coax/transformer/balun.

13

You guys beat me to it (and CurtC went one step further by supplying an actual number.)

Presumably you already know that a 3dB signal loss is half the power (6dB is half the amplitude). So, unless you’re fishing for signal strength, there should be no issue.

Try not to play the trick (changing impedance) twice in the same signal path, or you’ll get reflections at the receiver.

I remember using a TDR to see where a 2" diameter coax cable (for Ethernet-like datacom, back in 1980) was kinked by the building staff, who said they could lay the cable to spec but then didn’t bother to read the specs (ignoring radius of curvature). Most of it had to be redone (by us engineers, not the building staff) and even then we had to use the TDR to find the places where we’d goofed. 1960’s buildings are not fun to run thick cable though.

14

[TDR = Time Doman Reflectometer. Basically, this is the equivalent of radar/sonar, sending a pulse down the transmission line and showing the places with reflections on a graph, by distance from the device.]

15

And don’t forget to correct for velocity factor! Or do the expensive ones do that for you?

16

In 1980 they didn’t.

17

Cool, thanks for the info. A minor point of curiosity: I was thinking about how you would actually run really thick cable through a building, and particularly how you would run cable around a sharp corner (like going from one corner of a room to another). It occurred to me that if you put a kind of S-shape in the cable (maintaining the minimum radius), you can orient it so that the middle point runs axially along the edge you’re trying to transition across, and the cable can stay flat against the wall while still not getting kinked.

Seems like it should be a common enough technique that there should be a name for it. Is there?