Electrical – Connecting a 75 ohm device to a 50 ohm antenna

antennacablesimpedanceimpedance-matching

I have a device operating at low-frequencies (470 – 790 MHz, Television White Space), which works with a 75 ohm antenna. I want to connect this to a 50 ohm antenna instead. The 75 ohm end has an F-type female connection and the 50 ohm end has an SMA-type female connection. I've been looking for the appropriate 'adapter' for this situation but most items are difficult to come by, expensive, and have numerous names. For example, there are items like:

I am sure the plain F type to SMA type adapter is out of the question, but which of the other types is suitable for my application? Or is there a different type of device suitable for this.

Best Answer

Ok, first off that range is not LF(That would be below 300Khz), but UHF(which is 300Mhz to 3Ghz), to be more precise. :)

Second, your looking at give or take a .2 dB loss for the mismatch, which is not that big a deal for transceiver usage. This means you'll lose about 4% of your power.

You'll loose more signal by using a common coax type(anything less than semi-rigid) at that frequency than that mismatch. Examples of this are below. These examples are for looses at 500Mhz and 800Mhz per 100 Feet in dB.

RG-59 7.3 9.5

RG-6A 6.6 8.7

RG-11 5.3 7.2

This is for a high quality cable(Belden), no names/off brands will most likely be worse. A better cable might get better specs, as an example LMR-400 gets 2.8 and 3.6.

(This is from CO8TW's nice calculator at Coax Calculator.

The only time I would worry about it would be if I was using over medium power(over let's say 30 to 50W)(and maybe not even then if I had a decent antenna) or it was for something like a Spectrum Analyzer or some type of Network Analyzer where the actual numbers are important.

Then I would deal with it by using something from K6STI's Match Page or the equivalent commercial gear. But only if I could get less than a .2 dB insertion loss(not going to happen unless I get REAL lucky..:) ), most likely I would either correct it in software or ignore it.

Note, the SWR(standing wave ratio) in this case is 1.5 due to the mismatch, which is where the .2dB loss comes from..

In your case, I would just use a high quality F to SMA connector with the appropriate sexes and not worry about it.

What I mean by high quality is something like this one. Gold plate contacts and Teflon insulation. Not the $3.99 specials...

Related Topic