Mismatching impedance - is it REALLY not a big deal???

[LIScanner101]

This has probably been beaten to death, and I'm not even sure if I should be posting this here or in the coax forum,

But I see so many conflicting views on using the “right” impedance cable for a scanner - "right” in this case being 50 ohms to match the typical antennas being used (such as a rooftop vehicular whip, or a base discone etc).

But what about if you have a dipole like the Scantenna or the old Monitenna, which always come with a 300-ohm-to-75-ohm balun?

Do you run 75 ohm coax down to your scanner (which is 50 ohms - mismatch!!),

Or do you use 50 ohm coax that matches the scanner’s impedance (which then means you are forced to terminate the cable in a special F connector made for 50 ohm coax but then connect it to a 75 ohm balun on the antenna end - mismatch!!)?

To me it seems like you have to “compromise” at either the antenna end, or the scanner end.

Does it really make any freakin' difference for scanning?

Any thoughts on this?

 

[Thayne]

Not a big deal in using good quality RG6 for receiving only; I have seen guys use it to save money even in commercial applications (Like for GPS timing, Etc)
look at the low loss # for some of the available 75 ohm cables.

 

[LIScanner101]

Thanks. I always wondered why one would go through all the trouble of matching the scanner's impedance with the right coax and then negate that effort on the antenna end (or the other way around). I'm going to go with 75 ohm coax BUT I'm going with RG-11 instead due the lower loss.

 

[n0nhp]

The broad range of frequencies a scanner receives all but precludes an antenna that matches 50 ohms across the range. A discone probably comes closest. 25 ohm impedance mismatch in the cable will cause an SWR of about 1.5 to 1. (shock, you can use 75 ohm cable with a matched 50 ohm antenna for transmit) That should give you less than 3 dB of loss (1-2 S units). As it seems that most scanners are in Urban areas and running with the attenuation on anyhow, that should not be an issue.
I suppose if you are in the middle of Wyoming or Montana, 70 or 80 miles from the nearest repeater, that loss might be enough to make a difference.
The lower high frequency loss per foot will probably make more difference to the receiver unless you are going with something like LMR 400.
The bottom line is, what do you feel like spending money on? Cable or antenna? or both?
No body is saying not to use 50 ohm cable, it is just that you can get a whole lot more RG-6 than you can LMR 400 for the same money.

Bruce

 

[LIScanner101]

The broad range of frequencies a scanner receives all but precludes an antenna that matches 50 ohms across the range. A discone probably comes closest. 25 ohm impedance mismatch in the cable will cause an SWR of about 1.5 to 1. (shock, you can use 75 ohm cable with a matched 50 ohm antenna for transmit) That should give you less than 3 dB of loss (1-2 S units). As it seems that most scanners are in Urban areas and running with the attenuation on anyhow, that should not be an issue.
I suppose if you are in the middle of Wyoming or Montana, 70 or 80 miles from the nearest repeater, that loss might be enough to make a difference.
The lower high frequency loss per foot will probably make more difference to the receiver unless you are going with something like LMR 400.
The bottom line is, what do you feel like spending money on? Cable or antenna? or both?
No body is saying not to use 50 ohm cable, it is just that you can get a whole lot more RG-6 than you can LMR 400 for the same money.

Bruce

Hi Bruce,

When I really think about it, I'm really not looking to DX anything at this time. I live in the eastern edge of Nassau County on Long Island in New York. I did a 25 mile radius mapping from where I live, and it covers my entire county, part of Queens, and all the way out to the western 1/3rd of Suffolk County. This means I will have PLENTY to listen to even with such a small radius. With that said, I'm not going to fret over whether I should blow a wad on LMR-400 or not - I'm going to go with RG-11 and be done with it. That should give me plenty of signal with this relatively small radius of listening area.

Thanks!
John

 

[W2NJS]

It doesn't make a whole heck of a lot of difference if all you're doing is receiving and the signal strength is relatively high. OTOH, if you want to drive yourself nuts get an MFJ antenna analyzer and find out what exactly the impedance characteristics are for your particular antenna. In the end you'll probably end up at the same place as the guy who just connects a 50 or 75 ohm coax between his antenna and receiver.

 

[LIScanner101]

It doesn't make a whole heck of a lot of difference if all you're doing is receiving and the signal strength is relatively high. OTOH, if you want to drive yourself nuts get an MFJ antenna analyzer and find out what exactly the impedance characteristics are for your particular antenna. In the end you'll probably end up at the same place as the guy who just connects a 50 or 75 ohm coax between his antenna and receiver.

I have a Monitenna, which is a 25-1000MHz dipole. It comes with about 50' of RG-6/U and a 300 ohm/75 ohm balun, so I will assume I would be best off with 75 ohm cable.

 

[KC4RAF]

For receive only, you'll do just fine with

the mismatch. The difference that may occur would not be noticable to you what so ever. Look at all those long wire antennas. They by no means come close to 50 ohms. So as long as you are receiving only, don't worry about the mismatch. Just like others have posted, you're ok.

Just read your last post. Use 50 ohms if you have it. It will be ok.

 

[LIScanner101]

the mismatch. The difference that may occur would not be noticable to you what so ever. Look at all those long wire antennas. They by no means come close to 50 ohms. So as long as you are receiving only, don't worry about the mismatch. Just like others have posted, you're ok.

Cool! I'm all set then! All I have to do now is crawl into that attic....