Change 75 ohm input to 50 ohm input? - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: June 06, 2021, 08:55:35 pm
content: I have a radio service monitor with a 75 ohm bnc input but the problem is the radios I want to use it on are all 50 ohm PL259. Is there any device I can use to connect successfully?

author: mio83
author_rank: Regular Contributor
timestamp: June 06, 2021, 09:00:23 pm
content: Hi!Yes, you need an impedance matching pad, such as this one:https://www.minicircuits.com/WebStore/dashboard.html?model=BMP-5075%2BIt will cost you a few db of loss though (the one I linked seems to have -5.71db loss).

author: TimFox
author_rank: Super Contributor
timestamp: June 06, 2021, 09:08:05 pm
content: This may not be necessary: the PL259 is not an impedance-controlled connector (not like N, BNC, and SMA connectors), and the actual impedance of your radio's input may not be accurately 50 ohms.PL259s can be used (sometimes with the appropriate adapter) for RG-58, RG-59, RG-8, and RG-11 cables, both 50 and 75 ohms.Otherwise, look up "minimum loss pad" e.g.http://home.sandiego.edu/~ekim/e194rfs01/minl_atten/minlosatten.htmlfor a calculator application that can do minimum loss between specified impedances (two resistors).

author: edpalmer42
author_rank: Super Contributor
timestamp: June 06, 2021, 09:55:36 pm
content: The Minicircuits padISa minimum-loss pad. That's the problem you run into when trying to use them. You can also find 50 - 75 ohm transformers that would have a loss of only a fraction of a db.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2021, 11:22:08 pm
content: How much power is involved?https://www.balundesigns.com/model-15133-1-5-1-unun-75-to-50-ohms-3kw/

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2021, 08:19:21 pm
content: if you're want to use it with measurement equipment, you're needs to note that impedance match circuit have losses which affects measurement result

author: fcb
author_rank: Super Contributor
timestamp: June 07, 2021, 08:27:45 pm
content: Technically you should match impedances for optimium power transfer.In reality the 50-75 mismatch will make a small difference, and you likely won't notice it - where it matters is if you are transmitting, the outputstage and any filtering will be optimised to drive the 75ohm load, and driving 50ohms will mean a loss of power and you will potentially damage a transmitter stage (probably not an issue if its a watt or two).Balun would be my goto method if there is an issue.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 10, 2021, 01:41:34 pm
content: If you are just measuring radios for repair, like putting in a S9 signal to the receiver or monitoring the output of a transmitter, you might not notice a difference using a 75 ohm monitor. It is only when you get above 10 MHz that I think there is a real difference, and then it is probably not consequential till much higher freq.75 ohm BNCs are different from 50 ohm BNCs and may affect your connections. There are supposedly some instances in which you can damage connectors by using 50 into 75, or the other way. I think these are exaggerated. I think all connectors made today are compatible. I do not know if the BNCs on your Service Monitor are 50 or 75 ohm. 50 ohm BNCs were sometimes used on 75 ohm equipment since they are more common and the loss at low freq is not important.You could look at your BNCs and compare them to Wiki pictures. Also examine them for wear and damage.I did change some inputs in an old Marconi spectrum analyzer a while back.Many manufacturers made the same unit in both 50 and 75 ohm versions. This was one of them.The change was easy: a couple of about 75 ohm resistors were removed and something close to 50 ohm resistors put in place. I do not remember, I think the schematics called for 51.1 ohm resistors. The documentation I have clearly shows the different inputs and the different components.I also changed the BNCs. In Test Equipment, the connectors are likely to get used a lot and worn out, so I thought it was a good idea to use new good quality connectors, regardless of the probable 50-75 ohm mismatch. Besides, I had the thing apart anyway.If you have the schematics, they may have the different circuits for the 75 vs 50 ohm inputs...Might be an easy modification and those old BNCs might need to be replaced anyway.PL 259 connectors are not necessarily 50 ohm, I think they actually measure about 32 ohm at 30 MHz. They are ancient technology maybe 100 yrs old. Still they work well in proper applications and we are stuck with them on most HF radios.Most of the time, if you are servicing radios, there will be a difference in your readings that will not be of any consequence. Most of the time you will probably be looking for "Max" or "Min" signals.An example might be that you are setting the S meter. you feed in a S9 signal (neg 73dBm for HF signals) and set the meter to S 9.When you get to VHF there is a potential for more loss and more problems, that is why radios in VHF and above frequently do not use PL 259 type connectors.Having said all of this,The biggest problemI see in using test equipment is bad coax or bad connectors. Get some good coax and good connectors and good adapters. I use RG 223 coax that has double silver plated shield. You should use at least very good coax that will put up with being banged around a bit. Also no solid aluminum center conductor that can break.(and drive you nuts looking for a problem in the device under test that does not exist)Loss is not a big concern with the coax you would use. The lengths are too short to make any difference. Cost is really not important since you are not using 100 foot runs. Signal integrity (shielding) is most important. Being able to work with the coax making connections to the connectors is important also. I personally have trouble with the foil wrapped coax. (I guess I am a klutz)