SMA - soldering to coax with stranded center

[BucksGuyUSA]

I have a cheapo little magnet mount antenna - the small whip with the center coil that I wanted to switch from BNC to SMA on the skinny little coax cable that it came with. No big deal, right?

SNIP!

Now I have this coax which has a slightly-too-small-diameter for the SMA connectors I have, unbraided shielding wire, and a stranded center conductor that is a complete PITA to work with.
I can tin the stranded center, but when I try to solder that to the SMA center pin, no matter how quickly I try to heat it up, the center coax insulation melts and swells up and it won't fit the connector properly unless I trim it down with a razor. The sheilding just bunches up, I can't get the connector to slide in...

Beating my head against the workbench here.

Tips? Pointers? Just throw out the cheap-o antenna? Replace the coax with real coax?

 

[Project25_MASTR]

Buy the right crimping tools…

 

[rocky28965]

Could have just used an adaptor if you hadn't cut the plug off.

 

[kayn1n32008]

Buy the right crimping tools…

This is the correct answer.

 

[kayn1n32008]

I have a cheapo little magnet mount antenna - the small whip with the center coil that I wanted to switch from BNC to SMA on the skinny little coax cable that it came with. No big deal, right?

SNIP!

Now I have this coax which has a slightly-too-small-diameter for the SMA connectors I have, unbraided shielding wire, and a stranded center conductor that is a complete PITA to work with.
I can tin the stranded center, but when I try to solder that to the SMA center pin, no matter how quickly I try to heat it up, the center coax insulation melts and swells up and it won't fit the connector properly unless I trim it down with a razor. The sheilding just bunches up, I can't get the connector to slide in...

Beating my head against the workbench here.

Tips? Pointers? Just throw out the cheap-o antenna? Replace the coax with real coax?

Crimp. Crimp. Crimp.

 

[jwt873]

How are you soldering the center pin? What are you using?

I solder mine using a fine tipped hot iron and thin (.06mm) rosin core 60/40 solder. (The 60% tin content allows the solder to melt at lower temps than 50/50 solder.

All the SMA center pins I've seen have a small hole for applying the solder. (see below). Slide the pin over the center wire, and hold the iron tip on the side opposite of the hole. While doing that, hold the solder against the hole. When the pin gets hot enough to melt the solder, it will be drawn in the hole and into the wire. Do this quickly.

You do need a crimper to compress the ferrule in order to fasten the braid to the connector itself.

 

[mmckenna]

Tips? Pointers? Just throw out the cheap-o antenna? Replace the coax with real coax?

If you are going to be doing this more than a few times, get the right crimpers and connectors. Soldering coax takes some skill, but you can get connectors that have a crimp-on center pin. The crimpers don't have to be expensive. It's pretty easy to practice with some scrap coax and a few extra connectors and get pretty good at this quickly.

If this is a one off project, it's always a good idea to toss cheap-o antennas and get something decent. The antenna and feedline are the most important part of your system and Chinese Junk Amazon/E-Bay mag mount antennas are not doing you any favors.

 

[prcguy]

I've not seen crimp type center pins for SMA connectors on RG-174 cable and the pin pictured above is a solder only type. I've had a lot of mil spec solder training over the years and for a "legal" solder connection you must tin both the wire and any gold plated item, then remove excess solder then assemble and solder the connection. Pre tinning both the wire and inside of the center pin will make the final soldering go much faster and will do less heat damage to the dielectric. I usually deal with Teflon wire which does not melt but with other types of dielectric its best to have a very hot iron and work fast to avoid melting the cheap plastic dielectric. Basically get in and out quickly with the hot iron.

On small diameter wires with tiny center pins that have impossibly small solder holes I will usually prep and tin the wire at the perfect length to stop the center pin a few thousands of an inch before hitting the dielectric, then slide the pre tinned center pin on just enough to leave some bare wire between the dielectric and pin to get a soldering iron tip up against the bottom of the center pin, then heat, add a little solder that will suck up into the center pin then quickly slide the center pin down the wire flush with the dielectric before it cools.

With cheap plastic dielectric you will generally melt some and it may pull back from the wire and fatten up. For what I call a low frequency connection, something under 500MHz you can trim down the diameter of a fattened up dielectric with center pin attached so it will slide up into the connector and it usually tests just fine. I was certified to assemble SMA type connectors to 40GHz (actually 2.92mm K connectors) and you can't deviate from those instructions in any way, otherwise they will not meet VSWR specs. Even a .001" mistake in a cut will show up as a VSWR problem.