1
$\begingroup$

So, I'm currently tinkering with a RTL SDR dongle and want to put an external antenna in the proper way (i.e. not by soldering a random wire on the included mcx terminated coax)

I was about to buy a proper MCX-SMA pigtail and some soldering connectors, but I keep reading in descriptions things like "sma male connector (... random chinglish... ) RG XXX" where xxx designates a rg standard. It is my understanding that connectors have a specific width to put the coax core in. I had no idea about this. Is this critical in mounting the connector? Is it ok to get a bunch of RG-58 sized connectors so that thinner types of coax will fit as well?

Thank you and sorry for my English

$\endgroup$
  • $\begingroup$ Have you looked on the usual sites? See Glenn's answer below. They are easily available and no soldering required. $\endgroup$ – SDsolar Sep 24 '17 at 22:01
  • $\begingroup$ Here is a link that at least shows the proper names for some of the various types: miklor.com/COM/UV_Technical.php#antenna - it is written for the Baofeng radios and what you are looking for is the opposite, but the diagrams can be helpful. $\endgroup$ – SDsolar Sep 24 '17 at 22:14
4
$\begingroup$

Is it ok to get a bunch of RG-58 sized connectors so that thinner types of coax will fit as well?

Not if you want to do it right. Proper termination of coax requires a smooth transition from the coax shield to the connector. Mechanical compatibility is an issue as well if you don't want your cables constantly breaking.

I've never seen a solder-on SMA connector except for semi-rigid coax. Usually they come in two kinds: clamp and crimp.

Clamp

An example of the clamp type is Amphenol 901-128-11. This has a clamp nut assembly which the cable passes through, then the shield is folded back upon it. The plug then threads onto the clamp nut, with the folded back shield clamped between the two pieces.

enter image description here

If the coax is too large, it will not pass through the clamp nut. If the coax is too small, it might work well enough. The coax will flop around, which will eventually fatigue the metal in the shield. And the geometry of the connector will be out of specification, compromising high frequency performance. Below 1 GHz you may not notice.

Clamp connectors don't require any special tool to install. That's about the only advantage they have. In an application where the cables are frequently handled, expect them to fail either due to working themselves loose, or from the shield breaking where all the strain is focused at the clamp point.

Crimp

An example of a crimp connector is Amphenol 901-9511. These connector have an inner ferrule which slips between the connector shield and dielectric. An outer ferrule is then crimped over it, making both a solid mechanical and electrical connection.

enter image description here

If the coax is the wrong size in either direction then the ferrule won't fit at all.

Crimp connectors are the way to go. You'll have to buy a crimp tool, but they're less clunky, and the crimp provides a more solid mechanical connection which takes the strain off the cable braid. They will never come loose. They never fail.

$\endgroup$
2
$\begingroup$

Soldering coax to connectors is tricky because it is easy to melt the dielectric. I would not recommend attempting it as a beginner, especially for small coax as typically used for SMA connectors. Crimped connectors are easier and give more reliable results, but require the correct crimp tool and matching all dimensions of the coax to the connector.

If you are constructing an antenna, unless the antenna contains coax as part of its design, I would recommend avoiding working with coax at all and instead getting a connector intended for panel or PCB mounting, then soldering the antenna elements to that, then using a separate cable/pigtail to connect the dongle to the antenna.

And if you are specifically looking to work with random wire antennas, I would suggest not soldering anything at all and instead getting a MCX-BNC pigtail and a BNC to binding post adapter. These are both common items and will allow you to simply clamp in any wire with the binding post, or attach an alligator clip, etc. BNC connectors are also much more convenient for experimenting because you can quickly swap between two antennas, and connect a variety of other equipment and adapters.

$\endgroup$
  • $\begingroup$ For random or long-wire antennas I recommend the so-called 1:9 balun. Just note that as you look at it from the SMA connector end it is designed to have the shorter wire element or even a ground on the left wire connector. Looking at it from the spring connector end that is the right-hand one. The other is for the long wire. I think "1:9 balun" is a marketing term and not a technical one. It is made for seriously unbalanced antennas. $\endgroup$ – SDsolar Sep 24 '17 at 22:05
  • $\begingroup$ btw, I can't locate the source link for that comment. But I discovered it during my research into how to make a semi-decent antenna for 472 KHz for the SDR. The loading coil and whatever vertical element would go onto the long-wire connector and the other basically to ground. $\endgroup$ – SDsolar Sep 24 '17 at 22:10
1
$\begingroup$

There are so many, well made adapters and adapter cables available, it hardly pays to try to put one together. Most are available for < $10 USD.

enter image description here

enter image description here

$\endgroup$
  • $\begingroup$ If I could I would upvote this one twice. They are easy to find. $\endgroup$ – SDsolar Sep 24 '17 at 21:59
  • $\begingroup$ Actually I know about cheap pigtails, I was looking for connectors to connect a homemade antenna with the pigtail itself. I think I will just do the following: pigtail - > bnc-two cables adapter - > balun and maybe a simple ladder line - > balanced antenna. At least as long as I'm listening HF only $\endgroup$ – sEdivad Sep 25 '17 at 6:22
  • $\begingroup$ @sedivad If this is a receive only antenba, the ladderline is overkill. Use coax to a dipole antenna and have a 1:1 choking balun at the center of the dipole. A slight improvement is to have a second choking balun at the receiver. $\endgroup$ – Glenn W9IQ Sep 25 '17 at 6:49

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.