Upgrading coax on receiver with proper antenna

[ianparky]

Hi guys, new to the world of r/c flying and video downlink, hope you can help with my video receiver antenna problem.

I use a 10mW 2.4GHz camera on my aircraft, which is fine, but I've been struggling to get a decent, compact ground station. I recently got a pocket-sized wireless receiver (no brand, ebay) with built-in LCD screen, and it records video as MPEG-4 to SD card. Works very well, really pleased with it ... apart from the antenna. It's limiting my range to about 50m at the moment, but I'm thinking with a simple antenna upgrade I can increase that range significantly.

You can see in the pic I've removed the receiver module (ComTech?) to show the little bit of coax, which is the only antenna. I want to install a SMA connector instead of this coax, to give me a choice of patch or rubber antenna.

Now, I've already had a little go at this, but I don't know what I'm doing, so I've reverted back to 'factory default'. From reading around it, I understand the length and size of coax is important. I guess I just need to solder one end of the signal wire (of the coax) to the centre pin of the SMA, and the other end solders to the point on the comtech module. And the coax shield ... er ... soldered to the outside of the SMA and the outside of the comtech module ..?

Can this mod be done the way I describe? Anything to look out for particularly?

Thanks for any help,

Ian

[W3FJW-Ron]

Can be done if you have tiny fingers and good soldering skills. My suggestion would be to get a made up coax with SMA connectors for use between the antenna and the Rx. Keep an eye on the male/female ends and make sure they match up with your equipment.

The length of the coax (including connectors) should be a multiple of 1/2 wavelength as measured from the connection on the ckt board to the connection on the antenna. One half wave @ 2.4ghz= .195' (2.34"). Keep the coax as short as possible and be as accurate with your measurements as you can. You will have to take into account the velocity factor of the coax you use as well. In fact, mount the Rx to the patch antenna if at all possible, and run your video connections (if any) from there.

Edited May 8, 2009 by W3FJW-Ron

[Terry]

I would like to back up what Ron said but also add that I would not worry too much about the length of the co-ax but keep it as short as possable. What aerial are you going to use ?

Terry

[ianparky]

Thanks your prompt response guys, most helpful.

I have two antennas I want to use with it, a square patch antenna, and also a standard rubber duck antenna. Both have male sma connectors.

Limitations of space prevents me from soldering the SMA directly to the receiver, but I might have another look at that at some stage.

I'm now thinking of using a PCB-mount female SMA, fixed to the outside of the case, more or less as shown in the 1st pic below. No, my soldering skills aren't great, but this looks easier to fit and solder than other types of plugs. I'll drill 5 tiny holes in the case, solder the signal to the centre pin, and glue the other 4 pins to the case for support. Should work ... shouldn't it?!

Still a little puzzled about the braid shielding on the coax. Is it important to make a shield connection between the receiver case and the SMA connector? I've put my multimeter across the two points indicated by the blue arrow-heads in the 2nd pic, and was surprised to see a connection exists. Not just on this one, but I've opened up other wireless devices I have with the same receiver module, and they're all the same.

Regards,

Ian

Edited May 8, 2009 by ianparky

[Terry]

Yes it is very important to have the braid correctly connected to the outer part of the SMA and the other end will be connected to the case. Making a bad job of this will cost signal and at only 10mW you need it all

Its good to mount the SMA so that any twisting of the aerial dose not crack your solder joints, this is an easy way to loose signal.

EDIT, I just looked at your SMA and its not the best type to use, what you want is one that is designed to connect to a co-ax not a pcb.

Terry

Edited May 8, 2009 by Terry

[ianparky]

Fair enough Terry, makes sense. As Ron mentioned, best job would be to get a SMA connector with the coax already fitted. Do you know a source in the UK for such a fitting?

Ian

[Mr.RC-Cam]

I'll drill 5 tiny holes in the case, solder the signal to the centre pin, and glue the other 4 pins to the case for support.

If you decide to use an SMA on the sardine can, then *everything* needs to be soldered, even the four pins you mentioned. The RF ground on the SMA requires maximum surface contact to provide the proper RF grounding.

Here are some examples of how others hacked-on an SMA to different types of RF modules:

http://www.rc-cam.com/forum/index.php?showtopic=1495

http://www.rc-cam.com/forum/index.php?showtopic=1836

You might have noticed that great care went into soldering the SMA's body (ground) to the RF Module's RF ground. These little details are important.

[ianparky]

If you decide to use an SMA on the sardine can, then *everything* needs to be soldered, even the four pins you mentioned. The RF ground on the SMA requires maximum surface contact to provide the proper RF grounding.

Here are some examples of how others hacked-on an SMA to different types of RF modules:

http://www.rc-cam.com/forum/index.php?showtopic=1495

http://www.rc-cam.com/forum/index.php?showtopic=1836

You might have noticed that great care went into soldering the SMA's body (ground) to the RF Module's RF ground. These little details are important.

Thanks for that, just what I was looking for! Of course, to this noob it raises more questions, specifically ...

I've attached a pic from one of the examples, but how is the SMA centre pin connected to the RF module? If it's the same layout as my module, the antenna connection point is some way away, in the middle of a long side.

Ian

[Mr.RC-Cam]

The center pin is soldered directly to the pad on the RF module's PCB. Full details are shown in the first link.

[ianparky]

Sorry, I see it now.

I think I will have a go at mounting the PCB-mount SMA directly to the receiver, bearing in mind my soldering has got to be spot on. What can possibly go wrong?! I'll shuffle off now, before my questions get really stupid.

Thanks for all your help guys, what a friendly and helpful place this!

Ian