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Apr 19, 2012, 03:19 PM
Registered User

Can't get more range with Lawmate's 1.2 ghz TX/RX than with default rubber ducky

Hi all,

I just tried some antenna's for the Lawmate's 1.2ghz TX/RX videosystem. See picture.

Standard comes the Lawmate little rubber ducky antenna which is 9.40cm in length (measured from the point where the active part of the antenna comes out of the base just before the joint)

It's performance is pretty ok, but I don't understand the 9.40cm length. According to this antenna calculator, for a quarter wavelength the length should be 5.94 cm. There is a little coil inside the base though, so that's probably got something to do with it, but from what I've heard it's there more so the antenna can be rotated form the base without tearing a wire. Anyone know more about it's inner workings?

I also got these chinese el-cheapo antenna's from China. They don't have a coil at the base and the active antenna part (measured from the end of the gold colored metal tube) is 5.90 cm So that's exactly tuned to 1200 mhz.

For 1200 mhz both antenna's are pretty ok, but with the TX set to 1010 mhz I figured both antenna's would be performing sub-optimal. Testing reveals the usable range is indeed decreased a bit... So I soldered a little extension the the cheap chinese antenna so it measures 7 cm. Unfortunately it doesn't make any difference at all. I measure this by the way by placing the TX at the back of my backyard (with proper cooling ofcourse) and the RX without antenna on my computerdesk inside. This way I can just about make out the picture I'm getting with the default Lawmate antenna on the TX. I use this as a reference.

So why doesn't the 'tuned' antenna make any difference at all? Looking closely the standard Lawmates antenna always seems to perform just a hair better, even compared to the 'tuned' Chinese antenna!

Next: the long antenna's from China. They are rated 1200mhz and 7dB. Opening one up reveals the exact same length metal tube at the base as used on the smaller versions. The three radiating parts (excluding the coils - they are 1.3cm) are 5.8 - 7.9 and 8.5 cm long, where only the first part more or less equals quarter wavelength. Any idea how these other parts/coils relate? Anyway this antenna performs even worse... I don't get a picture at all! And when I put the Lawmate antenna on the RX it's even a bit more noisy compared to the default setup with both the Lawmate's antenna's.

By the way this is setup on a FPV truck, so that's why I got those 7dB antenna's. To get a more 'flat' donut shape and better horizontal range... Quite a bummer... Luckely they where pretty cheap and I can use some parts of them when needed.

To finish this up: What's the best thing to do to get more ground range? I already did the SAW filter mod on the TX (most annoying solder job ever haha!) and that did improve the picture a little bit! I'm thinking about a high gain omni, but I lack a bit of the knowledge for making it high gain... Oh and should't the metal tube found in the chinese antenna's equal the length of the active part? And last but not least, can I use the aluminum chassis plate of the car to increase range? I already put the TX flat on the chassis plate for cooling purposes but it might help with the grounding too?

Last edited by Bram1982; Apr 19, 2012 at 03:29 PM.
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