Noise to Signal ratio

[hawk1943]

Hi there,

It's been a while since I have used my scanner...

Basically I am an apprentice falconer, don't worry i'm 34 and taking things steady...

But I do not have £450+ to buy a dedicated reciever to receive the telemetry signal being transmitted on 173MHz...

What I have have done is coupled my Yupiteru MVT-7100 scanner brought in 1995 to a 3 Element Tuned Yagi, so that I can receive the incoming signal from a 2 stage transmitter on 173.237.0 MHz...

But after 500 yards the signal to noise ratio is starting to drown out the signal, with the squelch turned off I can pick the signal up at approximatly 2 miles but the noise is pretty much drowning out the signal...

I was thinking of using a band pass filter to help clean up some of the noise etc but not designed anything to work with a scanner... most things electronically I am fine with but for some reason I am a bit hesitant with this...

The BNC is 50 ohms so I presume the whole design needs to be below this thrershold but does that also include the Yagi?

Any help or advice would be greatfully received...

 

[unit_399]

What type of transmitter are you using to broadcast the telemetry signal and how much range do you need ?? If the telemetry unit has an external antenna, raising the antenna as high as possible will increase the range. Ditto for the receiver. If you can increase the output power of the transmitter, this will help too. You are probably at the mercy of high atmospheric noise, and this will be hard to overcome with a low power transmitter. If it is possible to connect your yagi antenna to the transmitter, and use a wire antenna on the receiver this may offer some improvement too.

 

[hawk1943]

Hi Unit_399,

Thanks for replying...

The antenna is approximatly 6inches long and attached by a plectrum to the back tail feathers of the bird I am flying, which this season is a Harris Hawk...


So depending upon whether the bird is on a kill (down at ground level) or perched in a tree depends upon the height of the antenna... this is one variable that cannot be controlled...

The rig cannot be controlled all that much, as the transmitter is limited by weight and only weighs 3 grams anything heavier can restrict the birds flying ability...

The units are brought in, at the pre-set frequency...

The usual distance required is around 5 miles although the manufacture does state that 20 miles is possible... but this must be line of sight and depends upon the bird it is being fitted to a Harris Hawk for instance will not usually travel too far upto 5 miles no more, but a sparrow hawk etc can go much further and 20 miles is possible...

The dedicated receivers can cope with the signal to noise ratio, hence I am convinced that a solution exists as how else can they overcome the same problem...

Only 173MHz is the bio-telemetry wavelength that we all use, the american version of 216MHz is I beleive now a DAB radio station the only other less used frequency is 433MHz as I beleive German and Finish Falconers use this frequency and a handfull of british falconers...

So will a pre-amp and filter arrangement fitted between the Yagi and Receiver help in any way...

I will gladly build any suggestions and give them a try as I do not wish to pay out £450+ for a receiver when I already have one to hand...

So what options are available to me...

 

[C2]

What mode is being used, NFM?

Your receiver is not very sensitive on FM, 0.75 uV. A decent receiver should be around 0.18 uV, about 4 times more sensitive.

A filter and pre-amp might help, but usually a pre-amp amplifies the noise as much as signal, so unless it is very quiet, you might not get what you expect.

 

[hawk1943]

Hi C2,

I am using the scanner in either USB or LSB mode, been swapping between the two to see which one give me the strongest reading...

I know it possible to use a scanner such as the yaesu vr500 and a 173 flexible yagi as these were untill recently available from falconry telemetrics.

So it is possible, just not sure how I can increase the range...

I know even with dedicated ones, you may still only get a couple of miles, it's just that they don't have the constant "hiss" that I am suffering with and drowning out the signal...

Attached is basically the schematic that most manufacturers use...

Telemetry Transmitter

 

[C2]

A filter should help, and you will also want a pre-amp. All components must be a 50-ohm match, optimized for the frequency used.

Raising your receive antenna should help a bit. A 4-element would help a bit more. You are going to want to start stacking gains, as little as they seem.

 

[hawk1943]

Hi C2,

Can you possible explain what is ment by a 50 ohm match, does this mean that all components must add-upto to 50 ohms including the ariel?

Only just a brief calcultion it appears I am going to be using much larger value resistors for instance...

I've not calculated anything on this scale since I did my electronics servicing course in 1995 so could do with a hand...

Also what is meant by stacking gains...

I know an amplifier has a gain etc are you referring to adding these together...

Any help on the circuit and values would be much appreciated...

Attached is a pdf that does appear to contain a pre-amp and filter for a band pass filter, would this be useable...

 

[C2]

your antenna system is 50 ohms, meaning you would not want to use 75 ohm coax because it would introduce a mismatch, causing signal loss.

By stacking gains I mean every small thing you do adds up. For example, you could get a higher gain antenna such as a 4-element yagi, lower loss coax, and those two improvements would combine.

The filter/amp designs are basic conceptual models. They will work, but could have the effect of increasing noise more than they increase signal. component selection is very important

 

[unit_399]

HAWK1943 -

The schematic of your transmitter calls for a battery voltage of 1.2 to 3.5 volts. If you are using a single 1.2v cell, increasing the voltage to 3.5 will increase the transmitter power considerably. Also, When the bird is flying normally, the antenna on his tailfeather will be horizontally polarized (parallel to the ground). This means your receiving yagi needs to be horizontally polarized too for maximum reception.

- 399

 

[hawk1943]

Thanks for that C2, so should I just be using a standard C - R circuit...

The Yagi Arial that I have obtained is made specifically for 173Mhz frequency, Falcon Telemetrics so not sure if building my own would help in any way...

I have tried to upload an image, just not sure if it's clear enough as I got it from the web... but basically it shows how I am attaching the telemetry and the way it hangs down the tail...

At the moment the end-cap only takes 3xLR41(AG3) batteries so 3.6V or I can use the other endcap and use 2xLR754 (AG5) as either is recommended by the manufactorer just not sure what the difference is between the two...

So what can I start off trying?

Would you recommend testing a band pass filter, but does the whole design need to be no more than 50 ohms...

 

[C2]

I should have thought of it earlier, but I would check out an off-the-shelf preamp. I've heard good things about Ramsey's low-cost preamp kits, such as the PR10 for 2-meters.

I would study the circuit, even buy the kit and change a few value to optimise for your frequency. They are about $15 direct from Ramsey or other outlets.

http://www.ramseyelectronics.com/downloads/manuals/PR10.pdf

If you do this, install the preamp as close to the antenna as possilbe, i.e., before the coax. This is because signal loss is generally higher in coax than noise loss.

 

[C2]

Now, you've likely cost me $15.

 

[C2]

Two more, the PR2 (much better) or SA7 broadband kits might just work well enough.

Either one can benefit from an input band-pass filter. The SA7 is a two stage compared to the single stage of the PR10, but they use the same low-noise transistor.

I think you just cost me another $20.

 

[hawk1943]

Shame the frequencys are either side of what I require...

I will see if I can purchase it here in the uk, thank you for that suggestion...

 

[hawk1943]

ooops...

Sorry about that, if your anything like me shortly you will have boxes filled with such stuff that never gets used...