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Terry

75 OR 50 ?

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I have been using my 2.4ghz set up for some time with very good results but the other day I was told that my receiver is 75ohm and I am using 50ohm aerial and co-ax. Dose it make a big differance to the receiver ? How much better would the range be if I swap to 75ohm gear ? Is there a simple way to match 75ohm to 50ohm gear ? What are you guys using and are you sure ?

Thanks, Terry

UK

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If you are sure your Tx's drive impedance is 75 ohms, then using 75 ohm coax will slightly improve efficiency. BUT, only if your antenna is constructed for 75 ohm operation too. Mixing a 50 ohm antenna into the 75 ohm RF chain just puts you back to square one.

Worse yet would be if your Tx was really 50 ohms, but you used 75 ohm cable, with a 50 ohm antenna. That is a double whammy. And many of the popular antennas are really 37 ohms, so matching is further affected when they are used with 75 ohm systems

Frankly, typical coax (examples: RG-174, RG-178) is very lossy at microwave frequencies. Several inches of it can affect range. Ten feet will cut practical range at least in half. Do what you can to get rid of it.

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You seem to be talking about the effects on my TX but it is my RX that is 75ohm !

I forgot to say that I am using a pre-amp on my aerial that is 50ohm and then 3metres of 50ohm co-ax to the receiver. Do you think it would be better to use 75ohm co-ax ?

Terry

UK

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The effects remain the same. The RF does not know if it is from a Tx or an Rx antenna.

If your preamp (or antenna) is 50 ohm then you will not gain much using 75 ohm coax. That will just move the mismatch from {here} to {there}.

FWIW, my comment about the coax losses should be noted. Common coax, at microwave freq's, is VERY lossy. About 0.6dB per foot. If you have three meters of it then your achievable range is cut at least in half. You would be better off dumping the preamp (noise figure issues) and installing the antenna directly on the Rx. You can always run video coax and power up to the Rx/antenna package.

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I see your point but the performance of my pre-amp is better than my RX, measured at 2320MHz, with NF=0.3dB and Gain=17.5dB, using an MGF4919 HEMT.

I am sure the pre-amp is making up for my poor mismatch as the range is doubled when I use it, I get a good signal at 1/2 mile on 10mW. Is it possable to use an aerial tuner type matching network at these frequencys ?

Terry

UK

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... aerial tuner type matching network

I have not seen any for sale. Keep in mind that the improvement of a better 50/75 ohm match would be minimal. Reducing the coax length a foot or two would probably help as much.

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Ahh thats very interesting, I was worried that 50 to 75 ohm was a long way out.

So there is not a problem ! Is there anyway to put a figure to the signal lost or should I just forget it ? Dose anyone else have any thoughts or comments ?

Thanks for your help.

Terry

UK

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Impedance mismatches result in VSWR issues. VSWR losses, on their own, are not always dramatic. For example, a moderate VSWR of 2.6:1 results in only about 1 dB of RF loss. That is about as much signal loss as 16" of common coax will cause. I do not know what the typical VSWR is when a 50 ohm component is used in a 75 ohm system, but it is not that nasty. I have used 37 ohm antennas in 50 ohm apps and managed very low VSWR's.

However, the standing waves (which can even occur on any antenna system) can produce grave results due to other effects, as outlined by cyberflyer's tests: http://www.cyber-flyer.com/main/testing.html

You will see that just a couple of mm's of coax length tweaking can affect the signal. Elimination of the coax helps tame these issues, as well as reduces the signal losses due to its poor efficiency at 2.4Ghz. So, that is why I do not promote its use in our low power app.

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You would be better off dumping the preamp (noise figure issues) and installing the antenna directly on the Rx.

Yesterday I tried doing as you surgest, I have not tested in the air but I have used a neigbours video sender signal about 200ft away. First I connected as normal (for me)'patch+pre-amp+co-ax+RX' resulted in a good picture.

Then 'patch direct into RX' which gave a poor picture.

Then I tried 'patch+pre-amp+RX' which gave a good picture again.

Then I tried 'patch+co-ax+pre-amp+RX' and this gave me almost no picture at all. Last I tried 'patch+co-ax+RX' and I had no picture what so ever, just some mush.

I can not tell you the make of my RX as it is not marked but it is simular to lots of others (on the outside). I believe that most of the receivers available are made for short range and are not of high performance, Im sure mine is not the only set up that would benifit from a low noise front end. Im not sure if the co-ax after the pre-amp dose any harm but it dose not show in the results. I would like to hear from anyone that has tested a pre-amp as I did and maybe got a different result ???

I will try and do some tests later with some figures to make thing clearer, when the weather is better, may be a long wait in the UK !

Terry

UK

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It sounds like your low NF amp is earning its keep. It's rare to see only a .45dB NF (they tend to cost more than the video Rx itself). It sounds like you might have the one from Down East uWave, which is an affordable kit.

Im not sure if the co-ax after the pre-amp dose any harm but it dose not show in the results.

It will reduce your achievable range. The loss of the picture, when the preamp is omitted ('patch+co-ax+RX'), is an example of that. At microwave freqs coax is a loss burden; it is best to do what you can to keep the RF gain budget as high as possible.

Edited by Mr.RC-Cam

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Yep, Ive had it a few years now and it transformed my set up at the time. I built it from this kit http://www.g3wdg.free-online.co.uk/preamps.htm and a freind tweeked it for me with his test gear. I have tried to leave it out to make my set up simpler but the fact is it dose perform better with it in. I think if I was using more power as most of you are ? then I could leave it out and connect direct as Im sure your right and that is the best way to keep it cheap. As it is I use as little as 10mW at times when only flying at around 1/4 mile.

Terry

UK

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Hi Terry:

I think what you might be missing is several key factors. Receive gain is best accomplished with the antenna. A simple co-linear antenna or PC board yagi can give as much gain as the amplifier. The coax you mention is terrible for 2.4 GHz. Rule of thumb is every 3 db is 1/2 the signal. The coax you mention has a loss of 3.2 db per foot! If you went to RG-6 it would be more like .8 db per foot.

Part of the "getting the best reception" formula is to have the transmitted signal be as strong as possible. Again, incorporating a good transmitting antenna, especially with a decent ground reference can make a big difference. No amount of amplification can make up for what is not there to start! It's the old engineering question: Is it better to blow through a straw or suck from it?"

One recommendation to solve your basic mismatch problem, is to write to the designer of the pre-amp and ask him/her how to change the output impedance to 75 ohms to match you receiver! You may find the modification very simple. Second is to make the antenna system as efficient as possible. This can be done by "tuning" the antenna, that is, cutting the antenna lenghts to be resonant for the exact frequency you are using or using larger elements to make the antenna more "broadband". Silver plating components with a product by Cool Amp, also helps.

Best, Ed (WB6NSN)

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Hi,

A simple co-linear antenna or PC board yagi can give as much gain as the amplifier.

I have used a yagi to get the gain but it has a very narrow beam. The point in putting the pre-amp directly on the aerial is to collect as much signal as possible and boost it so that the co-ax loss dose not cause a problem. Also the pre-amp I have has a better noise figure than my RX which was proofed by my tests.

The coax you mention is terrible for 2.4 GHz. Rule of thumb is every 3 db is 1/2 the signal. The coax you mention has a loss of 3.2 db per foot! If you went to RG-6 it would be more like .8 db per foot.

I did not mention the co-ax I use, I think you must have read the post from Mr RC-CAM and got mixed up, for the record I use URM67.

Is it better to blow through a straw or suck from it?"

I always tell my little boy not to blow through his straw !!

One recommendation to solve your basic mismatch problem, is to write to the designer of the pre-amp and ask him/her how to change the output impedance to 75 ohms to match you receiver!

Very good point but since doing the tests I think Mr RC-cam is right and the mismatch is not worth worrying about.

You may find the modification very simple. Second is to make the antenna system as efficient as possible. This can be done by "tuning" the antenna, that is, cutting the antenna lenghts to be resonant for the exact frequency you are using or using larger elements to make the antenna more "broadband".

I have now done this now by optimizing the sizes of my patch aerial for the frequency I use as since I got a reflectometer I have found the old designs off the web were not as good as I thought. But to be fair even the worst performer was not far under the best when used only for receiving.

Silver plating components with a product by Cool Amp, also helps.

Never tried this !!! Tell me more please.

Thanks Terry

UK

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Hi Terry:

If you go to www.cool-amp.com you can see the product. basically, if you have a clean surface of copper or brass, you take a damp cloth, add some of the powder and rub it onto the surface you wish to silver plate. In the order of conductivity, it's silver, copper, gold then aluminium. Silver is the best conductor and notice most high frequency components are silver.

Moving on to antenna's, you might look at using a colinear antenna for the transmitter. The advantage is you can get some very good gain and still remain omni-directional. At 2.4 GHz the lenghts are short, therefore with a colinear, the first section is 1/2 wavelenght and the other sections are 1/4 wavelenght. As you add sections, you increase gain so the only limit is how much lenght you can accommodate. A basic colinear takes the second section and you attach the center conductor to the shield of the first section and the shield of the second section to the center conductor. You alternate this for as many sections as will fit.

Most transmitter installations I've seen only use a whip antenna with no real ground reference as part of the antenna, thus the results are minimal.

Best, Ed

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That cool-amp sound like interesting stuff, im not sure if I can get it in the UK. Where dose brass come on the list of conductivity ?

Im not sure about a colinear as I feel that I would suffer more drop outs when banking at long range and as you say the length would also be a problem as I hang it out the bottom of the plane, the dipole I have works fine.

Most transmitter installations I've seen only use a whip antenna with no real ground reference as part of the antenna, thus the results are minimal.

I think you are mistaken, most of the whips are in fact dipoles that look like whips.

Thanks again, Terry

UK

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Hi Terry:

Brass is mostly copper so that is where is falls. What I was pointing out to you is that a dipole does not do as good a job as say a ground plane style of antenna. Dipoles are typically 300 ohms so again, I wonder if you have a true dipole? Most off the shelf systems I've seen use a single wire off the output of the transmitter so not seeing what you have or what you are working with does make it a bit difficult to come up with soild recommendations for improvements.

Generally, in airborne applications there is no advantage of transmitting up above the aircraft unless you are at a lower frequency and plan to bounce off the ionisphere. Best (IMHO) would be a ground plane type if a colinear is not practical. Again, pictures of your system would be helpful.

Best, Ed :)

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Dipoles are 73 ohms. They can be modeled as center fed 1/2 wave antennas. They are great for 50 or 75 ohm RF apps.

There is a quirky 260 - 300 ohm dipole made from twinlead that some hams like to build (I think they call it a Folded Dipole). But, it is not at all like the dipoles that we use.

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Im not looking to improve the set up, I was originally just asking if I was loosing much due to the mismatch and it turns out Im not. So Im happy !

Thanks for your interest though, its good to bounce these things about.

Terry

UK

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