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Sennheiser G3 sma mod and external LPDA issues


tonymuricy
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Hi Folks!

 

So I did my own SMA antenna mod on my Sennheiser G3 receivers.

And I said, ok, If they now have the sma plugs, it might be a good idea to have and external LPDA antenna to then, so this way I could have all my rx on my cart, and have then with longer range!

So I bough a rf passive splitter from 1 in to 2 outputs, and 2 passive splitter with 1 in to 4 outputs, this way I could send 1 antenna signal to 8 rx.

I bought a LPDA printed antenna which claims to have 18dB of gain. To compensate for rf loss on the passive splitters, I also bought a chinese RF amplifier circuit with more 20dB of gain. As I knew all this amplification this could overload the receivers, in the other hand, I bought a variable resistor, which went just after the RF amp circuit, before the passive distro circuits. This way I could turn down amplification before overloading my receivers, by checking effective RF gain in the receivers screens.

It did worked, in one side! I did have a strong signal with all my rx in my cart, the boost was very visible when I did power on the RF circuit amp! Great!

But there is one problem - everything is going fine, and suddenly rf signal drops in intensity violently, and audio is lost.

I couldn't find out if this was external interference or an internal problem with my system  - maybe it is overloading rf gain anyway. But the RF signal in the receivers looses intensity, it is not going up is going down!

This became happening very frequently, so I had to go back to my old system - to have my RX grouped on a tripod near the border of the set, and have a multicable bring audio to my cart. The old system proved more stable than the new one. 

Maybe I should try to have two sam antennas on each receiver - doubling this set up, so I could take advantage of the best signal on each antenna.

I'm not an engineer, so I have limited knowledge on these subjects.

What I am doing wrong? Everything maybe?

I know this whole thing may sound a bit crazy, but when you live in a country were 1 dollar is 4 times 1 real, and everything you bring officially in pays 100% in taxes,  it makes sense to try to get the most of the minimum, you understand.

I am attaching some pictures and any suggestion or ideas will be most welcome.

 

Thanks 

 

Tony Muricy - Rio de Janeiro, Brazil.

IMG_5663.jpg

IMG_5664.jpg

IMG_5665.jpg

IMG_5666.jpg

I forgot to say that I goy a 15m long sma cable to connect my LPDA antenna to my rf amplifier on my cart. This way I can move the antenna to the best spot.

 

Thanks

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Hi Tony,

Just some thoughts so you can plan a bit better. An LPDA with a claimed 18 dB of gain is almost impossible. Looking at the size of your LPDA and the number of elements it might have 8 dB of gain DBi (isotropic) or 5 dBd (5 dB of gain relative to a diplole antenna). You don't need to worry about the 5 dB of gain as far as overload goes since it is noise free gain, i.e., it's just a good antenna.

 

Assuming your splitters are decent, the 2 way split has theoretically 3 dB of loss to each port and the 4 ways have 6 dB of loss to each port. Let's say a total of 10 to 11 dB of loss to each receiver if there are some other losses (imperfect splitters, cables, connectors, etc.). So you need 10 dB of gain before your splitter arrangement to provide each receiver with the correct level that is above the noise floor of the receiver but below overload points. Again, don't consider the antenna gain, as that is noise free gain.

 

You need a strong 10 dB amplifier before your splitters. The amp you have is 20 dB and is not very robust. This means it can overload and produce intermod because it has excess gain and a low output third order intermod rating. It is designed for amplifying weak TV signals out in the boonies, not in an urban environment with multiple transmitters (yours and perhaps the film crew). I'm afraid that is asking for intermod and interference. The attenuators you have after the amp will protect the receivers but do nothing to improve the output overload rating of the 20 dB amp. 

 

If you were in the states, I would recommend this amp. It is high powered and low gain:

 https://www.lectrosonics.com/All-Accessories/product/ufm230-ufm230l-2.html   

If you can find something like that it would definitely improve your setup.

Best Regards,

Larry Fisher

p.s. Do you have specs on the amp you have?

Edited by LarryF
added p.s.
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Hey Larry!

 

I was thinking about this RF amp, if it could be the source of the problem.

I don't really have it's specs, it's a cheap chinese one I found in our brazilian Ebay equivalent.

I thought about trying the PSC RF Multi SMA as amp, but I'll take a look at your Lectro suggestion!

Do you know where I can buy it?

 

Thanks!

 

Tony

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The quandary of having to pay heavy import duties does present a challenge. Given that restriction, I would approach the matter differently. 
 

The Ramsey antenna you are using (the green circuit board shark fin) is a perfectly capable antenna. From my own tests, it has comparable performance to units from PSC, Lectro and others. It’s not very attractive, it has sharp edges and you must construct your own mounting hardware. But as an antenna, it’s fine and it’s available very cheaply. 
 

I suggest that you purchase several additional Ramsey antennas and rig them so that a single antenna serves only two radios. (Of course, with diversity radios you’ll need two antennas for two radios.) With a cluster of antennas, you’ll need to split the signal only once per antenna. That’s a minimal loss (3dB) that would not require any amplification at all. 
 

If your six radios are all diversity, you would employ six antennas and that is a bit of a cluster but all your hook-up hardware could be first class and the cost would be manageable. 
 

David

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  • 3 months later...
On 10/26/2019 at 10:35 PM, LarryF said:

Hi Tony,

Just some thoughts so you can plan a bit better. An LPDA with a claimed 18 dB of gain is almost impossible. Looking at the size of your LPDA and the number of elements it might have 8 dB of gain DBi (isotropic) or 5 dBd (5 dB of gain relative to a diplole antenna). You don't need to worry about the 5 dB of gain as far as overload goes since it is noise free gain, i.e., it's just a good antenna.

 

Assuming your splitters are decent, the 2 way split has theoretically 3 dB of loss to each port and the 4 ways have 6 dB of loss to each port. Let's say a total of 10 to 11 dB of loss to each receiver if there are some other losses (imperfect splitters, cables, connectors, etc.). So you need 10 dB of gain before your splitter arrangement to provide each receiver with the correct level that is above the noise floor of the receiver but below overload points. Again, don't consider the antenna gain, as that is noise free gain.

 

You need a strong 10 dB amplifier before your splitters. The amp you have is 20 dB and is not very robust. This means it can overload and produce intermod because it has excess gain and a low output third order intermod rating. It is designed for amplifying weak TV signals out in the boonies, not in an urban environment with multiple transmitters (yours and perhaps the film crew). I'm afraid that is asking for intermod and interference. The attenuators you have after the amp will protect the receivers but do nothing to improve the output overload rating of the 20 dB amp. 

 

If you were in the states, I would recommend this amp. It is high powered and low gain:

 https://www.lectrosonics.com/All-Accessories/product/ufm230-ufm230l-2.html   

If you can find something like that it would definitely improve your setup.

Best Regards,

Larry Fisher

p.s. Do you have specs on the amp you have?

Very kind of you Larry.

Now I'm learning.

 

"You need a strong 10 dB amplifier BEFORE YOUR SPLITTERS"

Does it mean the amp attached before splitter, after the long coaxial cable from antenna? Or the amp should be close to the antenna?

 

Many thanks

 

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