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 Post subject: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 3:42 am 
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paulg1953 - FlightAware user avatar

Joined: Mon Feb 13, 2017 10:25 am
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I have a Flightaware antenna followed (via a very short cable) by a LNA, between the LNA and dongle I have around 30 feet of RG58 coax, this cable has a loss of around 22db per 100 feet at 1GHz so I'm loosing around 8db. As this is after the LNA is this important?
Would I see much difference in performance if I were to replace this cable with, say, RG6 which has around 6db loss per 100 feet at 1GHz - RG6 is 75 ohm so I think I would see a small loss due to impedance mismatch.
Just to clarify, I using a laptop/windows setup, so I can't move it closer to the antenna.
Thanks,
Paul


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 7:40 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1980
Location: Toronto CYYZ
I live in an apartment, so my installation (including antenna) is strictly indoor.

When I started this hobby in 2013, the DVB-T was plugged into my Windows desktop in my room, connected to the antenna near a large window by 45 ft of RG6 cable. The LNA was located close to antenna. The LNA was a narrow band Satellite Amplifier (K-band 950 to 2050 Mhz, 18 dB RCA D903).

The system performed very good with various types of home brew antennas.

In 2015 I purchased RPi, placed it under antenna, plugged dvbt into RPi, connected to antenna with a few feet long piece of RG6 cable and removed Satellite amplifier.


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 7:55 am 
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ToyonADSB - FlightAware user avatar

Joined: Tue Feb 21, 2017 10:27 pm
Posts: 17
Any additional coax loss will cost you something in terms of SNR for the more distant signals. Some of that depends on the quality of the LNA, so it's hard to quantify how much you'll lose.

If you want to eliminate coax losses as much as possible (without costly low-loss coax), put the receiver as close to the antenna as possible. I used a 32' active USB extender cable and put my FlightAware Pro+ in the PVC antenna mast (sealed from the weather), so it's just a couple of feet from the antenna -- pretty much eliminates any coax loss. And my PiAware hangs on the wall of my ham shack at the other end of the 32' extender.

Typically, even a passive USB extension cable will work well out to 20' -- and active USB extenders can be as long as 85' (or more with a few tricks).

The downside to this is if you're powering your LNA through the coax, you'll have to find another way to feed it power. But the improved performance might be worth the effort.


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 8:08 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1980
Location: Toronto CYYZ
Image
2013 Photo: My first DIY ads-b antenna.
Length of RG6 coax between Satellite Amplifier and DVB-T = 45 ft

Note: The write-up on picture mentions amplifier #2 down the stream. This was initially done. Later I discovered that 2nd amplifier is actually overloading dvbt, and when I removed 2nd amplifier, the performance improved.


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 6:36 pm 
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paulg1953 - FlightAware user avatar

Joined: Mon Feb 13, 2017 10:25 am
Posts: 3
Regarding long usb cables, I thought I read in another forum that they interfered with mlat operations, something to do with timing issues. I assume you've never had these issues.


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 8:09 pm 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1980
Location: Toronto CYYZ
paulg1953 wrote:
Regarding long usb cables, I thought I read in another forum that they interfered with mlat operations, something to do with timing issues. I assume you've never had these issues.

RG6 is not a costly cable. The length involved is also 30 ft only, so the total cost will not be much (around $15). Why not replace RG58 by RG6 and see the result?


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Wed Mar 15, 2017 8:26 pm 
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FlightAware Staff
obj - FlightAware user avatar

Joined: Tue Sep 30, 2014 7:14 pm
Posts: 3459
paulg1953 wrote:
Regarding long usb cables, I thought I read in another forum that they interfered with mlat operations, something to do with timing issues.

More specifically it is USB noise - if the USB bus starts dropping messages due to noise, which is more likely with a longer cable, then mlat timing will suffer (the dongle does not appear to buffer samples, if a block of samples gets lost to noise it's gone forever). Long USB runs aren't inherently a problem so long as they are good quality and don't cause noise issues.


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Thu Mar 16, 2017 3:27 am 
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paulg1953 - FlightAware user avatar

Joined: Mon Feb 13, 2017 10:25 am
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abcd567 wrote:
RG6 is not a costly cable. The length involved is also 30 ft only, so the total cost will not be much (around $15). Why not replace RG58 by RG6 and see the result?

Yes, I think this is what I'll do, It'll be a simpler solution for me than trying to feed a long usb cable through a door frame


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 Post subject: Re: Cable losses after LNA - do they matter?
PostPosted: Sun Mar 19, 2017 11:36 pm 
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mattbna - FlightAware user avatar

Joined: Tue Jan 17, 2006 4:00 am
Posts: 98
Location: Nashville, TN USA (KBNA)
abcd567 wrote:
RG6 is not a costly cable. The length involved is also 30 ft only, so the total cost will not be much (around $15). Why not replace RG58 by RG6 and see the result?



If anyone spends $15 on a 30ft section of RG-6 - they're getting royally screwed. You can pickup a 500ft roll of quality RG-6/U (quad-shield) on Amazon for $50. You can then cut the cable to the exact length you need. You'll have to supply your own fittings (of course), but even that is cheap.

Your cheapest option would be to find a Comcast/Charter/whoever cable tech somewhere around your town and ask them to cut you a 30ft length and install the compression fittings for you (crimp and screw-on coax fittings are garbage). I did this all the time for random people when I was a field tech for Comcast for almost a decade (easy to do when you carry around a few thousand feet of have hundreds of fittings in the truck). I also handed out splitters to people all the time as 99% of what you can buy at a big box or other electronics store is almost always junk - including the ridiculously priced Monster and Belkin goodies. Their cables aren't bad, but you're paying for the name and spending $20-$30 on the exact same cable you can get online for $5 without their name on it.

I really hate seeing people get screwed buying pre-made cables. The markup on cables is absolutely insane and retail stores and online outlets take full advantage of this. Monoprice.com is one of the few places where you can buy cables online and not be taken to the cleaners. Amazon can be cheap if you look around long enough.

Matt

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