Would this be worth trying?

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VK4WDM
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Would this be worth trying?

Post by VK4WDM » Sun Jan 26, 2014 8:33 pm

I have a commercial ADS-B antenna that is is mounted at 6m high. The run of coax from the antenna to the PlaneGadget receiver is 16m and I am using LMR240. The results are reasonable but I would like to improve them. I have tried a pre-amp at the antenna end but the improvement was hardly noticeable. It is impossible to use a thicker coax such as LMR400 or helix, so would the following idea work?

Connect the receiver (mounted in a weather-proof box) directly to the antenna and run a USB cable back to the computer in the shack. I guess the question is: what would result in lower losses, a 16m run of LMR240, or a 16m run of USB cable?

73

Wayne VK4WDM

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VK3ALB
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Re: Would this be worth trying?

Post by VK3ALB » Sun Jan 26, 2014 8:52 pm

Perhaps you ought to work out why the preamp made no difference. Lot's of things to check there before you start changing coax and running USB leads.

Can you test it on a bench?
Lou - VK3ALB

I'll decide how I enjoy my hobby.

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Re: Would this be worth trying?

Post by VK6ADF » Sun Jan 26, 2014 9:04 pm

Just wondering what distance you are receiving without the preamp and what distance with?
73 Phil...VK6ADF

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Re: Would this be worth trying?

Post by VK3KH » Sun Jan 26, 2014 9:08 pm

Wayne,
Something wrong there!
What freqency is the preamp?
I was running an ADSB receiver system using a 16 element collinear, and I added a Minikits 1296MHz preamp, modified to 1090MHz. This is a really easy conversion. I was getting 26db of gain, <1db NF and using a T-bias fed the 12v up the coax.
This made a remarkable difference to my receive coverage, with regular coverage (ie no propogation) being out to 350-400km.
If your preamp is working correctly you should see a dramatic increase.

Michael VK3KH

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Re: Would this be worth trying?

Post by VK4TI » Sun Jan 26, 2014 9:11 pm

VK4WDM wrote:I have a commercial ADS-B antenna that is is mounted at 6m high. The run of coax from the antenna to the PlaneGadget receiver is 16m and I am using LMR240. The results are reasonable but I would like to improve them. I have tried a pre-amp at the antenna end but the improvement was hardly noticeable. It is impossible to use a thicker coax such as LMR400 or helix, so would the following idea work?

Connect the receiver (mounted in a weather-proof box) directly to the antenna and run a USB cable back to the computer in the shack. I guess the question is: what would result in lower losses, a 16m run of LMR240, or a 16m run of USB cable?

73

Wayne VK4WDM
anything over 4 or 5 metres for usb becomes very marginal , 16 would be heighly problematic and likely to fail if it worked at all

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Re: Would this be worth trying?

Post by VK2OMD » Sun Jan 26, 2014 10:03 pm

VK4WDM wrote:I have a commercial ADS-B antenna that is is mounted at 6m high. The run of coax from the antenna to the PlaneGadget receiver is 16m and I am using LMR240. The results are reasonable but I would like to improve them. I have tried a pre-amp at the antenna end but the improvement was hardly noticeable.
"Hardly noticeable"?

It is not clear, but I am guessing that this might not be an objective assessment of sensitivity, but rather observation of the effective range of the system.

But, lets look at the sensitivity angle because that is where you are going with the proposition of eliminating the 16m run of coax.

The matched line loss in 16m of LMR240 at 1090 is 4.4dB.

Lets say your receiver had a NF of 2dB, then the NF at the antenna connector would be 6.5dB, or Teq=1005K.

You didn't put numbers on the preamp, but a good masthead preamp (that does not suffer from seriod IMD problems) of say 1dB NF (Teq=75K) and gain of say 20dB (100x) would result in a system Teq=75+1005/100=85K.

Wow, that is a dramatic improvement in Teq at the antenna connector!

Now this analysis is purely hypothetical because you didn't provide the relevant data for your own scenario... but it does illustrate that a good preamp should dramatically reduce the system equivalent noise temperature and Noise FIgure, and unless the external noise was so high to mask that improvement, there should be a significant difference in receiver performance... quite likely better than obtainable by eliminating the 16m of coax as you proposed.

In practice, a lot of preamplifiers depend on antennas for a large part of their front end selectivity, and may be quite prone to serious degradation by IMD when used with a less selective antenna. It is entirely possible that a poor preamp degrades system performance in a real world application.

I have seen a number of designs for ADSB antennas that were seriously flawed (one recently in Silicon Chip for example), hopefully your commercial antenna is a good one.

Owen

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Re: Would this be worth trying?

Post by VK4WDM » Mon Jan 27, 2014 9:14 am

Hi All

As I guessed, the idea of a longer USB cable is not a good one. The antenna is a ZMX technology dedicated ASD-B antenna. The pre-amp I tried is one I was given by another ASD-B user. Its pedigree and specs are unknown. I do have a Mini-Kits 23cm preamp which I can tweak to frequency. That is the obvious way to go. Thanks, guys :D

73

Wayne VK4WDM

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