Friday, December 11, 2015

LNA + DVB-T = FAQ


I get the LNA connected to the DVB-T dongle but the reception is bad, worst than without the LNA. The noise floor is really high comparing to the signal.

If you connect the LNA strait to the dongle with the SMA adapter the LNA picks up the noise from the dongle or the PC. Move the LNA few meters away and connect it with the dongle using the coaxial cable. Instead, you can use the shielded LNA in the proper metal housing.

Should I position the LNA close to the antenna ?

This should be always done if you aim for the weak signal reception on the VHF and up frequencies. Positioning the LNA close to the antenna will give us lower noise figure of the system and better reception. Other benefit is that after the LNA we can have a long run of cheap, not high quality coaxial cable to the dongle that will not spoil the overall system performance dramatically.

I've heard that I can power supply remotely my LNA using the Phantom power and Bias-T, is it true ?

The remote (close to the antenna) LNA may be power supplied using the coaxial cable, the same one that is delivering the signal from the LNA. If your dongle/receiver have the Phantom power option on antenna connector you can inject the DC into the coax. The LNA should be enabled for such operation.
If you do not have this option on the dongle nor on the LNA you may use so called Bias-T, one close to the receiver and the other close to the LNA. The function of the Bias-T is to inject the DC voltage on the receiver side and to extract the same DC voltage on the LNA side, isolating the RF leads (connectors) from the DC power.

Can I print the housing on my 3D printer ?

3D printers usually use the plastic materials. The ESD can build up on the plastic and damage the ESD sensitive device in the LNA. Using the plastic housings will not help shielding the LNA from the electromagnetic noise/sources. Not recommended.

I use the metal shield/box for the LNA but the reception is bad and the noise is very high!

You probably end up with the oscillator rather than the amplifier. The probable cause are ground loops created due to long antenna connector leads at the input or the output. The best way is to use already soldered connectors and run a small jumper coaxial cables to the shielding walls. The oscillations may occur also due to housing dimensions, creating the resonator at some higher frequencies.

My reception is still bad!

You may have the problem with the desensitization. Your receiver is overloaded by the strong blocker (radio transmitters) in or out of the band. Reduce the gain in the dongle manually. You may have to turn also the AGC off. Use the filter to filter out of the band blockers.

I bought the filter, should I put it before the LNA ?

Inserting the filter before the LNA (between the antenna and the LNA) will spoil the noise figure roughly for the filter insertion losses expressed in dB. So the S/N will also be degraded for the same figure.

I bought the filter, should I put it after the LNA ?

Inserting the filter after the LNA (between the LNA and the dongle) will leave the LNA widely opened to the strong signals. This may be fine if you have the high IP3 LNA.

OK, where should I insert the filter then ?

If you aim for the weak signals reception and you have the high IP3 LNA you need to install the filter after the LNA. If your LNA can not stand the strong blockers (broadcast TV, Radio, Cell towers) you need to protect it with the filter, otherwise the LNA may produce reach intermodulation products that may be spread all over the frequencies masking the other signals on the band. More over the result may be a bunch of phantom signals that are not existing on the bands.

I bought the SAW filter, any good ?

The saw filters are compromise, small and sharp filters but with the higher insertion losses (2db and up) than the other filters. They may be quite expensive too. Avoid positions between the antenna and the LNA for the SAW filters if you can. They are perfect solution for the post LNA filtering.

What does the high IP3 LNA mean?

IP3 stands for 3rd order intermodulation products. You want to have this number as high as possible. The LNA with the IP3 (OIP3) number higher the 30dBm is already considered the high IP3 LNA.

What figures I should look when buying the LNA ?

Depending the purpose where you want to use your LNA you may choose some characteristics better than the others but generally you are looking for a low NF (noise figure) number, high IP3 number, and the higher – the better gain number.

I do not see the IP3 figure declared, how can can estimate the IP3 performance ?

If you can find the P1dB figure, than as a rule of the thumb you may estimate the IP3 some 10-15dB higher than this value. If you read the P1dB of 20dBm, you may estimate the 30-35dBm IP3. The latest modern LNAs have even bigger difference then the mentioned 15dBm. High IP3 LNA draw much more current (50-120mA) for the single stage comparing to the low IP3 LNA where the consumption may be not higher than 10mA.

My LNA stop to work after the first storm, what is wrong ?

Depending on the antenna type used, the ESD may build up and damage the ESD sensitive parts in the LNA. The LNA should be repaired.

How can I protect the LNA from the ESD ?

The best protection is to disconnect the antenna from the LNA or any other device. This may not be simple if the LNA is close to the antenna. The most common solution is using a pair of anti-paralel diodes (BAV99). They can introduce the IMD products if the strong signals are present in the antenna.
Good protection is to use the antennas designed to have the DC grounded hot end antenna connector.
Another option is to use the shorted quarter wavelength long coaxial stub resonant on the frequency of the interest.

I would like to use cheap RG-6 coax from antenna to LNA

The RG-6 is 75 ohms coaxial cable widely used in TV/SAT and radio installations. The losses in the cable are 6.2dB for a length of 100ft (30 mtrs) at 1.000 Mhz so let say close to ADS-B frequency. 50Ft long run of the coax from the antenna to the LNA will introduce roughly 3dB loss in the signal or 50% loss of power.

My LNA has the impedance of 50 ohms and the RG-6 has the 75 ohms, will it work ?

It should work. The mismatch/SWR between the 50 and 75 ohm is 1:1.5, means reflected power of only 4%. In terms of dB, the losses are lower then 0.2dB due to mismatch. You can not see the difference in the reception for such a small loss. If you run the long run of the coax, the coax impedance mismatch losses are negligible comparing to the losses introduced by the coax itself.

I would like to use the F connectors on my setup

The F type connectors are designed for a 75 ohms coaxial cable, but they should work also with the same diameter 50 ohms cables. The losses are bit higher than the standard SMA, BNC or N type connectors but if you are not in the weak signal operation the connectors can perform good enough up to 1GHz range.

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