Adding an LNA Made No Difference

[Dominic1967]

I have a question about where in my signal chain to place LNA, FM Bandstop, DC block, etc.

I have an attic-mounted Diamond D130J Discone (thanks, HOA!) and am interested in very general listening- nothing above 900 MHz. The Diamond is ultimately connected to an A/B coax switch in my office via 75 feet of RG6. The switch "A" goes to a Stridsberg MCA204M for my "nicer" SDR and scanners and "B" simply goes to a 4 way splitter and 4 Realistic scanners, a Pro-2004 and three Pro-2006s. (I get nostalgic.) I'm not really concerned about loss with the older scanners, thus the splitter.

I feel that my setup has been adequate for my occasional hobby interests, but wanted to experiment with making up for coax and adapter loss, so I bought an LNA, some attenuators to prevent overload, DC block and FM trap. I'm guestimating that I may lose around 6dB of signal, but I could be wildly off. I was thinking that I would see a dramatic change (good or bad) by adding the LNA.

Here's how things are now set up; Diamond to FM bandstop (Nooelec Flamingo) to LNA (Nooelec Lana, powered by a Jameco linear 5V wall wart) to 75 feet of RG6, ending in a DC block, then 18dB of attenuation (I bought several pads of differing values) to the A/B switch. Again, I just wanted to make up for loss. And maybe give myself a little bump in signal strength. I was thinking the Lana averaged about 24dB of gain.

I'm not noticing much of an improvement or change, if any at all. Even after removing the attenuation. Do I have everything in the right order? It's fun to experiment but going up and down those attic stairs is a pain.

 

[Ubbe]

That LNA seems to be okay and can take a -10dBm signal without starting to compress, so no need to filter anything on its input if you don't have a powerful FM broadcast transmitter nearby.
There's no DC voltage coming out from the LNA at either end of it, so no need for a DC blocker.

Put the LNA at the antenna and set the FM filter on its output. It has 25dB gain at 150MHz and 20dB at 900MHz.
That Stridsberg multicoupler do not like high signal levels.
The 4-way splitter will have something like a 8dB loss and the coax 4dB at 900MHz and less than 2dB at 150MHz.

Try and get a 3dB gain at the input of the Stridsberg and a 15dB gain at the input to the 4-way splitter. Those scanners can handle a lot of signal so it can be a +6dB gain at their antenna inputs.

For 150MHz and the Stridsberg you will need a 20dB attenuator at its input.
For 900MHz you will need a 12dB attenuator.

For the 4-way splitter and 150MHz you will need a 12dB attenuator and for 900MHz a 6dB attenuator.

I would suggest to get a variable 0-20dB attenuator to really test how much attenuation are needed to give the best reception while listening to the background noise in analog reception and adjust for minimum noise, or for minimum databit errors from a digital signal.
I would ditch that Stridsberg and replace it with a passive 4-way splitter as you have a much nicer LNA to use.

/Ubbe

 

[Dominic1967]

That LNA seems to be okay and can take a -10dBm signal without starting to compress, so no need to filter anything on its input if you don't have a powerful FM broadcast transmitter nearby.
There's no DC voltage coming out from the LNA at either end of it, so no need for a DC blocker.

Put the LNA at the antenna and set the FM filter on its output. It has 25dB gain at 150MHz and 20dB at 900MHz.
That Stridsberg multicoupler do not like high signal levels.
The 4-way splitter will have something like a 8dB loss and the coax 4dB at 900MHz and less than 2dB at 150MHz.

Try and get a 3dB gain at the input of the Stridsberg and a 15dB gain at the input to the 4-way splitter. Those scanners can handle a lot of signal so it can be a +6dB gain at their antenna inputs.

For 150MHz and the Stridsberg you will need a 20dB attenuator at its input.
For 900MHz you will need a 12dB attenuator.

For the 4-way splitter and 150MHz you will need a 12dB attenuator and for 900MHz a 6dB attenuator.

I would suggest to get a variable 0-20dB attenuator to really test how much attenuation are needed to give the best reception while listening to the background noise in analog reception and adjust for minimum noise, or for minimum databit errors from a digital signal.
I would ditch that Stridsberg and replace it with a passive 4-way splitter as you have a much nicer LNA to use.

/Ubbe

Wow, thanks so much for that reply- it's really helpful. I do also have the Nooelec variable gain amplifier I haven't gotten around to playing with yet. It has a digital display but no case- it's bare bones.

How close is too close for an FM transmission tower? I live roughly 8.2 miles from the closest tower and that station broadcasts with 4.8kW of power. They are not a powerhouse. The next closest station tower is 20 miles away- they have 280kW of broadcast power. I read somewhere on this forum that you may not even realize a strong nearby transmitter is causing issues until you add a filter and discover stations you never heard before.

 

[dave3825]

You could verify the LNA is working properly by running it directly to any of the sdr dongles. Scan around prior and take some measurements on different freqs, then add the LNA and see if signal and noise floor are higher. Also make sure ant is connected to LNA input.

 

[Ubbe]

I do also have the Nooelec variable gain amplifier

How close is too close for an FM transmission tower?

The other LNA starts to compress at a -10dBm level but the variable gain one, that can do 46dB gain, at -23dBm, pretty bad. You can easily get intermodulation issues in it and for sure you will need the FM filter at its input and hope that you have no strong transmitters at other frequencies. There's usually a relationship between how much signal a LNA can handle and its max gain. The more gain the less signal level it can handle. LNAs that can handle a transmitter on the same roof have only a 10-12dB gain.

How much broadcast transmitters will reduce a receivers performance depends of the receiver. Those Pro2004-5-6 are more or less unaffected, that's why they are still good to use in difficult RF environments. Modern scanners have better sensitivity but are also much more prone to overload. FM trap filters are cheap enough to always be used, and when used at the LNA output you do not have any negative impacts from it but requires a good LNA. But you could just as well have issues from TV and cellular transmitters that needs much more expensive filters.

When using a variable attenuator you can set it to 0dB and listen in analog mode for the background noise from a weak signal, or look at the bit errors from a digital signal, and then add attenuation and check where you get the least noise and less bit errors. Attenuate quickly back and forth on the adjustment to hear or see where the best signal are received. Unfortunately it will usually be a different attenuation needed for different frequency bands but at least you will know if you suffer from overload issues that degrade reception. Discone antennas have it's best gain at 100MHz so they need FM trapfilters more than any other antenna type.

You could verify the LNA is working properly by running it directly to any of the sdr dongles. Scan around prior and take some measurements on different freqs, then add the LNA and see if signal and noise floor are higher. Also make sure ant is connected to LNA input.

SDR dongles are terrible receivers. You can adjust their own gain to a level where they overload and don't need additional amplification. But they also have terrible NF noise figures and if that 1dB NF LNA are used at the antenna then it can be used if the signal gain are not more than 6dB at the SDR dongle and then adjust the dongles own gain to maximum signal noise ratio, not maximum signal or lowest noise floor. A program like SDR# will show SNR when the mouse pointer are over the signal in its spectrum view. Then both coax and splitter losses and their added noise figures can be eliminated, as well as most of the dongles own noise figure.

/Ubbe

 

[Dominic1967]

When using a variable attenuator you can set it to 0dB and listen in analog mode for the background noise from a weak signal, or look at the bit errors from a digital signal, and then add attenuation and check where you get the least noise and less bit errors.

SDR dongles are terrible receivers.

/Ubbe

I was confusing a Nooelec Vega that I have on hand with a variable attenuator. Yeah, I need to get one of those. I realized I also need to measure the output of the Lana to see if there's any voltage. If not, I imagine I can eliminate the DC block.
As far as dongles, I don't really have any. Besides the Realistic scanners, my Stridsberg connects an ICOM R7000, ICOM PCR1500, Uniden BCD436HP and the only SDR I use for UHF/VHF- the Afedri LAN-IQ. I do have other SDRs but they're connected to loop antennas for HF- RSPduo, RSPdx and Airspy HF+ Discovery. I'm not sure which would perform better at those higher frequencies, the Afedri or RSP models.

 

[Pape]

I got a Lana and yes, it does push voltage on the line because of the integrated Bias-tee. As you mentioned, validate it, my lana is couples of years old
From experience the best way to see the change is to test multiples different configuration and so on.
If a way to record quality level for each setup you did. I did it using audio recording and image from the 800 mhz waterfall.
I have a similar setup currently in operation and found out that the sweet spot for me is 7 DB and this only help with 1 or 2 frequency.
I documented by journey here
As for the FM band Stop, remove it and see how is the 120+ MHZ look. The 800 MHZ band also opened up good. If you see no difference with and without the FM band stop remove it as it adds more issue than it solve.

 

[Dominic1967]

I got a Lana and yes, it does push voltage on the line because of the integrated Bias-tee. As you mentioned, validate it, my lana is couples of years old
From experience the best way to see the change is to test multiples different configuration and so on.
If a way to record quality level for each setup you did. I did it using audio recording and image from the 800 mhz waterfall.
I have a similar setup currently in operation and found out that the sweet spot for me is 7 DB and this only help with 1 or 2 frequency.
I documented by journey here
As for the FM band Stop, remove it and see how is the 120+ MHZ look. The 800 MHZ band also opened up good. If you see no difference with and without the FM band stop remove it as it adds more issue than it solve.

I measured the coax with the DC block and got 0VDC, as expected. Without the block, I read 0.071VDC. I'm thinking that should be safe enough to go block-less. Without the FM band stop before the LNA, my air band is full of broadcast FM images (if that's the right term.) It just makes it unlistenable. That filter is crazy good- only a handful of the very strongest stations come through at all from 88 to 108MHz.

 

[Ubbe]

If it's line of sight to those FM transmitters then the 2.8 mile one will have a -27dBm level outside your house and the 20 mile one -17dBm. The roof will attenuate some and also if there are something blocking the signal to the towers. It's the sum of all signals that the antenna can receive that are important. Maybe you have a VHF TV transmitter nearby that usually have much higher power output, or a cellular tower.

What happens if you set that FM filter after the LNA? Still overload issues, then that FM filter needs to be used for the LNA. But I suspect the Stridsberg to be overloading, it has worse specification than your LNA.

That Nooelec starts to compress and have issues at -10dBm on its input and compare that to a PGA103+ based amplifier that has +6dBm at its input where it compress the signal. It's 30 times better in strong signal handling.

/Ubbe