This is my experience as discussed in a few other threads.
If the “built-in” filter isn’t the first component in the receiver chain (after esd protection of course) then the LNA can simply “give up” with a strong signal anywhere in it’s frequency range.
I have yet to confirm, but someone from FlightAware should be able to easily chime in on whether or not this is correct:
I believe the FA Orange and FA Blue radio use the SKY67150-396LF for their amplification. Per spec @5v, this chip has a nice, high IP3 of +38dBm (In a nutshell, the higher the better for saturation purposes). Most alternative low(er) noise amps fall between +30-35dBm for comparison.
That said, I think they opted to feed the chip 3.3v to help keep the power consumption down (remember the USB port needs to power all this on top of the tuner). Unfortunately Skyworks doesn’t have 3.3v specs in their datasheet, but besides some positive effects (lower power consumption, lower noise factor), doing so also lowers the IP3. Comparative chips have measured 8-10dBm drops when being fed 3.3v vs. 5v.
So now we’re looking at an amplifier running with a lower linearity intercept making it much more predisposed to saturation. This is why both of these radios tend to perform better with some sort of upstream filter.
This is my theory and still need to take one of my metal cases apart to confirm (stuck mine in metal cases for cooling and shielding). I suppose anyone from FA could save me the trouble…
I suppose if this is the case, and some run their radios though powered hubs, there could be another thread on how to mod these things
Just experience the same issue after removing the FA filter, I’m using the Prostick Plus with the “refurbished” Jetvision antenna.
Probably my setup is very close to saturation.
Does anyone ever tried to install a variable attenuator before the RTL-SDR dongle?
It showed that at my location, the RF interference is so bad that without an external filter, Pro Stick Plus’s front-end LNA gets saturated, and performance drops to near zero.
However if attenuaters are added, these dampen the interfering signals and performance shoots up even without an external filter.
I’ve tried it, but discovered my uW variable attenuator passes nothing below 2GHz.
They can be had cheap for TV if you still have an interest
An attenuator will “attenuate” the signals you want by exactly the same amount as the noise you don’t. By adding an attenuator, you’ll be trading long range (weak) signals for local.
An attenuator compliments a filter, but it does not replace it.
I still have a rough time assimilating a single graph without a watermark to illustrate whether or not a change in setup is having an affect. Example:
I’d be looking to take fish out of my diet and scratch my head over ham. In reality it’s the same untouched feeder dealing with air traffic.
That said, I can somewhat identify from the previous post that Pro Stick Plus seems to present an issue in that setup, but so does fish on mine. As constructive criticism, I think it would help to show a side-by-side against a watermark setup since air traffic and environment are so unpredictable.
I’m comparing apples to oranges, but that’s my whole point. Please don’t take this the wrong way, I mention this with full respect.
(1) The drop and rise is drastic and very steep, almost a vertical rise/fall, very distinct as compared to your ham & fish example.
(2) Removal & insertion of filter and attenuaters was repeated several times, and each time a steep change took place.
(3) That test was done 4 years ago . I have done another test 2 months ago with same steep drops & rise. You can see it here:
Up to 21:20 EDT > Antenna connected to Pro Stick PLUS through FA Light Blue Filter
From 21:20 to 21:50 EDT > Antenna connected to Pro Stick PLUS directly (Filter removed).
From 21:50 to 22:00 EDT > Antenna connected to Pro Stick PLUS through FA Light Blue Filter
From 22:00 to 22:25 EDT > Antenna connected to Pro Stick PLUS directly (Filter removed).
From 22:25 EDT to end of graph > Antenna connected to Pro Stick PLUS through FA Light Blue Filter
NOTE:
Every time I removed and inserted the Filter, I made sure that all connectors are fully tightened, so the drastic drop is not due to loose connection.
I’m not dogging you amigo, just trying to help with an idea to make your tests that much better is all. I know you have gobs of equipment laying around, may as well put it to use.
I never took anything you mentioned as “dogging me”. I always take it as a constructive advice / criticism intended for help, correction & improvement, and I am thankful to you for it.
This test involves FA Antenna and ProStickPro (Blue).
Unfortunately I have only one FA antenna and one ProStick Plus (Blue), so I cannot do a true side-by-side test.
I dont have “gobs of equipment laying around”
My Dongles are:
3 x Generic
2 x Orange ProStick
1 x Blue ProStickPlus
1 x Green FlightStick (RB24)
1 x Red FlightStick (RB24) for 978 Mhz
My Filters are:
2 x FA Light Blue
1 x RB Blue
1 x Triple-filtered LNA (RTL-SDR Blog) + Bias-T
My Antennas are:
1 x FA Antenna
2 x Stock Mag mount whips of DVB-T
2 x Coiled Mag mount whips of different dimensions $4 each
2 x Magmount with V-Stub Whip
1 x V-Stub QuickSpider (made of coax only, no SO239)
Not specific to you, but I’ve wondered about about this point. N-connectors are very tolerant of tightening because the mating surface isn’t the RF connection. SMA’s are quite a different design and can be a bit finicky.
For interest, how do you satisfy " fully tightened". (finger tight?, 12" shifter? …?)