30 dB amp with DVB-T: Overkill?


#1

The two amps I’m currently using are rated at 20 dB. I’m planning to buy a new amp and saw this on eBay. Thoughts?

http://www.ebay.com/itm/IN-LINE-SATELLITE-AMPLIFIER-DIRECTV-DISH-NET-30DB-GAIN-PERFECT-VISION-PVLA-30-/141043086029?pt=LH_DefaultDomain_0&hash=item20d6d2b2cd


#2

6dB is kind of noisy – the single-stage LNAs I use are 18 to 20dB gain with a nf under 1dB at 1 GHz ($20 kit on eBay with surface mount parts).

I’ve tried an LNA with 38dB gain and a 4dB nf – filtering on the input is mandatory otherwise cell towers and DTV saturate the poor little bugger. No significant improvement over the 18dB LNA.

For the price, probably worth a try. The specified output is -10dB, which implies to me a pair of low power high-gain stages. I’d worry saturation is going to be an issue. Saturation drives it nonlinear, distorting everything. This typically shows by inserting the amp inline and having your numbers drop. The cure for that is to filter the amp input to prevent saturation; once signals are distorted in the amp, that distortion can’t be removed by post-filtering. And of course the closer to the antenna you can put the amp, the better.

Are you running both your sites from the same antenna - RF chain, or using separate antennas and amps? If you’re using a single chain and a splitter, beware – garden-variety 75-ohm splitters for cable TV, the kind sold at home improvement stores, typically have horrible performance above 700 MHz or so – you need to be sure you’re using a splitter designed for satellite work as those will have low attenuation around 1GHz.

If you do get one, I’d be interested in an rtl_power spectrum especially below around 900 MHz, to see if the amp really does drop off below that (which to us is a big benefit).

bob k6rtm


#3

Could you provide the link for this?

Thanks!


#4

Bob, as someone said in another post, if you come up with the filter kit, I’ll buy one. I have looked at a few on eBay and I’m not sure which one to go with. As for my setup, I have two separate RF chains. Each has a 20 dB amp, about 40 feet of RG6, power supply and injector. Here’s a couple of shots of my antenna mast.
http://i57.tinypic.com/1zguv12.jpg http://i60.tinypic.com/j7xj6e.jpg


#5

Same here.


#6

http://www.ebay.com/itm/PSA4-5043-low-noise-amplifier-kit-902-903-1296MHz-Pout-18dBm-/121560532074

It’s a kit using surface mount parts – which are damn tiny. While it’s (barely) possible to do surface mount assembly with a really fine tipped soldering iron, it’s not recommended! I use an illuminated magnifier, tweezers, solder paste, and a reflow oven.

Following the link for the 30dB amp, eBay showed me a similar 20dB amp with a really low noise figure –

http://www.ebay.com/itm/ASKA-Inline-Amplifier-20dB-Gain-For-Satellite-FTA-Coax/350455332373

cheers–

bob k6rtm


#7

Very nice!

Tip for others: unless you get freak-grade 50 Ohm cable, a good quality RG-6 coaxial cable sold for satellite use will be lower loss that common garden variety 50 Ohm stuff. For ADS-B, line loss at 1GHz is the important factor, and for cables such as RG-8, RG-8X, RG-58, RG-59, it’s really bad.

bob k6rtm


#8

As Bob said: a Noise Figure (NF) of 6 dB is noisy! I have been using a BVV-250 for a while. This uses a MAR-6 amplifier chip having a NF of 3dB at 1 GHz at 20dB gain. The results greatly improved using this preamp.
On january 25 i changed this for a G4DDK LNA i borrowed from a friend. This unit has a measured NF of 0.4 dB (!) and 35 dB gain. Message rate dropped to half of the original but to compensate for the high gain i lowered the gain setting of the dongle down to 19 dB for optimal results. Improvement was 5% more aircraft and message rate but range did not improve and stayed at 200nm.
A week later i changed to dump1090-mutability which again resulted in +5 % more aircraft.
Maybe it’s not only the good NF but also the point that the RTL gain can be turned down and thus making it less sensitive for noise pickup from other sources through the USB which plays a role here.

/paul


#9

Thanks. And also thanks for the tip on the 20 dB amp with low noise factor. I ordered one and will report back my results.


#10

Paul –

You are in a truely “target rich” environment! You are also in what I would suspect is a very dense electromagnetic environment.

Have you ever done spectrum scans with rtl_power to see if there are strong signals that would overload an amplifier?

Yes, I’m on the “put a filter on it” bandwagon yet again… But only if it helps, and doing some scans will give you an idea if it would or not.

cheers–

bob k6rtm


#11

Bob,

The ebay listing may not be correct on the ASKA Inline AMP. The listing states a Noise Figure of 0.4dB. However, the ASKA product information states a Noise Figure of 4.0dB Max. http://www.askacom.com/product.php?productid=13

This is still better than my noisy RCA D903.

–Steve


#12

Have you ever done spectrum scans with rtl_power

Ha! Sure did! After wondering why my 8 element coco would work just below the rooftop and would not work 2 meters up i ran RTLScanner without preamp or filter at 0dB gain which showed a strong cell tower spike at 937 MHz.

I did some write-ups here how the filter is constructed (a bit bigger than your SAW). Actually i built a second one which will be measured on a VNA this wednesday.
The filter is after the preamp but it seems the IP is high enough to cope with the strong signals from the cell tower at 200m.
And this is how the coco is mounted. The receiver is in the attic below with no more than 8 feet of cable:

/paul


#13

I’d love to find out if anyone has done anyone has done anything with the nunu (hairpin) filter here rtl-sdr.com/update-homemade- … -same-pcb/

Bigreid (who did most of the work on it) has put the design in a git repository with a GNU license on his work - what it needs is someone to verify the tuning set up manufacturing on the board and to sell it at an affordable cose.

(I can feel a group buy coming on!)


#14

grumble grumble – while at HP/Agilent I worked with a number of practitioners of the Microwave Dark Arts. Haripins are high-order magic. The design software is expensive, and fabricating repeatable, stable designs means using a substrate like Rogers Duroid (a PTFE substrate) which is an order of magnitude more expensive than FR4. FR4 isn’t environmentally stable for microwave use, is lossy, the performance can vary batch to batch, and yet it’s used in zillions of boards for WiFi and Bluetooth, which are 2.4x or 5.6x the ADS-B frequency.

Take a look at

http://www.w1ghz.org/filter/Recipes_for_Printed_Hairpin_Filters.pdf

for a good writeup of one ham radio operator’s experience with haripins. This guy is very good (as his callsign, W1GHZ, would suggest).

bob k6rtm


#15

@paulmerkx
Ha! Sure did! After wondering why my 8 element coco would work just below the rooftop and would not work 2 meters up i ran RTLScanner without preamp or filter at 0dB gain which showed a strong cell tower spike at 937 MHz.

I’ve just had a look for RTL Scanner but the file doesn’t seem to be on SourceForge (or anywhere else) Can you point me to a download?
Thanks


#16

Take a look at;

http://kmkeen.com/rtl-power/

It’s rtl_power you’re looking for, as in

sudo rtl_power -f 800M:1200M:200k -g 40 -i 10 -e 1h ohmy.csv

to scan from 800 - 1200 MHz in 200 KHz steps.

As to knocking out the cell spike, I suggest looking into stub filters… cheap and easy to make by hand; make one a bit long and cut to desired performance watching successive spectral scans.

bob k6rtm


#17

[quote=“k6rtm”]

You’ve given the link to a page about stub filters in the past (maybe worth a repeat). Just to be clear, the stub is cut to 1/4 wavelength of the frequency you’re trying to block? Insert the stub using a T into your feed line? Stub is open or shorted? Before or after the amp? Thanks.


#18

Hi Bob,

rtl_power is in the directory that was loaded with the Osmocom drivers etc. paulmerkx mentioned a Windows product called RTL Scanner which produces nice waterfalls. I found a link on SourceForge but keep getting a 500 error.

Ahh, I’ve just been to SourceForge to include a link here and it looks like the whole site is down …


#19

@trigger

Ahh, I’ve just been to SourceForge to include a link here and it looks like the whole site is down …

Yeah you’ll probably find it when sourceforge is back on. It was eartoearoak.com/software/rtlsdr-scanner i used.
But prepare for a long install. It’ll just put a lot of python stuff on your disk so maybe the suggested rtl-power is a better option.
/paul


#20

The machined interdigital filter is a thing of beauty – if I had access to a machine shop, that would be the way to go. I’d be tempted to machine enclosures for the LNA and the SDR on the output side… I have a spectrum analyzer and scalar analyzer that goes to 2.9 GHz; that works well for a lot of things. One of the great things about SDRs is that for $20 or so you have basic spectrum analyzer functionality – you’re not going to make precision measurements, but you can find out what’s causing you grief, and work to tune that out!

bob