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Re: [IRCA] Brief RTL-SDR review/modification



Thanks for that extensive report Tim.

If I understand you correctly, your modified version is quite sensitive on medium wave, as judged by how it performs with a short bit of wire indoors, and I guess your 30' vertical outdoors proves it has poor signal handling ability,. A tuned loop or an antenna tuner for an outdoor antenna would get past that problem, but by sacrificing the broadband nature of the SDR.
Did you try just attenuating the vertical's signal until the MW band 
becomes useable?  If so, does that just make the band deaf, or is 
there still reserve sensitivity in the unit?


best wishes,

Nick




At 19:39 02-06-15, you wrote:
I decided to go cheap getting my feet wet with SDR. Saw the RTL-SDR on Ebay for less that $50 so I ordered one.
It was advertised as having 100KHz to 1.7GHz coverage and I've been 
playing with it using SDR# and HDSDR.  I much prefer the SDR# 
interface, seems more intuitive to me.
I can't say much about the upper end but it does quite well with 
FMBC and VHF Air/Weather. It outperformed my Grundig S450DLX for 
sensitivity and selectivity on FM with a 31" rod.
On SW it outperforms my R-71A both for sensitivity and with the 
digital filtering adjacent channel interference and noise. Both were 
tested using a 30' vertical feeding a 4:1 BALUN and RG6QS feedline. 
The RTL-SDR does suffer from overloading and a preselector helps 
there a lot. It's dynamic range of 50dB seems to be the main 
limitation on SW. It can display/record up to a 2MHz bandwidth so I 
thought it would be handy for MWDX.
I was wrong. In the upper part of the MW band the sensitivity leaves 
much to be desired and becomes nearly non existent below 1MHz. I 
decided some exploratory surgery was in order.
The RTL-SDR  has a LPF on the HF input to keep strong FMBC/TV 
signals out but the internal BALUN was found to be the limiting 
factor for lower frequencies. It's a very small binocular core wound 
in a 1:1:1 single sided input and balanced output. After removing it 
and replacing it with a home-rolled transformer with the same ratio 
on an Amidon FT-37-75 core I decided to remove the LPF elements 
since the 75 material would choke them anyway. The upper end of the 
SW band suffered a bit from the transplant and the MW band was still 
unusable, hetrodynes and overloading all over the band, again a 
preselector cleared up the problem and a tuned loop would probably 
be best (next on my to-try list). I also put in an F connector since 
SMA cables/ends are very $pendy.
The sensitivity is now rather impressive, a 6" wire shoved into the 
F connector gets the usual daytime locals and semi locals sitting on 
my bench in the basement, it starts to overload with the 31" FM rod. 
It takes about 10" of wire on the R-71A to equal the reception I get 
from the 6" wire. With no antenna it doesn't pick up the 10KW 
station 6 Miles away at all and the spectrum is flat all the way down to DC.
Tapping the of the 2nd mixer (9MHz) on the R-71A works surprisingly 
well, coupling with a 33pF disk provides the right level to the SDR 
and doesn't interfere with the operation of the Icom. The radio 
provides the tuning, preselection and AGC and the SDR /software do 
the demodulation. The main drawback is the 20KHz maximum useable 
bandwidth with this setup. For broadband MW recording this is not 
the SDR to have. I now have a DSP IF that doubles as an FM radio. 
Still not too bad for the price. The 455KHz IF out on the Grundig is 
the right level feeding it directly from jack to jack with RG6.
Overall I think it would make a good FMDX receiver for tropo 
openings with it's 2MHz display/recording capability or monitoring VHF/Air.
For SWL it works very well for the money but your going to want to 
put a HPF in line with it to keep the MW band from overloading it.
For MWDX and Longwave as it is out of the box, forget it. I plan on 
hooking it up to a high Q loop and will post the results of that 
when I get the time. Maybe with a laptop and an FSL on a cliffside? ;)
Not sure about the UHF performance but it does pick up what I assume 
are digital cell, TV, etc. but I have no other receivers that pick 
up those frequencies to compare it to.
I would suggest a dual core processor and 4GB memory at a minimum if 
you want use the 2MHz sample rate and USB2 is a must. Windows7 does 
not recognize it when plugged into a USB3 port. According to the 
performance monitor Processor usage on a 3.4GHz quad core is about 
23% total recording at 2.048 MS/S. Using it as an IF at 250KS/S the 
processor is barely ticking over even when recording. It crashes 
when I try sample rates above 2.048MS/S, just like the manual said it would.
I plan on experimenting with it some more and will post the results.

Tim Hills
Sioux Falls, SD
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