[IRCA] Frankenradio

["T. Hills" <thills@xxxxxxxxxxxxx>]

After a long hiatus I'm getting back into MWDX/NDB chasing. Due to 
family and other obligations my budget and time are limited for now.
Right now the antenna is just some wire and a balun and I have the parts 
to put the remote tuned loop V2.0, a modified Lear 2320-A, together as 
time permits. Add a hombrew phasor it should work pretty well. I have 2 
of the 2320-As the first will be for MW and the second will be set up 
for LW. Nothing really "new" there but an upgrade from my last antenna 
system.
What is "new" is the rig at least for me. I wanted to try SDR but 
couldn't justify spending a Month's rent and utilities on a Perseus and 
most of the budget SDRs didn't seem to be very good. At that I did wind 
up spending US$50 on an RTL.SDR. After a Month in transit from China I 
finally got it. It seems to do OK with strong signals but the lack of 
dynamic range (50dB) makes it poor for DX. I did make a few catches but 
nothing new or noteworthy. Part of that is the antenna but it overloads 
too easily. I was planning on tapping the IF of the homebrew but I'm 
still short a couple of parts for it.
Then I remembered the scope output on my R71-A. It worked, sort of. The 
output from the first mixer (70.4515MHz) has a lot of hash outside the 
desired passband and overloaded the RTL.SDR I put an attenuator in line, 
again, locals were clear but weak signals were lost in the mush. I think 
part of that is the quadrature mode used above 30MHz though it does a 
decent job on FMBC. Still looking into that but what I wound up doing is 
tapping the 2nd IF between the second mixer and the filters through a 
100pF ceramic cap.
Bingo! Adjusted the filter in SDR# and heard some sports talk right 
around LSR on 1330 fading in and out with 3 or 4 other stations and no 
hash from 1320, 5KW directional 6 Miles away aimed right at me. I'm 
assuming the dominant signal was WLOL Waterloo IA. but can't confirm, 
also heard what sounded like religious programming and CW in the mix.
One of the problems I've had at this QTH is 2 of the local stations 
directional arrays converging in my back yard and another local that I 
can see the tower from the porch leaving big gaps in reception even with 
tuned loops, preselecters etc. Nearby power lines make nulling 
practically impossible on strong signals. I have all summer to tweak 
things and looking forward to the fall DX season.
Some observations about the RTL.SDR.

It's advertised as covering 100KHz to 1.7GHZ but using a signal 
generator I've found it's pretty deaf below 1MHz or so. It took almost 
30mV at 100KHz to get a usable signal through the software. Was planning 
on using it at 455KHz but it's much more sensitive at 9MHz. I've only 
tested it to 432MHz. using the remote starter key fob. Not really 
impressed but it does fine with a 31" rod for local FM listening on the 
V/U input.
If it's going to be used by itself on MW/SW I'd strongly suggest a 
preselector/preamp/attenuator. On SW it's the least sensitive (by orders 
of magnitude) of any of my receivers and even overloaded on 5MHz WWV :P
It should work well with any radio that has an IF in the HF spectrum. 
Tried it at 455KHz on a DX-160. It just doesnât seem to want to work 
well below 1MHz.
It works on USB2 but the USB3 bus doesn't recognize it. Installation was 
very easy if you remember to RTFM first. I use SDR# and have HDSDR 
installed too. I prefer SDR# because the interface is more intuitive 
(for me). Processor load and performance seem to be about equal with 
both programs. YMMV.
Using SDR# it runs fine on a Dual Core laptop if you keep the FFT at 
65536 or lower and the old XP box with a 2.4GHz P4 at 1.024 MSPS. It's 
currently running on a Win7 AMD Phenom II Quad @ 3.4GHz and using ~22% 
of processor time. That's with the sample rate at 2.048MSPS and the FFT 
set at 131072, spectrum, waterfall and IF displays enabled. A P4 or 
equivalent would be the least I'd recommend so the old laptop is out of 
the question.
As a receiver it leaves a lot to be desired but used as an 
IF/filter/detector I'm quite happy with it. For less than half the cost 
of a single _good_ crystal or mechanical filter, a $0.10 cap and a bit 
of solder I have a tunable, infinitely variable filter and multi-mode 
demodulator, not to mention adding FM and Air bands to the HF rig.
Tim Hills
Sioux Falls, SD
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