Re: [IRCA] Frankenradio

[Brian Rachford <azswdxing@xxxxxxxxx>]

Tim,

I got back into DXing via the inexpensive SDR route myself last summer.
I've also heard (no pun intended) that the RTL.SDR unit is rather deaf on
the low end.  Instead, I'm using a cheap RTL-SDR ("dash" not "dot") dongle
with a separate upconverter to get LW/MW/SW and that option may currently
be the best way to do SDR under $100.  But, it sounds like you've done some
good mods.  I'm lucky in that my local stations are low power, but I need
filters or a tuned antenna for optimal performance due to the low dynamic
range.  I'd have a more expensive SDR by now, but OS X and Linux software
support for anything higher-end is pathetic.

Good luck and welcome back to DXing.

Brian Rachford - Prescott, AZ
azswdxing@xxxxxxxxx
http://azswdxing.wordpress.com/


On Sat, Apr 4, 2015 at 7:05 AM, T. Hills <thills@xxxxxxxxxxxxx> wrote:

> 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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