Category Archives: SDR

Update on the rooftop amp

Last time we visited the roof, the amp followed by the FM BCB notch filter was now in the die-cast enclosure, but not actually attached to anything.  Now it finally has a home, at least for now, on the tripod leg. It required a visit to Artie’s Ace Hardware in Phoenix at Tatum and Thunderbird, which until about 8 hours ago was unknown to me as a purveyor of a near infinite number of different kinds of metric fastener! Only 4 miles away, it’s a treasure to know that I can get an M4x8 mm pan head screw even late in the afternoon.

The metric hardware was required to install the steel mounting ears on the die-cast enclosure; those mounting ears accept the muffler clamps that hold the whole thing to the leg of the tripod. Later on this winter I’ll bend up some 0.032 Al sheet to act as a sun shield and remount the box on the north leg with the shield to keep it cooler during the summer. I still need to do something permanent about the power for the amp, it’s currently the solar power setup I made a couple weeks ago.

Left is input, right is output. Runs on any voltage up to about 32 vdc and down to about 7 vdc. The internal dc-dc converter keeps the amp supplied with an even 5.0 volts.

With this amp in place, my stack’o-scanners is just bangin’ along. I’ve got great reception, and no FM BCB interference. And, there’s space in the enclosure for a future Arduino or Raspberry Pi, as well as the necessary network connection.

Shocking new amplifier

Is finally built. I bought three of these a couple years ago from kuyaya520 on eBay and they’ve languished since then, heat-shrink protected and tie-wrapped in place, like for the truck’s half-deaf GRE PSR-600 scanner.

Finally put one in my new die-cast case that I got 10 of last month, and used a 82 ohm resistor to set the operating voltage to around 10 vdc when running off 13.8 vdc. According to the eBay page about the amp, its best gain and noise figure is around 9-10 vdc.

Checked out the gain, and it’s pert darn near what the vendor says it is. Don’t have a simple way to do noise figure. Need to get myself an ENR noise diode so I can do y-factor.

This amp will likely go into the truck to replace the tie-wrapped kludge… I’m starting to get reasonably good at assembling these things.

10 MHz bandpass filter from China

Saw this 8 to 11 MHz band pass filter (item number: 201406314462) on eBay a few weeks ago and thought it might come in handy to help my SDR radios hear WWV and WWVB while not being overwhelmed by other signals.

Here’s what the 1 to 100 MHz band looks like using the PCR-1000 connected through the new antenna multicoupler, without the new filter. Remember, this antenna setup already has some significant attenuation below about 100 MHz since it’s using a VHF/UHF discone and the 10 to 2000 MHz LNA.

Here’s a plot using the ICOM PCR-1000 using the new BPF. Indeed, it has good performance, especially for the US$11.48 incl shipping.

LNA unhappy with 10 uH Coilcraft 0805 chip inductor

Today I finally synced up with my friend Doug and collected from him quantity five Coilcraft 0805 10 uH chip inductors. Tiny things. Had little caffeine today, so by the time I’d returned home I was pretty steady.

Brought the LNA down from roof, opened lid, clamped assembly down to bench so it wouldn’t move, put a drop of 60/40 on one inductor pad, and with my TU-10b tweezer I picked up the part and set it in place, then tapped the one end with the soldering iron. It was harder than I thought; the part weighs nothing and has no surface friction with the tiny bead of molten solder, so it instantly moved on me.

After some re-approaching of the problem, I got the part securely attached. Checked continuity, everything looked good! Lid back on. Connected it to network analyzer, and everything did NOT look good. 20+ dB gain above about 200 MHz. The whole 10-200 MHz output level was badly attenuated, and there were strange artifacts in the low end of the spectrum. Removed the inductor and everything returned to normal. Posted a note to the designer over on gpio.com and am awaiting a response. 10 uH at 100 MHz is 6k  ohms impedance, so it should be fine. The LNA was drawing its typical current (~ 160 mA).

Took another identical  inductor and pressed it down on the pads with a plastic tuning stick, and did exactly the same thing as soon as it made contact. There’s something about that output circuit that doesn’t like the chip inductor.

 

 

LNA mounted – time to test!

To recap: I’ve taken the LNA that I purchased from iseeabluewhale on eBay and put it into a diecast aluminum box (purchased 10 of those from wonderco_buy on eBay as well). The TNC pigtails came from my friend Chris’ stash. The other bits and pieces came from the fossil beds of the garage.

Turned out to not be so tricky to get the thing mounted and connected.

Put it up on the roof behind the FM notch, and powered by 4 NiMH batteries.

First test – sweep 100 – 200 MHz and see what it looks like. Hopefully it looks just like the sweep from last night, just 20 dB higher.

100 – 200 MHz Pre-LNA, with filter, from last night (red peak, green instantaneous):

100 – 200 MHz w/LNA, with filter:

No obvious instability or oscillation. All the signals I can hear, like NOAA weather, Arizona DPS, aviation AM, even the residual signals from FM broadcast, all clear and crisp. My my my. Looks like pretty good fidelity.

Now I’ve just got to get a source of stable dc power to the amp! And I like these little boxes…

Spectrum scanning (again)

Now that the filter setup appears stable, it’s time for another assay of the spectrum around the QTH with the discone.

001 – 100 MHz (old):

001 – 100 MHz (new):

Pretty big difference! And just to check the ICOM demodulator, here’s the same spectrum using the AM demodulator.

100 – 200 MHz (old):

100 – 200 MHz (new):

Low noise floor, more better! Still some strong interference, but livable.

200 – 300 MHz (old):

200 – 300 MHz (new):

So much better.

With most of the big interferers suppressed significantly, tomorrow I will put one of the LNAs in a case and get it up on the roof as part of the lineup.

FM broadcast band notch filter update

Wasn’t able to spend any time on this the past week or so, but today I finally tracked down the pesky problems that I was having with my temporary installation.

I’d damaged one of the original TNC pigtails used to connect to the filter, and that created a sometimes 30 dB additional loss. Also, I finally found a marginal to no-good TNC-f to TNC-f adapter that I was using temporarily to connect the TNC-m connector on the box to the UHF-m connector on the cable that runs from the roof to the shack. I don’t like UHF connectors of any type, but the discone has a UHF-f connection and this cable was originally connected directly to the antenna. Until I finish my next phase of project (getting LNA into a die-cast box as well) I will leave it this way.

Using the ICOM PCR1000 receiver, here’s a plot of 50 to 150 MHz. Pretty serious FM broadcast band suppression!

n7uvqth_pcr1000_50-150mhz

Still a few broadcast stations making it through, but much better than before. The plot below is the one from a few weeks back when I first got the filter assembled into the box.

n7uvqth_pcr1000_fmbroadcastband_filter_in_box_1120sun

 

The following is what it looks like today.

n7uvqth_pcr1000_fmbroadcastband_filter_in_box_1807sat

What’s changed? Certainly the filter/coax assembly is doing a better job of rejecting the FM broadcast stuff, but what’s curious is the new noise floor rise that wasn’t apparent before. That noise floor rise corresponds to the band notch characteristics of the filter, but I don’t know why I didn’t see it before.

Also, the installation needs to be rid of the coax adapters that I’m using while the project is midway. Once I get the LNA into a cast box, then both the LNA and the filter into a bigger cast box, I’ll install N-f connectors on the outside of the big box and will no longer need the adapters.

Finally getting somewhere…

While I’m at it, what TV channels are detectable here?

A couple posts ago I ran a scan of radio spectrum observed from a discone atop the house. There were a bunch of TV stations observed.

This FCC site comes in handy to check these.

Channel Callsign Start Freq
(MHz)
Stop Freq
(MHz)
XMTR location Distance (km)
8 KAET 180 186 South Mtn, PHX 31
10 KSAZ 192 198 South Mtn, PHX 31
12 KPNX 204 210 South Mtn, PHX 31
15 KNXV 476 482 South Mtn, PHX 31
17 KPHO 488 494 South Mtn, PHX 31
20 KPAZ 506 512 South Mtn, PHX 31
22 KNAZ 518 524 Mormon Mountain 158
24 KTVK 530 536 South Mtn, PHX 31
26 KUTP 542 548 South Mtn, PHX 31
30 KUAT 566 572 Mt Bigelow 174
31 KSAZ 572 578 South Mtn, PHX 31
33 KTVW 584 590 South Mtn, PHX 31
35 KFPH 596 602 South Mtn, PHX 31
36 KAZT 602 608 South Mtn, PHX 31
38 K38IZ-D 614 620 South Mtn, PHX 31
39 KTAZ 620 626 South Mtn, PHX 31
40 KEJR-LD 626 632 South Mtn, PHX 31
41 KPDF-CA 632 638 South Mtn, PHX 31
42 KVPA-LD 638 644 South Mtn, PHX 31
44 K44CN-D 650 656 Mingus Mtn 122
45 K45MW-D 656 662 Sentinel, AZ 143
46 KDPH-LD 662 668 South Mtn, PHX 31
47 KDFQ-LP 668 674 Mingus Mtn 122
48 K48NH-D 674 680 Globe 104
49 KASW 680 686 South Mtn, PHX 31
50 KFPB-LD 686 692 South Mtn, PHX 31
51 KPPX-TV 692 698 South Mtn, PHX 31

Some of these are a little hard to believe, like the low-power transmitters at Sentinel and Globe. Mingus, Bigelow and Mormon are somewhat more likely…

Using a FM notch filter

Earlier I mentioned that I’d gotten an FM notch filter via eBay  from iseeabluewhale. That filter is now up on the roof in a metal box, wrapped with aluminum foil. The discone has a 6′ pigtail of LMR-240 coax, that then goes into the FM notch filter, the output of the notch filter goes into the LMR-240 that runs down into the shack. It’s not a super installation, but let’s see what it looks like from the receiver’s point of view.

n7uvqth_pcr1000_fmbroadcastband_wfmtrap

Looks bad, but still better than the original (red, below) but significantly worse than the terminated version (green, below).

n7uvqth_pcr1000_dcterminationatantenna

So, the filter does seem to do a fairly good job of cleaning things up.

Next, I removed the cast enclosure from the mix and just wrapped aluminum foil around the filter assembly and over the coaxial cables connected to the filter.

n7uvqth_pcr1000_fmbroadcastband_wfmtrap_b

Even better than before. Filter blow-by is a real issue here. We are talking a filter that advertises 85 dB rejection, after all.

n7uvqth_fmnotchfilter_from_iseeabluewhale

As well, I realize that the 25 cm RG-178 coaxial jumpers that I’m using to get from the LMR-240 to the filter are partially exposed to the world, so I’ll go do something about that right now.

n7uvqth_pcr1000_fmbroadcastband_wfmtrap_foilshieldingtoendsofcoax

More shielding is better, so it appears! I’ve wrapped the Al foil from LMR-240 connector to the other, covering all the RG-178. Perhaps I’ll fix that next. I believe I’ve a small die-cast box in the garage that I could use.