Tag Archives: spectrum analysis

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…

FM broadcast stations

The last post involved sniffing the spectrum around the house from 1 to 1300 MHz. It was obvious from the scan that the strongest signals were all in the FM broadcast band. Time to dig in a bit more and identify these culprits.

A very handy tool is radio-locator.com, which is chock-full of useful information on commercial radio stations. If you’ve never used it, I encourage you to check it out.

n7uvqth_pcr1000_fmbroadcastband

Sweeping the FM broadcast band again, the strongest carrier is at 98.7 MHz. This is KMVP atop South Mountain, with an effective radiated power of 97 kW.  Another very strong one for me is 88.3 KNAI, located over on Shaw Butte, only 5 miles west.

Let’s now remove the antenna and replace it with a dc – 18 GHz termination. Ideally, there should be nothing but a flat line at the receiver’s noise floor. Sadly, this is not the case.

n7uvqth_pcr1000_dcterminationatantenna

 

There are two pieces of LMR-240 that make up the coax run from the antenna to the shack. The first, from the shack up to the roof, is about 30′. The second, up on the roof from the cable outlet to the antenna, is about 60′. The following plot shows the reception with the second piece disconnected from the first and the termination attached to the roof end of the 30′ piece.

n7uvqth_pcr1000_dcterminationatrooftopegress

It appears that there’s not even 60 dB of isolation between termination on and antenna. Rats – there should be around 90 because it’s pretty good coax, and the connectors seem to be installed well. Let’s look more closely to see whether these are external or internal.

n7uvqth_pcr1000_dcterminationatantenna_labels

It appears that all the signals are from over-the-air transmissions, except the one around 96 MHz, doesn’t appear to correspond to any OTA signals.

Station KNAI (88.3 MHz) broadcasts from Shaw Butte, only 8 km or so west, while all the others have transmitters atop South Mountain, over the Phoenix mountains from this location.  The big guns are 100 kW ERP.

In any event, it appears that my setup is somewhat leaky, with only about 60 dB through-cable loss.

Local radio spectrum scan

Interested in installing a low-noise wideband amp on the rooftop discone to allow me to aggregate together a bunch of receivers and reduce the amount of clutter in the shack.

I have an old Diamond discone on the roof, only 5 – 6 m above ground, and about 30 m of feedline to bring it down into the shack where all the scanners and other receivers are.

I’d purchased on eBay a super-hot LNA from iseeabluewhale with NF of 0.5 dB at 400 MHz, 20 dB of gain, and a useable bandwidth of 10 – 2000 MHz.  Most importantly, it’s got an output IP3 of +42 dBm at 1 GHz, so it should be able to withstand nearly any level of signal that I can throw at it and stay linear.

First thing is to see how noisy the environment is. I set up the Icom PCR-1000 radio to scan frequencies from 1 MHz to 1300 MHz and see what kind of average noise levels I was dealing with out there. The discone is about 1 – 2 dBi or so, the cable loss varies over the frequency span. The Icom was configured to step in 25 kHz increments with a 50 kHz bandwidth.

The antenna is really only competent from about 100 or 120 MHz up, but it has a bottom-loaded coil atop the disk to supposedly get down to about 50 MHz or so.

n7uvqth_pcr1000_001-100mhz

1 – 100 MHz: No surprise here, the strong signals are all FM broadcast band. Some are exceedingly strong, although I’m pretty sure the closest one is at least 8 km distant. No digital TV signals between 54 and 88 MHz.

n7uvqth_pcr1000_100-200mhz

100 – 200 MHz: The rest of the FM broadcast band, some peaks in the aviation, 2 m ham, and VHF 150 – 170 MHz bands. The big rectangular humps at the right are digital TV channels 8 and 10.

n7uvqth_pcr1000_200-300mhz

200 – 300 MHz: Here, there’s just TV channel 12, then it’s pretty quiet beyond, with the noise floor about as low as the Icom can report.

n7uvqth_pcr1000_300-400mhz

300 – 400 MHz: The innards of the Icom must switch receiver paths or filter setups at 350 MHz, since the floor was rising smoothly and then made the 5 dB step down. Otherwise, it’s pretty quiet still.

n7uvqth_pcr1000_400-500mhz

400 – 500 MHz: Still quiet, random narrow-band carriers here and there until the 450-470 MHz two-way band, plenty of carriers there, and then TV channels 15 and 17.

n7uvqth_pcr1000_500-600mhz

500 – 600 MHz: A bunch of TV channels, from left, 20, 22, 24, 26, 30, 31, 33, and the lower edge of 35.

n7uvqth_pcr1000_600-700mhz

600 – 700 MHz: All DTV still, 36, 38, 39, 40, 41, 42, 46, 47, 49 and 50, and maybe 44, 45, 47, and 48. Otherwise, quiet noise floor.

n7uvqth_pcr1000_700-800mhz

700 – 800 MHz: Mostly quiet, notice the noise floor’s come up from the last plot, and there’s LTE cellular for Verizon and AT&T.

n7uvqth_pcr1000_800-900mhz

800 – 900 MHz: In a normal receiver, there’d be all sorts of signals between 824 – 849 and 869 – 894 MHz, but because this receiver is not allowed by law to tune those frequencies (stupid law; nothing to hear but digital noise) there’s nothing there. However, my $10 USB SDR dongle can tune this spectrum, so maybe I’ll look there to see how strong the signals might be.

n7uvqth_pcr1000_900-1000mhz

900 – 1000 MHz: The 902 – 928 MHz part 15/18 band has plenty of small carriers, lots of activity, but it’s all pretty weak. There’s more carriers up in the business bands all the way up to about 960 MHz, then gets quiet again.

n7uvqth_pcr1000_1000-1100mhz

1000 – 1100 MHz: Between cable loss and the absorption by foliage, the signals here are all generally pretty weak. Don’t know what that plateau is at about 1015 MHz, or the hump around 1025 MHz. The one at 1015 MHz looks suspiciously like a digital TV carrier… The ADS-B stuff at 1090 MHz barely shows, though there’s hundreds of transmissions per second.

n7uvqth_pcr1000_1100-1200mhz

1100 – 1200 MHz: Really not much here that my receiver can detect.

n7uvqth_pcr1000_1200-1300mhz

1200 – 1300 MHz: Same here.

So, it appears that the bulk of the stronger signals in range are all FM broadcast or digital TV stuff. FM broadcast really beats everything else. Let’s take a closer look at that next.