Paul Thurst – Page 36 – Engineering Radio

The Realtek 2832U

In my spare time (lol!) I have been fooling around with one of those RTL 2832U dongles and a bit of software. For those that don’t know, the RTL 2832U is a COFDM demodulator chip designed to work with a USB dongle. When coupled with an R 820T tuner a broadband RF receiver is created There are many very inexpensive versions of these devices available on Amazon, eBay, and other such places. The beauty of these things is that for around $12-30 and a bit of free software, one can have a very versatile 10 KHz to 1.7 GHz receiver. There are several good software packages for Windoze, Linux, and OSX.

The one I recommend for beginners is called SDR-Sharp or SDR#. It has a very easy learning curve and there is a lot of documentation available online. There are also several worthwhile plugins for scanning, trunking, decoding, etc. At a minimum, the SDR software should have a spectrum analyzer, waterfall display, and the ability to record audio and baseband PCM from the IF stage of the radio.

Some fun things to do; look at the output of my reverse registering smart (electric) meter (or my neighbor’s meter), ACARS data for the various aircraft flying overhead, a few trips through the EZPass toll lanes, some poking around on the VHF hi-band, etc. I also began to think of Broadcast Engineering applications and a surprising number of things came to mind:

Using the scanner to look for open 950 MHz STL frequencies
Inexpensive portable FM receiver with RDS output for radio stations
Inexpensive Radio Direction Finder with a directional antenna
Inexpensive Satellite Aiming tool

Using SDR sharp and a NooElec NESDR Mini+ dongle, I made several scans of the 945-952 STL band in a few of our markets. Using the scanner and frequency search plugin, the SDR software very quickly identified all of the in-use frequencies. One can also look at the frequency span in the spectrum analyzer, but this takes a lot of processing power. The scanner plugin makes this easier and can be automated.

Analog and digital 950 MHz STL frequencies, Albany, NY

I also listened to the analog STLs in FM Wideband mode. Several stations are injecting their RDS data at the studio. There is one that appears to be -1500 Hz off frequency. I’ll let them know.

Next, I have found it beneficial just to keep the dongle and a small antenna in my laptop bag. Setting up a new RDS subcarrier; with the dongle and SDR# one can quickly and easily check for errors. Tracking down one of those nasty pirates; a laptop with a directional antenna will make quick work.

Something that I found interesting is the waterfall display for the PPM-encoded stations:

Not only can you see the watermarking on the main channel, you can also see the HD Radio carriers +/- 200 KHz from the carrier frequency. That is pretty much twice the bandwidth allotment for an FM station.

WDPA using RTL 2831U with SDR Sharp — WDPA using RTL 2831U and SDR Sharp

Those two stations are simulcasting. WPDA is not using Nielson PPM nor HD Radio technology. There is all sorts of interesting information that can be gleaned from one of these units.

Aiming a satellite dish at AMC-8 can be a bit challenging. That part of the sky is pretty crowded, as it turns out. Dish pointer is a good general reference (www.dishpointer.com) and the Dish Align app for iOS works well. But for peaking a dish, the RTL 2832 dongle makes it easy to find the correct satellite and optimize the transponder polarization. Each satellite has Horizontal and Vertical beacons. These vary slightly in frequency, thus, but by tuning to the correct beacon frequency, you can be assured that you are on the right satellite. All of the radio network programming on AMC-8 is on vertically polarized transponders, therefore, the vertical beacons are of interest. Here are the vertical beacons for satellites in that part of the sky:

Satellite	Position	C band Vertical beacon (MHz)	L band (LNB) Vertical beacon (MHz)	Comment
AMC-8	139W	4199.5	949.25
AMC-7	137W	3700.5	1450.25
GOES15	135.4W	2209.086	N/A	NOAA WX
AMC-10	135W	4199.5	949.25
Galaxy 15	133W	4198	949.00
AMC-11	131W	4199.5	949.25
Galaxy 12	129W	3700.5	1450.25

For those in the continental United States, there is not much else past 139W, so AMC-8 will be the westernmost satellite your dish can see. Of course, this can be used in other parts of the world as well, with the correct information. Bringing a laptop or Windows tablet to the satellite dish might be easier than trying to drag a XDS satellite receiver out.

In order to use the RTL-2832U, simply split the output of a powered LNB, and install a 20-30 dB pad in between the splitter and the dongle. Using the vertical beacon on 949.25 MHz, adjust for maximum signal.

For some other uses; look for the nearest and best NOAA Weather radio station. Several times the local NOAA weather station has been off the air for an extended period of time. Sometimes, another station can be found in the same forecast area. Heck, couple these things to a Raspberry Pi or Beaglebone black, and a really nifty EAS receiver is created for NOAA and broadcast FM. One that perhaps, can issue an alarm if the RSL drops below a certain threshold.

I am sure there are plenty of other uses that I am not thinking of right now…

The isocoupler and the SX2.5

The second post in the series, “things to do with a truck body tool box.”

We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping. The SX2.5 transmitter was of an age when, apparently, VSWR fold-back circuits were just a gleam in Hilmer Swanson’s eye. The correct description of the sound made over the air during this event would be “motor boating,” because that is what it sounds like. Obviously, very undesirable.

Thus, the isocoupler was removed from the tower, dried out, waterproofed, and replaced. That lasted about six months.

Once again, the isocoupler was removed from the tower, a capacitor was remounted, drain holes and a small vent were added to the top of the unit and it was replaced. That lasted about a year.

I am getting a little tired of this and so is the client. Time to rethink the entire setup.

We had several leftover parts from various AM decommissioning over the last few years which included these nifty sample loop isolation coils:

AM antenna system sample loop isolation coil

Why not repurpose one of these to make an isocoupler for the translator?

Enter; the truck body toolbox. This one is slightly smaller than the last one, measuring 23.5 x 18 x 16 inches (60 x 45 x 40.5 cm).

The isolation coil consists of 35 turns of 3/8 coax on an 11.5 inch diameter form. The coil length is 15 inches. I calculate the length of the coax on the coil to be out to be right around 100 feet using the π x D x (turns) formula. I measured the inductance with my analyzer, which came out to 200 μH. Not to shabby.

Checking length of cable with TDR — Checking the length of cable with TDR

The coax is Cablewave FCC38-50J which has a velocity factor of .81 and the TDR shows it to be 100 feet also.

Coil impedence and reactance — Simple coil impedance and reactance

At 860 KHz, the isolation coil presents 1,200 impedance. I don’t think that will be good enough for that cranky old SX2.5. I decided to make a parallel LC circuit (AKA a tank circuit) to bring up the impedance.

Tank circuit formula:

Where:

F_R = Resonance frequency in Hertz
L = Inductance in Henrys
C = Capacitance in Farads

Given that I have two leftover capacitors, one is a .001 μF and the other is a .0012 μF, those values determine where the coil needs to be tapped. I also wanted to have a good bit of coil in the circuit on the tower side before the capacitor tap to dampen any lightning strikes on the tower. Thus the inductance needs to be about 28 μH.

Using Wheeler’s coil inductance formula:

L= (d² x n²)/(18d+40l)

where:

L = inductance in micro Henrys
d = coil diameter in inches
l = is the coil length in inches
n = is the number of turns

I removed a small portion of the outer jacket on the coil at approximately the 28 μH point (12 turns) then installed a .0012 μF capacitor. I used a small variable capacitor to tune for resonance on the carrier frequency. With this setup, at 860 KHz, there is >47,500 impedance. That goes down to about 16,000 ohms +/- 10 KHz.

That should make things better.

Then I mounted the coil and capacitor in the truck body toolbox. There is a fair amount of stray capacitance from the box itself, which raised the resonant frequency by 5 KHz.

Device Under Test, initial testing of isocoil after fabrication — Device Under Test; initial testing of isocoil after fabrication

Resonance is slightly above the carrier frequency with the permanent fixed .0012 μF capacitor. I think this will change once the unit is connected to the station ground plane. The network analyzer indicated there is too much capacitance in the circuit. Unfortunately, this may be as good as it gets, however, the analyzer shows the impedances are still pretty high:

Frequency (KHz)	Impedance (Ohms)	Deviation from Carrier (KHz)
850	9,950	– 10
855	14,720	– 5
860	28,590	0
865	59,580	+ 5
870	24,780	+ 10

The base impedance of this tower is 34 ohms on the carrier frequency, so the isocoupler should be invisible to the transmitter across the 20 KHz occupied bandwidth of the station.

The FCC38-50J cable has a loss of 1.04 dB per 100 feet at 100 MHz, which is the figure I will use to calculate the insertion loss on the FM translator antenna system.

The old isocoupler is made with RG-214, but likely a somewhat shorter length. RG-214 cable has a loss of 1.9 dB per 100 feet at 100 MHz.

Installation:

Before and after measurements with the network analyzer show a very slight change in the reactance at the tower base. Nothing major and easy enough to tune out with the series output inductor of the ATU.

If I were to do this again, I would simply tap the coil at ten turns from the bottom, measure the inductance and install the proper value capacitor. Since this had to be constructed with the parts on hand, less the truck body toolbox, it because a bit cumbersome to get close to the resonant frequency.

All this got me thinking; there are other possible uses for such a design. Crossing a base insulator with an Ethernet cable always presents some unique problems. I know the WISP forum that I read, they are always talking about how difficult it is to mount an antenna on an AM tower. What if… armored Cat5e or Cat6 cable was used with waterproof RJ-45 jacks? Something like that could carry Ethernet data and DC voltage past the base insulator to a three or four around sectorized access point and an edge switch or router mounted on the tower.

Armoured category cable specifications — Armored category cable specifications

just thinking…

Anyway, it would not be hard to make coils and install capacitors for the right frequency

Прощаться!

This information is from an occasional reader who wished to remain anonymous.

Another AM station surrenders its license, this time from north of the border. CKSL, London, Ontario, Canada is gone for good. The current owner, Bell Media, has determined that it would cost more to repair the deficiencies with the antenna system than economically feasible, especially considering its low ratings. Here is their filing with the CRTC:

Bell Media is the licensee of CKSL-AM 1410, assuming stewardship of the station in 2013 as part of the Astral Media acquisition.

A technical review of the transmitter site was recently completed both by Bell Media and contractors, which has resulted in the determination that the AM array poses an unacceptable risk from a health and safety perspective. The five towers are experiencing serious structural degradation and also require repairs to the aviation safety lighting system. In addition, the building which houses the transmitter has shifted off its foundation (as have several of the individual tower sheds).

Given these problems, Bell Media would need to make a significant financial investment to bring CKSL-AM’s transmitter up to compliance with Human Resources Development Canada, Industry Canada and NavCanada operational codes and standards, all of which is estimated to exceed $3 million dollars.

From a market perspective, CKSL-AM has consistently ranked last out of all ten commercial stations in the London market, both in audience share and revenue generation, over the last several years. In fact, since 2013 the London market has seen radio revenues drop 4% and CKSL-AM generates the least amount of revenue of the stations in the market. Even with a significant investment in programming, this trend is unlikely to be reversed.

In light of the significant capital costs coupled with the absence of revenue and audience share, Bell Media is respectfully requesting the revocation of the CKSL licence.

Well, 24/7 comedy will do that to you. Somebody in the business said to me recently “The listeners are abandoning radio!” No, it is the broadcast station owners who are abandoning their listeners and their cities of license. I have a news flash for all current broadcast station owners; as surprising and radical as this might sound, bland, boring, canned, completely irrelevant, dismal, uninformative, unimaginative, unentertaining, dreary, stale, unenjoyable programming will drive away even the most loyal listeners. People really want to listen to the radio, it is an easy habit and readily accessible. Radios are ubiquitous; they are in our kitchens, bedrooms, cars, hotel rooms, offices, restaurants, barber shops, etc. That, however, may not always be the case, as more and more people move to Spotify, Pandora, or Apple radio when they are tired of the disappointment. I was listening to a certain sports radio format the other day and I kept waiting for something interesting to happen. I waited and waited. I would say to myself; okay, this will be the segment when I will learn something or be entertained. This upcoming guest will say something interesting. Sadly, those expectations were never met and I will never tune into that station again. Elevator music would have been better. Worse than sports radio, 24/7 comedy is the absolute death knell. This is like saying; we are out of ideas and we do not care.

Here are a few pictures of the former CKSL-AM transmitter site:

CKSL_transmitter — CKSL transmitter building

CKSL_transmission — CKSL transmission line bridge

Actually does not look too bad, at least the field is mowed. I have seen much, much worse. Those bolt-together towers, though. I would bet that they are the real problem, bolts are deteriorating faster than the tower steel. Very likely all the towers need to be replaced and that is why the license is being surrendered.

If you are a radio geek, get out there and take some pictures of your favorite radio station. If the current trends continue, eventually they will all be gone.

A Linux based remote control system

We are extending LANs out to transmitter sites for many reasons; backup audio, control and monitoring, security systems, VOIP phones, etc.

I am casually (very casually) toying around with creating my own Linux-based remote control system. The ongoing Windows 10 upgrade debacle continues to not end, I can’t help but think that there are many potential clients who could use a reliable transmitter/studio remote control and monitoring system based on a stable operating system. Hmm, sounds like a sales pitch 😉

Anyway, I have run across several Ethernet board manufacturers that offer a variety of boards with 8-12 contact closures and a variety of analog and digital inputs. Most new transmitters have some sort of web GUI which are great for transmitter control and monitoring. As we all know, there is more than just a transmitter at any given transmitter site. In addition to the transmitter, I would like to control and monitor things like tower lights, interface and control of coax switches, temperature monitoring, generator status, the old non-web interface backup transmitters, STL signal strength for those old 950 MHz links, etc.

Since Google is my friend (when they are not storing my search data), I came up with this: Internet-ethernet-12-channel-relay-board

That particular PC board is made in Bulgaria, which is home to this: Mount Buzludzha

What I like about these particular boards is the DRM software (DRM has, apparently, many different meanings) which will run on Linux or Windows. There are also iOS and Android applications that can be used as well. It appears that the GUI can be customized for various uses. This seems like it is written in Java, so perhaps I could have some Java expert customize it for radio use. It looks like up to 32 boards can be controlled by a single instance of the DRM software. Alarm reporting would be via SNMP trap and email.

I don’t know, there is one particular cluster of stations that needs new remote control gear at almost every transmitter site. Perhaps a little alpha testing is in order? It could be fun…

Anyway, just a thought…