tech stuff – Engineering Radio

After 10 years, it is time to move

W277CJ, Pittsfield was originally a translator for WUPE-AM 1110 KHz. Since that license has been surrendered to the FCC, it is now a translator for WBEC-FM HD2 which is simulcast of WUPE-FM, North Adams.

Confused yet? Don’t worry, it is a reshuffling of signals because the land under the 1110 KHz transmitter site was sold and the license turned in to the FCC. Something that I think will happen many more times to many more stations in the coming years. This translator was first put on the air in June of 2015. This is the third part of a series, the first two parts are: More AM work, Part V and The Bext TFC2K broadband antenna.

The translator recently moved it off of the Holiday Inn (formerly Crown Plaza) in downtown Pittsfield, MA to the WBEC AM tower. In order to make that move, we needed to do several things;

“sufficient measurements shall be made to establish that the operation authorized in this construction permit is in compliance with the spurious emissions requirements of 47 C.F.R. Sections 73.317(b) through 73.317(d). All measurements must be made with all stations simultaneously utilizing the shared antenna.”

These are intermodulation products, or third order products, between the two signals being transmitted, in this case W277CJ on 103.3MHz and WUPE-FM 95.9 MHz. The antenna side mounted on the AM tower serves as a back facility for WUPE-FM.

WBEC-FM backup and W277CJ connected to Bext FDCSDC-2 combiner

Those measurements are as follows:

(F1-F2) + F1 or (103.3 MHz – 95.9 MHz) + 103.3 MHz = 110.7 MHz
F2 – (F1-F2) or 95.9 MHz – (103.3 MHz – 95.9 MHz) = 88.5 MHz
F2 +F1 or 95.5 MHz + 103.3 MHz = 199.2 MHz

In order to make those measurements, I used two Microwave Filter Company MFC-6367 notch filters to attenuate the carriers on 95.9 and 103.3 MHz. This keeps the spectrum analyzer from overloading, thus lowering the analyzer noise floor and giving better results.

Various tools for proofing FM installations

Over the years, I have collected various parts to assist in getting good measurements for FM proofing. Going clockwise and starting at the top, the Rhode Schwarz NRP-Z11 power sensor, the MFC-6367 FM notch filters, directional couplers with power extractor element, various attenuators including the HP 255C variable 0-12 dB unit, and in the middle are two Mini-Circuits NHP-200+ high pass filters. The high pass filters are great for measuring harmonics.

To measure the third order products noted above, I first measured the carrier without the filters and an appropriate pad to get a carrier reference level. Then installing the MFC-6367 filters to measure the third order products. In addition to that, harmonics of both FM transmitters out to the 10th harmonic. Of particular importance is anything in the cellular or mobile data bands. All of these measurements were well below the -80 dBc threshold required by the FCC.

All of these measurements were well within the limits established by FCC part 73.317.

Also, because this is mounted on an AM tower, there are some AM things that needed to be completed:

“The AM station identified below may be affected by the facilities authorized by this construction permit. Pursuant to Section 1.30004 of the Commission’s Rules, at least 30 days prior to commencement of construction of the facilities authorized herein, the permittee must provide notification of the construction to the AM station licensee. As part of this notification, the permittee must examine the potential impact of the construction of the authorized facilities on the AM station using a moment method analysis. The analysis shall consist of a model of the AM antenna together with the potential re-radiating tower in a lossless environment. The model shall employ the methodology specified in Section 73.151(c) of the Commission’s Rules, except that the AM antenna elements may be modeled as a series of thin wires driven to produce the required radiation pattern, without any requirement for measurement of tower impedances. If the AM station was authorized pursuant to a directional proof of performance based on field strength measurements, the permittee may, in lieu of the moment method analysis, demonstrate with measurements taken before and after construction that field strength values at the monitoring points do not exceed the licensed values.”

Since this station was proofed several times, we did about ten readings along the monitor point radials, both before and after.

The new isocoupler was properly mounted:

This is simply a large coil of 7/8 Coax wound inside of a PVC form.

These AM antenna systems are a regulatory nightmare. Although the Moment Method is an improvement over the system of field measurement proofs, it is still complicated. Part of the issue with AM in general is the expense of the the antenna systems, particularly anything that is directional.

No real research into Medium Frequency antennas and propagation has been done since the 1930’s. Perhaps we know all there is to know about it, then again, perhaps not. I am currently working on a project which will study Medium Frequency propagation, which I feel, is the first step into revisions of antenna design.

The General Electric T1000C Stereo Receiver

My parents had one of these units on the side table in the dining room. My father put up an FM antenna outside on the roof so he could listen to more stations. In the early 1960s, there were not as many around as there are today. Our house was on the wrong side of a hill for the NYC stations, although Peekskill seemed to come in just fine. What is fascinating to me is the timing and cost. These stereos were made in 1963, not long after the Zenith/General Electric FM stereo system was adopted and first broadcast on WGFM (now WRVE) in Schenectady, NY (June 1, 1961). Not every FM station rushed out to install the new system.

General Electric T1000C Stereo receiver marketing

For a bit of a reference, $180.00 in 1963 is worth $1,868.65 in 2025. At that time, my father was an installer/repairman for New York Telephone. My mother was not working and six of us lived under one roof. That was quite a bit of money for an AM/FM radio.

The radio was normally tuned to 100.7 WHUD, which initially went stereo in 1972. Other stations that could be received: WGFM, WROW-FM, WSPK, WEOK-FM (now WPDH), and WGFH (later WINE-FM now WRKI).

General Electric T1000C stereo (Walnut cabinet)

I purchased this one on eBay for $70.00. It turns on (in fact, it did not turn off), there is a hum, the pots are scratchy, etc. However, if I tune it to one of the local AM stations, I can hear music under the loud 60-cycle hum. In other words, it works! So, I spent time fixing all the defects and enjoyed some nostalgia. According to this date stamp, the wood enclosure was made in January 1963. I would think the rest of the unit was made about the same time, which means this is one of the first radios in this model. This may have been manufactured in Bridgeport, CT, or Syracuse, NY. The serial number is missing from the back of the chassis.

The main source of the hum appears to be this capacitor, which clearly has seen better days.

The on/off problem was the selector switch, which stuck in the on position because it was gunked up with dried-up lubricating oil and dust. I cleaned it with denatured alcohol and DeOxit.

The parts list included about $15 worth of capacitors, $1 for a new rectifier diode, a $7 telescoping FM antenna from Amazon, and $6.32 for two PLT 12 6.3 volt miniature lamps for dial light.

All of the tubes look to be the original GE units. After the recap, I turned it on and there was nice sounding AM, but no FM. The FM RF section has a triple triode (V2) which is the AFC, 10.7 MHz Oscillator, and mixer. This tube was loose in its socket and needed to be reseated. After that, everything worked.

All of the pots were scratchy. I cleaned them with DeOxit and worked them back and forth many times. After a while, they all are working.

FM Stereo receiver MPX decoder block diagram

I found a Sam’s Photofact (basic service manual) on this set. What is very interesting is the schematic for the multiplex receiver. This section decodes the L+R/L-R signals and produces the stereo audio. Unlike modern FM stereo receivers, in which the broadband multiplex signal is fed into one side of a chip and the discrete L/R signal comes out of the other side, the signal path through the various processing stages can be followed.

The broadband MPX signal comes from the IF stage via wire #27. The signal is amplified by V6. The L+R (20 Hz to 15 KHz) or mono signal goes through a low-pass filter L17/C40; the 3dB cutoff should be around 16-17 KHz. The L-R and 19 KHz pilot goes to wire 34, thence through a high-pass filter C37/L16/C38; the cutoff should be 20 KHz or so. The L-R and 19 KHz pilot are Amplitude Modulated subcarriers on the FM signal. Wire 38 routes the MPX signal to V6 which recreates the 38 KHz subcarrier by doubling the 19 KHz pilot. This is filtered by a bandpass filter C13/L14. The L-R and the 38 KHz subcarrier are sent to the product detector.

Diode product detectors X4 and X5 (1N541) demodulate the lower sideband (23 – 37.98 KHz) and the upper side band (38.02 – 53 KHz) respectively. Those signals are summed in the matrix subassembly K4 with the L+R. Mathematically, the results are:

The Left and Right audio is then sent to the first audio stage V7 through a deemphasis network. If no 19 KHz pilot is detected, no 38 KHz carrier is recreated and this stage remains silent. In other words, you have to find an FM station in mono first, then flip it to stereo to see if there is enough signal to decode the L-R. One of the limitations of the first generation of FM stereo receivers. Newer versions of this set have a stereo light, or “Stereo Eye” so the listener knows when stereo reception is possible.

The front of the cabinet is covered with glass, which I cleaned with soapy water. The glass has part of the gold leaf trim rubbed off. I think this radio got a lot of use.

I let the knobs soak in soapy water overnight then cleaned them off with an old toothbrush. I believe that this radio was once in a smoking environment, based on the amount of yellow, gooey substance covering everything. I ended up disassembling the entire unit to clean it. I used a paintbrush and the shop vac to get all of the dust out of the cabinet.

The speakers and speaker cones are in good condition. The speaker cabinets needed a little work; in both cabinets, the fronts (the part that is seen when both speakers are “closed”) were popping off. I had to glue a bit of wood back together and fix the metal holding brackets. The cloth on the speaker side is a little faded.

General Electric T1000C restoration complete

The wood finish is in good shape with a few scratches and dings. I decided to use Howard Restore-A-Finish. This is not the same as stripping and refinishing but rather repairing the existing finish. There was a water ring on top of the cabinet, which was removed with the Restore-A-Finish and light use of steel wool.

Three power supply capacitors, held down by a ty-base glued to the chassis

Reassembly went about as expected. I glued these tie bases to hold up the new capacitors.

The receiver is fairly sensitive and the dial is accurate. There is an alignment procedure in the repair manual, but I think everything is working as it should. I have spent enough time trying to fix things that are already working to know that for a 1963 tube receiver, this is good enough. Perfection, as they say, is the enemy of everything else.

So, how does it sound? Pretty darn good, as it turns out. I am working on a brief YouTube video with some religious music (I’ll post it when it is done). On the FM side, I can get WAMK, WBPM, WKXP, WJUX, WDST, WPDH, WFSO, and WPDA clearly with the whip antenna on the radio. AM, I hear WGHQ and WJIP.

I can hear the old man now, humming along to his favorite tune…

The GPSDO; what is it and why do I have one

I purchased this GPS Disciplined Oscillator a few weeks ago. The reason being, I wanted to make sure that this frequency counter was accurate.

This Hewlett Packard 5315A was last calibrated in 1990. That made me suspicious. While I could send it back to Agilent and have it recalibrated, I thought it might be interesting to check it with a known standard.

When I connected the frequency counter to the 10 MHz GPSDO, it was -2.1 Hz off. At first, I thought perhaps the GPSDO was off; however, the spec for the LBE-1420 is 1 x 10^-12 with a resolution of 1 Micro Hz. I let the HP unit warm up for 3 hours thinking maybe it was cold and would come back in tolerance. Nope, the frequency stayed about 2 Hz low.

It took about five seconds to find the full service manual online, which gives the alignment and calibration procedure in detail.

The first step is to use a DVM and check the +3, +5, and -5.2 power supplies. If they are off then adjust each accordingly. The next step is to check the +5 VDC pin on the Option 4 OCXO module and adjust as needed.

‘scope lead connected to reference oscillator pin

The calibration procedure for the HP 5315A is to connect a known 10 MHz reference to one channel of an oscilloscope and the output of the frequency counter OCXO to the other channel and look for slippage of the two signals. If the counter is on frequency, there should be no movement between the two waveforms. This is more accurate than trying to adjust the counter while looking at the frequency display on the counter.

Frequency alignment HP 5315A, Yellow squarewave trace LBE-1420 GPSDO, Violet sinewave trace 5315A reference oscillator

When I first connected it, the HP’s waveform was running backward at a pretty good clip. I adjusted the OCXO until there was no movement relative to the two waveforms. I let it sit like this for about three more hours before buttoning the HP unit back up. I am confident that the frequency counter is accurate +/- 0.3 Hz, which is good enough for my purposes.

What I like about this Leo Bodnar GPSDO is that you can change the output frequency to any value between 1 Hz and 1.4 GHz. The output level is +13 dBm (per data sheet) with low phase noise, making it an excellent portable signal generator. The output is a squarewave, however, installing an LC type bandpass filter such as a Mini-Circuits SBP 10.7+ will round that out into a nice sinewave.

The Leo Bodnar website has a portable Windows executable for download, which can be used to program the output frequency and monitor performance.

I measured the output with my precision power meter; at 10 MHz it was +10.35 dBm. The low power output setting is about +5 dBm.

WWV carrier measurement with LBE-1420 as external 10 MHz reference

Another use for the LBE-1420 is as an external 10 MHz reference for test equipment. My Network Analyzer (and many other pieces of test gear) has an external 10 MHz input and if I use the spectrum analyzer to measure carrier or pilot frequency, it is nice to know that the test equipment is exactly on. I confirmed this by measuring the WWV carrier with my Siglent SVA-1032X spectrum analyzer using a long wire antenna.

Continuing with this interesting topic, I purchased a fairly cheap version from Ebay for further research. This particular unit is a clone of a BG7TBL, which is itself a clone. The interesting thing about these units is that they are using recycled OCXOs, which appear to be from decommissioned telecom equipment.

This diagram shows how these units work. The GPS signal is received by the GPS module, in this case, a uBlox M-7. The NMEA sentences and 1PPS are fed into the CPLD (Complex Programmable Logic Device). The NMEA sentences are also available on the RS-232 DB-9 connector.

GPSDO component side, GPS module lower left

The CPLD takes the output of the OCXO, in this case, a CTI OC`12SC38A, and compares the 1PPS from the GPS module to the 10 MHz from the OCXO module and adjusts the OCXO module by varying the voltage on the frequency adjust pin to keep it on 10 MHZ. It then sends the corrected 10 MHz and 1PPS signal out to BNC jacks. I found the 10 Mhz output level was +13.58 dBm as measured with my precison power meter. There is a built in bandpass filter, so the output is a good looking sinewave.

Judging by the CTI model number, it was made before 2015. There should be a date code on the bottom of the unit, but I did not feel like unsoldering it.

The one issue with this; OCXOs frequency drifts over time and eventually it will be out of the adjustment range. A closer look at the circuit board shows that it will accept several different OCXO modules. These modules run about $40-60 US new and $10-15 US used.

If an OCXO is suspected of being out of adjustment, they can be measured using the osciliscope method noted above.

Does this thing work?

I found this obviously used GPS antenna in a storage room at one of our client’s transmitter sites.

How often have you asked that same question about some older piece of equipment lying around? There is a trend among engineers to hold off on getting rid of old equipment because someday, perhaps, it can be used again. Often, these treasures so lovingly stored away for many years or decades do not work when that day finally comes along, leading to disappointment and despair.

This GPS antenna falls into that category.

What to do, what to do…

Fortunately, there is an easy way to test this antenna and do other things with GPS. I had one GT-U7 GPS receiver module left over from a previous project.

Couple that with an FT232RL FTDI USB to serial converter and a bit of software from u-blox. The GPS receiver is a clone of a u-blox M6 GNSS chip, meaning the u-center software will work with it. That is a free Windows software application. The u-center software is great because you can access all of the options on the GPS receiver chip. Since this is to be used for testing, I enabled the LNB voltage sensing and protection features in the antenna configuration menu. Thus, the software will notify if there is a short circuit or open circuit in the GPS antenna under test.

I had this nice Hammond 1590WB diecast enclosure left over from a previous project. It’s probably a bit of overkill, any small enclosure would work, but why buy something new?

Here is a complete list of parts:

GT-U7 GPS Module Satellite Navigation Positioning GPS Receiver, u-blox NEO M6 clone, Amazon B07P8YMVNT, $12.99
FT232RL Mini USB to TTL Serial Converter Adapter Module 3.3V/5.5V, Amazon B00IJXZQ7C, $6.49
IPEX to SMA jumper, RG-178, 4 inch, Amazon B0B9RYL56H, $8.78
Hammond Manufacturing 1590WB, Amazon B005T59VNS, $9.63
Mini USB 2.0 Cable, USB A to B Cable, 3 feet, Amazon B00006B6PH, $3.95

The cross-connect between the two modules is fairly straightforward:

GT-U7 pin	use	FT232RL pin	use
2	TXD	2	RXD
3	RXD	3	TXD
4	GND	5	GND
5	VCC	4	VCC

Pin out

This particular FTDI module has a jumper to set the VCC to 3.3 or 5 volts DC. I left it on 5VDC to run the GPS receiver and provide LNB power to the connected GPS antenna.

I used a piece of packing foam tacked into the inside of the enclosure with gorilla glue, then tacked the FTDI module to the foam with gorilla glue. The GPS module is tucked under the header for the FTDI chip.

The software is pretty easy to use. The most difficult thing is figuring out which com port and baud rate to use. To find the com port, open the Windows device manager then plug the FTDI module in. The new serial device should register automatically. Click on the new device to get the com port information. I find the GT-U7 modules are most often set to 9600 from the factory, but it could be anything. I suggest trying different baud rates until you start seeing data.

Putting all of those things together, we get this miniature USB power GPS receiver with software that can show how well a GPS antenna is working and whether or not the location has good (or good enough) reception. One could also check the coax going to a GPS antenna and make sure that it is working right and not too long. Or check and see if a line amplifier is working, etc.

The answer to the above question is, yes!

u-center software screenshot, GPS antenna under test

The used GPS antenna is picking up plenty of signals from a less-than-optimum position. I’d say this is a keeper.

Device under test; GPS antenna on window sill

A little bit of orange paint, also left over, and a few labels and it looks like a professional unit. Not bad for some leftover parts I had lying around.