tech stuff – Engineering Radio

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.

Weak Signal Propagation Reporter

Slightly off-topic, but includes radio.

The antennas are the most interesting aspect of Radio Frequency Engineering to me. The transfer of power in the form of voltage and current to the magnetosphere and back again is where the rubber meets the road. Any opportunity to experiment with the art of antenna design and fabrication is welcome.

This is for the Amateur Radio community. With the upswing of Solar Cycle 25, predicted to peak in July of 2025, I decided it would be fun to get back on the air with some type of HF setup.

My past experience with HF radio and peak solar cycles is that wild fluctuations can occur creating band openings at unusually high frequencies or no propagation at all. The geek in me finds this very interesting. HF Propagation is a complex matter. Long-distance communication can be carried out with very low power levels provided the ionosphere is bouncing signals back to the earth instead of absorbing them.

Weak Signal Propagation Reporter (WSPR) is an HF beacon system, where stations transmit a digital signal containing your call sign and Maidenhead Gird locator for several seconds. The challenge is to have an efficient antenna and use as little power as possible. In this case about 200 mW (0.2 watts) or 23 dBm. The modulation type is MFSK and the bandwidth is 6 Hz. According to Wikipedia, which is mostly accurate about things like this; WSPR uses a transmission protocol called MEPT_JT. That sends messages composed of:

28 bits for callsign, 15 bits for locator, 7 bits for power level, total: 50 bits.
Forward error correction (FEC): non-recursive convolutional code with constraint length K = 32, rate r = 1⁄2.
Number of binary channel symbols: nsym = (50 + K − 1) × 2 = 162.
Keying Rate is 12000 ⁄ 8192 = 1.4648 baud.
Modulation is continuous phase 4 FSK, with 1.4648 Hz tone separation.
Occupied bandwidth is about 6 Hz.
Synchronization is via a 162-bit pseudo-random sync vector.
Each channel symbol conveys one sync bit (LSB) and one data bit (MSB).
Duration of transmission is 162 × 8192 ⁄ 12000 = 110.6 s.
Transmissions nominally start one second into an even UTC minute: e.g., at hh:00:01, hh:02:01, etc.
Minimum S/N for reception is around –34 dB on the WSJT scale (2500 Hz reference bandwidth).

Distant stations report reception to a database. Several good websites display reception in a map or table format.

This map shows a good path to coastal Maine on 40 meters. The received signal-to-noise ratio is -2 dB at a distance of 423 KM.

80 Meter End Fed Half Wave antenna supported by trees

My antenna is an End Fed Half Wave (EFHW) cut to 3.568 MHz which can be used on any harmonically related frequency (7, 10, 14, 18, 21, 24, and 28 MHz). To accomplish this, a 49:1 Unun (Unbalanced feed to unbalanced feed) transformer is used to transform the 2,400-ohm impedance of the wire to the 50-ohm impedance required by the transmitter. The antenna works best against a ground system that is not less than 0.05 wavelength or 18 electrical degrees on its lowest frequency. That works out to about 4.2 meters (14 feet). A little bit longer is a little bit better. Six 20-foot long 14 gauge bare copper ground radials are attached to an 8-foot ground rod.

Diecast aluminum box containing 49:1 Unun

The Unun is two FT240-52 (not an affiliate link) cores with 14 gauge enamel wire consisting of 2 turns on the primary and 14 turns on the secondary. The antenna is 40 meters (132 feet) of 10 gauge hard-drawn stranded copper wire. This should be good for about 800 watts CW/SSB on HF if I want to use it in that capacity.

Unun wire tied to a DIN rail with 100 pF 5 KV capacitor

There are several guides on how to make the unun available via Google search. There is some debate on whether a 64:1 transformer should be used. Most indicate a 49:1 is the best match. The diecast aluminum (not an affiliate link) enclosure is a nice feature. It cost $33.00 on Amazon.

I used the network analyzer to trim up the antenna a bit. I made a few measurements, the first was just the wire with no ground connected. The next was the wire and ground system after trimming the length for resonance on 3.5 MHz.

The transmission line is LMR-400 with N connectors. I loath PL-259s and use N connectors whenever possible.

I did a series of broadband SWR sweeps. The first was just the wire prior to trimming.

First sweep, frequencies are a little low, SWR is a little high

The next was with a ground rod and six ground radials, #14 bare copper wire twenty feet long.

EFHW trimmed up and looks good on everything except 60 Meters (10 MHz)

This demonstrates the effect of a good ground system. It is worth the effort (and it is an effort) to put in some buried ground radials with this type of antenna. I think above-ground radials would work too.

Here is a screenshot of the little Zachtek desktop WSPR beacon transmitter I bought. This is a great addition to the toolbox and works well for testing the radiation efficiency of an HF antenna. It has a GPS antenna input for timing and location reference. The frequency bands are selectable if you are testing a mono-band antenna. It will work into a fairly poor load, so I suggest sweeping the antenna first with an analyzer.

This shows that my signal is getting out. So far, the furthest distance is 17,030 km with an SNR of -10 (Australia, VK5ARG). That is quite amazing when you think about it. I am letting this run overnight to see how the propagation changes. Overall, this was a good recreational project and now I have a known working HF antenna.

How long should a transmitter last?

This Broadcast Electronics FM3.5A is 40 years old. There was a small problem that took the station off the air for a couple of hours this morning. The high voltage shorting solenoid fell apart, causing the 40 amp breaker in the service panel to trip.

These types of failures will become more frequent as the transmitter ages. Things like air switches, blower motors, tuning and loading mechanical assemblies, circuit breaker fatigue, plate rectifiers, screen and plate bypass capacitors, exciter and controller fans, etc. The list of potential failure points can get quite long. The fact is, nothing lasts forever.

There is no backup transmitter for this site and there is no easy way to get a temporary unit on line, if needed. This is not the oldest main transmitter that we service with no backup. That honor goes to a CCA DS-3000 built in 1970.

The question is; how long should old tube transmitters be kept in service? Also; how long should we (an independent service company) agree to maintain them? The temporary solution for the above failure was to remove the broken shorting bar and turn the transmitter back on.

That creates a safety issue for anyone who may need to work on the transmitter before the replacement arrives. It also creates a potential liability issue for my company.

I put a big label on the back door indicating that anyone doing service needs to discharge the power supply capacitor with the grounding stick (which they should be doing anyway). But I will feel better when the shorting solenoid is working again.

Rohde Schwarz Test & Measurement Fundamentals

I found a great resource for learning about test and measurement on Rohde Schwarz’s YouTube channel. Each video is about 5 to 15 minutes long and covers the basics of RF test equipment, measurement parameters, and definitions.

Rohde Schwarz Test and Measurement Fundamentals

Measuring RF systems is an important part of Broadcast Engineering. Many folks think that RF plants are going away, replaced by all IP content distribution. I disagree; Terrestrial Broadcasting will be around for a while yet. AM and FM radios are still ubiquitous in cars, homes, businesses, etc. There is no other information distribution method that is as simple and robust as over-the-air broadcasts. That is why Federal Emergency Management is still spending money on hardening broadcast facilities.

The Internet and Mobile Data in particular are susceptible to failure in emergencies. Cellular networks were almost useless due to congestion or system outages during the 9/11 attack or a natural disaster such as Hurricane Sandy.

Radio still has a role to play.

As the older Broadcast Engineers retire, there is a dearth of qualified RF specialists who can make accurate measurements on antenna systems, filters, and other transmission system components. There are very few mentoring opportunities, especially in commercial broadcasting. Gone are the days of several engineers on staff, when there was time to teach the younger people some hard-learned lessons. One of the reasons I write this blog is to pass along some of that knowledge to others so that the industry might survive.