Paul Thurst – Page 2 – Engineering Radio

The Shively 6025 Broadband Log Periodic Antenna

Several companies make variations of this antenna; Scale FM-CL is a lower-power version that is used mostly by translators. They are highly directional and can be installed in a vertical, horizontal, or cross-polarized (45-degree slant) manner. This model input power is 5 KW per bay and the manufacturer’s specification is for 1.28:1 or less VSWR across the entire FM band. In the slant configuration, which Shively states is right-hand circularly polarized, the gain is 4.03 dB.

I recently did some work onsite for WXMD, California, Maryland. They were having some issues with high reflected power readings on their transmitter and suspected an antenna or transmission line problem. The station has been on the air for about 10 years and began having issues late last year after a thunderstorm passed through the area.

WXMD California, MD South East Bay Shively 6025 antenna

The main issue was that the transmitter was showing 243 watts of reflected power with 9800 watts of forward power, while the inline watt meter showed 37 watts. As part of the repairs, a new 1 5/8 transmission line was run up the tower replacing the old line which was damaged at the power divider input connector. A new power divider was also installed. Was the antenna still defective? Was the new transmission line and/or power divider defective? Was there an issue with the inline watt meter? Questions, questions, questions…

Thus, several sweeps were needed to verify things:

1 5/8 inch line terminated at the power divider with known good load

This antenna has a power divider that splits the power between a southeast-facing antenna bay and a southwest-facing antenna bay. To be sure that we were not dealing with a bad connector or transmission line, the line was swept in isolation from the input of the inline watt meter to the input of the power divider. This showed that the transmission line, connectors, elbows, and inline watt meter were all good.

Next, each antenna bay was swept individually. The power divider port going to the disconnected antenna was terminated with a known good 50-ohm load.

Once the individual bays, jumpers, and power divider tested good, the entire antenna system was swept.

With everything connected, the SWR showed 1.19:1. Not ideal but not terrible either. The inline watt meter readings were verified with a precision watt meter and the final SWR calculated by hand was 1.16:1.

Therefore, the antenna system is performing within the manufacturer’s specifications.

The American Amplifier Technology inline FM watt meter was then checked with a precision power meter. The readings on that device were more or less in line with the precision power meter, thus the transmitter directional coupler is out of calibration.

Mini-Circuits Precision Power meter, Forward Power

Mini-Circuits Precision Power meter, Reflected Power

The transmitter shelter is just large enough for one rack. Thankfully, the weather was cooperative, we were able to work outside. Overall, it was a productive trip and an enjoyable experience.

SAS studio installation work

I receive several emails a week from interested readers. One noted that the blog seems to be focused on RF. Yes, that is what I do most, but the company does studio installs as well.

This was from a few years ago.

WDST moved out of their Bearsville studio location into the former Methodist Church in West Hurley.

We installed a new SAS audio router and console system.

Small Rack Room for NextGen servers and STL equipment

Pictures of their old Bearsville studio can be found here:

WDST, Woodstock, New York

If folks are interested, I will post some more pictures of more recent studio-build projects.

The Nautel J-1000

I Finished up this installation of a J-1000 in Brookfield, Connecticut for Nossa Radio. That is a Portuguese broadcaster that owns three other stations in the US.

These Nautels are fairly simple affairs; a controller and two RF amps with incumbent power supplies.

Nautel AM-RF and AC mains surge suppressor

Be sure to install the surge suppressor that comes with the transmitter.

The J-1000 is replacing the 43-year-old Harris MW-1A which will function as a backup. Like all new transmitter installations; some things must be done to complete the job.

Harmonic measurements out to about the 5th or 6th harmonic need to be documented and compliant with NRSC-2 (AM mask requirements). Although NRSC-2 measurements are required, I don’t see how they can enforce that specification after AM HD radio came into being. Nevertheless, it was measured and passed. With the station carrier power of 680 watts, I used the RF monitor port on the back of the transmitter to make the measurement. Otherwise, I would need to find an empty field somewhere 1 KM away and stand in the middle of it to reduce all of the electrical noise.

The NRSC-2 mask is mainly a function of High-Frequency limitation in the audio processor—certain transmitters, like the aforementioned MW1A did make some contributions to out-of-tolerance measurements.

The antenna is a skirted tower that has many other services colocated on it. At the top is WRKI.

Driving away from this site, I would have to agree with the predicted contour map above, at least on the highway. I think it may be a bit different driving around in town.

Another Low Power TV installation

One of the many projects we are currently finishing up. Over-The-Air TV is making a comeback.

A few things about LPTV; These stations usually have an ERP of 15 KW or less, and they are a secondary service, like FM translators, which a full-power TV station can displace.

Alive Telecom ATC-BCE48BB-V3-31 UHF slot antenna

This is an ATT microwave site built in 1977 according to county records. This may have been one of those VHF Mobile Telephone sites which existed before cellular telephone systems. ATT owned it until 2022 when it was sold to a private business.

This station is on channel 31 or 575 MHz center frequency. UHF TV stations often use slot antennas, which have gobs of gain. Slot antennas are simple designs that have a broad bandwidth and until recently were mostly horizontally polarized. This particular antenna is elliptically polarized which is becoming more common as TV providers are looking at mobile video applications.

Example of UHF slot antenna with Radome cover removed

Slot antennas are the inverse of a dipole antenna. A dipole is two conductive poles approximately 1/2 wavelength surrounded by free space whereas a slot antenna is 1/2 wavelength of free space surrounded by a conductive plane. The width of the slot determines the bandwidth of the antenna. Radiation from a dipole is in the plane of the two poles versus the radiation from a slot that is perpendicular to the slot. At UHF frequencies, many slots are placed on the radiating plane, giving large gain figures.

Transmitter rack with 6-pole Comtech mask filter

All TV transmitters require a bandpass or mask filter. This is to keep out-of-band emissions out of the tightly packed TV spectrum.

S11 return loss, looking at the antenna through the mask filter

These filters need to attenuate the upper and lower shoulders of the digital carrier by 46dB +/- 3.25 MHz from the center frequency.

These are fairly straightforward filters, this one has six cavities with plungers that slide in and out to adjust the tuning. I watched one of these get retuned in the field, it takes quite a bit of time and patience to complete and requires a two-port network analyzer.

400-watt UHF amplifier, exciter, and IT gear; WZPK-LD

With the TPO of 400 watts, the ERP is 4.7 KW horizontal and 2.35 KW vertical.

Longley Rice coverage map; green is easy indoor, yellow is outdoor, red is difficult outdoor antenna

So, why bother with all of this? Indeed that is a good question. As cable companies continue to raise their rates (the average cable TV bill is $250 or so) people are looking for alternatives. Cord cutting is a thing and OTA (over-the-air) TV as well as OTT (over-the-top or direct streaming) are popular alternatives.

This station will run France 24 English service and NASA TV to start. Other things you can find on Low Power TV stations; Heartland (mostly country music with some cooking shows mixed in), Retro (old movies), Rewind TV (Old TV shows), Buzzr (old game shows), Court TV, Weather Nation, News Net, etc. More information on OTA TV networks can be found here: https://en.wikipedia.org/wiki/List_of_United_States_over-the-air_television_networks

Rabbit Ears has a good signal search page if you are interested in OTA TV: https://www.rabbitears.info/searchmap.php

More information on Low Power TV (or Local Power TV) and be found here: https://www.lptvba.org/

For a sometimes interesting discussion on cord cutting in general, try this: https://old.reddit.com/r/cordcutters/