LPTV – Engineering Radio

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/

Filters for Over The Air Television

Many people are surprised that OTA TV (Over The Air Television) is still a thing. I am here to say that there are lots of TV stations still broadcasting. OTA is alive and well, especially around big cities. To wit; I noticed this older TV antenna on the roof of a transmitter building in Lodi, NJ. Being curious, I connected an ATSC 1.0 TV to the antenna lead in the kitchen. One scan captured 62 TV channels and sub-channels OTA in the NYC market.

Somewhat aged TV/FM antenna pointed at Manhattan

That site is 10 miles northwest of the Empire State Building.

I also noted that the satellite dishes on site have had Terrestrial Interference (TI) filters on the LNBs for many years. Recently, 5G filters were installed as well. Thus, I added a 5G/LTE filter made by Channel Master (part number CM-3201) to the TV antenna splitter. A rescan captured 79 channels. Interesting.

I began ordering TV receiver filters and testing them with my network analyzer. There are many different units made by different manufacturers. The smaller, cheaper units do not have as good performance as the larger, more expensive ones. Go figure.

Here are a few sweeps of various filters:

Channel Master CM-3201 5G/LTE filter. Cut off 608 MHz

Silicon Dust USA LTE LPF-608M. Cut off 608 MHz

There is also an FM band-stop (Channel Master CM-3202), which is effective for blocking out 87 to 113 MHz.

Sometimes I get questions from non-technical readers, thus for the uninitiated; these sweeps are return loss. The higher the line on the right-hand graph, the less signal will get through the filter. A flat line at 0dB means that little or no signal is getting through on those frequencies.

These filters are helpful, especially with inexpensive consumer-grade TV receivers. If you live near an FM transmitter site, then an FM band-stop filter may help, especially with low and high-band VHF stations. If you live anywhere near a cell site (and most of us do) then a 5G/LTE filter will likely help.

Happy cord-cutting!

A little bit of catching up…

I regret not having enough time for writing these days. There are a number of reasons for this. Firstly, much of what I do running my business is mundane and not worth noting. For example; today I am going over work reports and reconciling the bank account. Necessary, but about as exciting as watching the grass grow or reading about drying paint.

However, the rest of the time I have been working on various projects around the northeast, to wit:

In Boston, I took part in converting an LPTV station to ATSC 3.0. That was interesting and I am enjoying the TV work.

Boris Johnson resigns, also one of the first ATSC 3.0 images transmitted OTA in Boston, MA

In Syracuse, we had to lower a TV transmitter from the 23rd floor to the 22nd floor on the outside of the building. The transmitter itself became marooned because an electrical conduit for an alarm system was installed restricting the size of the stairwell.

Carefully lowering a 2KW UHF TV transmitter, State Tower Building, Syracuse, NY

Fortunately, we hired a moving company to do this. I am pretty sure that our insurance does not cover damages from transmitters falling 22 stories.

In NYC, I installed two FLX-40 transmitters for GatesAir.

WXBK FLX-40 transmitters, Rutherford, NJ

“Do you see over yonder, friend Sancho, thirty or forty hulking giants? I intend to do battle with them and slay them.” *“Take care, sir,* those over there are not giants but buildings on the island of Manhattan.”

In Kingston, NY a used BE AM1A (along with a coax switch and dummy load) was installed at WKNY.

Slightly used BE AM1A installed as backup at WKNY, Kingston, NY

7/8 inch coax switch and 2.5 KW oil cooled test load suspended from the ceiling

It is nice that this station has a decent backup transmitter to buttress the aging, yet very reliable Nautel ND-1.

Even though it is a short drive away, I had never visited the 1969 Woodstock Music and Art Fair site in Bethel, NY. It was interesting and being sort of an audiophile, I enjoyed this exhibit in particular:

Your author, standing in front of a “Woodstock Bin.”

From the display:

This speaker stack sat on scaffolding high in the air… festival sound engineer Bill Hanley custom-built eight speaker cabinets for Woodstock, amplifying music and stage announcements across the large festival site… Afterward, the design would be known in the industry as the “Woodstock Bin.”
Bethal Woods Performing Arts center Museum, August 10, 2022

The high-frequency horns used Electrovoice diaphragm S/A compression drivers. I don’t know which driver was used for the bins.

Site of Woodstock ’69 stage, looking up into the field

Looking down from the field to the stage area. Person(s) for scale.

I am also writing articles for Radio Guide, I hope that you are enjoying them!

Doing it with sound AND pictures

AKA: Television!

I am talking about the type of TV you need an antenna for. I have been installing a few of these low-power digital TV transmitters at various places and it is good work.

Anywave MPTV 2.2 KW digital TV transmitter, WCRN Boston

LPTV antenna side mounted on tower in Quincy, MA ERP is 15 KW.

Anywave MPTV unpacking, rolling into the building, Ellenville, NY

This transmitter’s dry weight is about 500 pounds, which was a little bit too much for our appliance hand truck, so we built a ramp. They have a nice set of wheels on the bottom, so they roll into place.

Carrier power after pre-correction files updated

There are a few differences in the way things are done. First of all, there is a different set of acronyms:

ASI- Asynchronous Serial Interface, format for MPEG transport stream, max speed 270 Mbps.
SDI – Serial Digital Interface – Similar to ASI but can run much faster, up to 12 Gbps.
TS – Transport Stream, Encoded video and audio streams into the exciter. Same idea as composite audio input on an FM exciter.
TSID – Transport Stream ID, a unique number assigned to each DTV station and encoded with the transport stream.
PSIP – Program System Information Protocol, carries program and system data about the transport stream.
ATSC 3.0 – recently updated ATSC standard that allows TV stations to do more with their transport streams than before. Will also change the modulation from 8VSB to COFDM.

Then, some things that look familiar are called by different names, BNC cable vs ASI cable… I am a neophyte in the TV world, so there are still many things to learn.

EAS is still EAS, but now there is a locally generated video to go along with the audio.

A while back, some fool wrote their congressman because THE COMMERCIALS ON THE TEE-VEE WERE TOO LOUD, so there is something called CALM Act compliance.

However, at the output connector on the exciter, through the amplifier, filters, directional coupler, etc; it is RF and behaves like RF. Even more interesting; Mr. Doherty’s name is used to describe the RF amp. Doherty amplifier or Doherty modulation was designed by William Doherty for Western Electric in 1936. It was not until Continental Electronics began using it in their AM (317B) and Shortwave (420A) transmitters that it became known broadly. Old things are new again:

Continental Electronics 420A 500 KW Shortwave transmitter, Greenville site B

That picture is from my visit to Greenville, NC in 2017. This is the control console, the transmitter is behind the glass and takes up half the building. It was installed when the site was built in the early 1960s. The new Doherty UHF amplifiers use LDMOS devices, notably the BLF888E in the Anywave units.

Antenna sweep, channel 30 UHF slot antenna, WCRN Boston

Antenna with channel band pass filter, Channel 30 WCRN Boston

The Channel Band Pass Filter is required by the FCC, basically, it ensures that the TV transmitter is staying in its allotted 6 MHz channel. They add phase rotation, as noted above. This is why pre-correction is needed to keep the ATSC signal linear across the entire 6 MHz bandwidth. RF is RF and we like RF.