Feedline impedance matching for the WSM 50 Kilowatt AM station in Nashville TN

[AA6EG]

TL;DR Summary

WSM, 50KW Nashville TN. Blaw Knox Diamond 5/8 wave vertical antenna 650khz since 1929, Matching house, coax feed, pictures.

The WSM 50 Kilowatt AM station in Nashville TN, has been transmitting using the same ~950ft 5/8 wave Blaw Knox Diamond antenna, since 1929. The 5/8 wave vertical had some horizon gain over a 1/4 wave vertical so theoretically will serve distant listeners better. It was found though that multipath RF routes combined and canceled at the distant ranges, and some years ago the antenna was shortened a bit to 850-some feet to optimize distant reception. Antenna plus guy weight, weighs 600,000 lbs to the ceramic insulated base insulator The antenna is base fed with a single conductor coming from a tuning house a short distance from the base, that matches the antenna impedance to the coaxial cable characteristic impedance.

Every aspect of the antenna, tuner, coax transmission line, and transmitter is massive. A recent really large lightning strike did major damage to the matching house, and it was rebuilt, since the pictures were taken.

 

[davenn]

HI and welcome to PF

Summary:: WSM, 50KW Nashville TN. Blaw Knox Diamond 5/8 wave vertical antenna 650khz since 1929, Matching house, coax feed, pictures.

Every aspect of the antenna, tuner, coax transmission line, and transmitter is massive. A recent really large lightning strike did major damage to the matching house, and it was rebuilt, since the pictures were taken.

So are you actually asking for some help or just showing some fun pic's ?Dave
VK2TDN

 

[anorlunda]

ping @dlgoff . Have you ever considered large scale restoration projects?

 

[Baluncore]

The long wave was probably chosen for maximum ground wave range.
650 kHz; λ = 461.5 m; 5λ/8 = 288.5 m = 946.5 ft; check ≈ 950 ft element.

I am surprised there is any coaxial cable there. It looks like parallel cage feed lines to me. As an end fed, λ/2 dipole it would have a very high input impedance, without a ground plane. The Z of a 5λ/8 vertical would also be quite high Z, too high for coax I expect.
So just how was the antenna fed? Against what?

Those early diamond verticals are hard to tune because the impedance along the element is tapered, so they had a continuous mismatch reflection.
A λ/4 wave conical dipole above a ground plane would be easier to match.

 

[zoki85]

Nice and interesting photos. Thanks for posting them. Are you employee/technical staff there?

 

[AA6EG]

HI and welcome to PF

So are you actually asking for some help or just showing some fun pic's ?
Dave
VK2TDN

No need for help, the Engineering staff at WSM rebuilt the Tuner house, and apparently all is well now.

Nice and interesting photos. Thanks for posting them. Are you employee/technical staff there?

No, but I visited the transmitter site after discovering it just off the freeway, on my way to Huntsville from Indianapolis. I stopped by, un announced and the southern hospitality of the place ruled, and the Ch engineer, Everett Lawson, gave me a grand tour and a cup of coffee. I am a ham radio operator, know and appreciate the technology, of Transmitters and antenna systems. The WSM Transmitter station and TX components blew me a way, in size and technical perfection. I got to know some of the the guys there, and discovered Physics Forums here because of earlier questions at Physics forums about WSM which I learned about over the years. Physics was one of my favorite classes and studies in college. I understand radio antennas pretty deeply and was impressed by the magnitude of the hardware at this site, and the power of mother nature (the lightning hit), recently. I consider WSM TX site, a wonder of the world.

 

[AA6EG]

The long wave was probably chosen for maximum ground wave range.
650 kHz; λ = 461.5 m; 5λ/8 = 288.5 m = 946.5 ft; check ≈ 950 ft element.

I am surprised there is any coaxial cable there. It looks like parallel cage feed lines to me. As an end fed, λ/2 dipole it would have a very high input impedance, without a ground plane. The Z of a 5λ/8 vertical would also be quite high Z, too high for coax I expect.
So just how was the antenna fed? Against what?

Those early diamond verticals are hard to tune because the impedance along the element is tapered, so they had a continuous mismatch reflection.
A λ/4 wave conical dipole above a ground plane would be easier to match.

I think you are right "Baluncore".
A 5/8 wave vertical has more effective radiation to the horizon which is the goal, but has some lobes of radiation at high angles, that interacted negatively with the ground wave, and I think, but am not sure, that the 5/8 needs less of a ground system, than the more common 1/4 wave AM verticals.
But the results at long ranges were not up to expectations, and they determined that the taper of the tower, fat at the center, and needle like at the top and bottom, changed the RF current flow in the radiator from the more predictable current in a straight sided tower, and that accounted for decreased long range signal strength.

performance expected from a 5/8 wave vertical

 

[AA6EG]

I think you are right "Baluncore".
A 5/8 wave vertical has more effective radiation to the horizon which is the goal, but has some lobes of radiation at high angles, that interacted negatively with the ground wave, at important distances, and I think, but am not sure, that the 5/8 needs less of a ground system, than the more common 1/4 wave AM verticals.
But the results at long ranges were not up to expectations, and they determined that the taper of the tower, fat at the center, and needle like at the top and bottom, changed the RF current flow in the radiator from the more predictable current in a straight sided tower, and that accounted for decreased long range signal strength. View attachment 257669performance expected from a 5/8 wave vertical

Also, in recent years, they replaced the very old but operational 5 wire coaxial-parallel wire transmission line with a large coaxial line, buried. When I visited, it was 1989, and the open 5 wire line was in use. There are now some very interesting videos on youtube with drone inspection flights way up the tower. search youtube for "WSM"
I am interested in where WSM is heard world wide, or any long range reception reports. These will normally only be when the entire path between your radio and WSM is in the dark. So tune your AM radio to 650 on the dial at night, say 2AM, and give a listen, let me know what you hear. WSM has a distinctive preference for twang-y southern accented country music.
(Dolly Parton, etc)

 

[Baluncore]

As I understand it, 650 kHz was reserved in the USA for only one transmitter.
In Britain that channel was used by Radio Caroline.

In 1978, Australian broadcast stations were moved from multiples of 10 kHz to multiples of 9 kHz. The closest occupied channel to 650 kHz would now be 648 kHz. It should be possible to detect the 2 kHz audio beat of the WSM carrier with carriers from the Australian stations, 2NU Tamworth or 6GF Kalgoorlie.

Is WSM still on 650 kHz, or was it harmonised to 648 kHz at some time around 1980?

 

[sophiecentaur]

performance expected from a 5/8 wave

That is dependent on a good Earth Mat. From memory, for reasonable efficiency you need at least λ/4 radius of fairly close spaced radial copper (or aluminium for temporary use) wires. Longer for a 5λ/8 mast, probably. Not all sites have that amount of room.
A 5/8 radiator will help with 'mush, fading and give a wider service area at night.

Smashing pictures of the equipment!

 

[AA6EG]

650 khz since 1929 in the case of WSM made it one of less than 10 FCC authorized "Clear Channel" stations in the USA. At one time I think it was the ONLY AM station on 650. As time passed, "Clear Channel" no longer meant exclusive single stations, but few stations in USA. Extensive history of Clear Channel covered here:
https://en.wikipedia.org/wiki/Clear-channel_station

Included is an image I collected with the 650 khz clear channel stations in CONUS, Alaska, and Hawaii.
Also a Gray line image that shows the magical propagation-grayline where it might be easy to hear WSM at extreme distances when it is near sunrise or sunset at Nashville. If the grayline is crossing, or close to Nashville, then radio propagation sometimes allows vast distances that WSM is heard, along or near to the grayline, worldwide, at that moment, or for a short time window. The appearance and timing of the appearance and disappearance of the Ionospheric D layer, and changes F1-F2 layers or region, can cause almost a waveguide channel that MF AM band waves can travel long distances.

I am trying, hoping to get some people to tune into 650 in Mexico to see if WSM can cross the Gulf of Mexico, Straight south.

I built a simple AM radio antenna that works very well to enhance reception of table top AM radios without any physical attachment to the radio. See picture. A resonant coil-capacitor circuit surrounding the radio, and tuned across the AM band to bring out the very weak AM signals at night. It is simple and works very well.

 

[Baluncore]

It would be difficult to identify the WSM carrier in Australia when KENI in Alaska has similar power. There are a dozen others in Mexico, Central America, Canada and lower power local in USA on the same frequency.

The biggest problem with 650 kHz DX will be the presence of switching power supply noise at the receive site.

The area around the foot of WSM is sufficient for λ/4 radials.
Google Earth Lat/Lon 35.998052° -86.790817°

 

[zoki85]

That is dependent on a good Earth Mat. From memory, for reasonable efficiency you need at least λ/4 radius of fairly close spaced radial copper (or aluminium for temporary use) wires. Longer for a 5λ/8 mast, probably. Not all sites have that amount of room.

I have heard that for beefy powers (50 kW and more) AM transmitter sites sometimes use a complete zinc-coated mesh buried in surrounding terrain. Anycase, great attention should be payed to grounding systems installation. Good RF ground means really good.
And 5λ/8 operation is better for space radio wave radiation than for ground wave

 

[dlgoff]

ping @dlgoff . Have you ever considered large scale restoration projects?

Not really. But I admire those who are.

... matching house,

Love those matching coils.

 

[AA6EG]

I am still seeking information about ground screens, etc;
Some more about WSM structure, from a Retired Ch Engineer, Watt Hairston, who succeeded Everett Lawson.
Watt Hairston "The top part of the tower was removed in early 40's, part of it was the flagpole at Lipscomb Elementary School across the road until it was replaced in 1997. The exact information on the total weight is conflicted by records with varying data and anecdotal misstatements as to tons or lbs. We know that each of the eight guys are 55,000 pounds dead weight plus tension for total of 440,000 pounds. Everett T. Lawson said that he thought the steel was somewhere between 150,000 and 175,000. I never found that exact data but knowing and working with ETL for many years, I found that he was right about stuff 100% of the time. Based on that, let's call it 615,000 lbs. There are specifications in a file out there that are for the Lapp insulator and its maximum compression stress is listed at 1,200,000 lbs or about 2 X the max estimated weight. In 2004, we replaced 43 diagonal members in the top part of the tower and I can tell you that the galvanization is still in very good condition and rust is not a problem Scott Baxter or John Almon may correct me on any of that but I think the numbers are close. "

 

[AA6EG]

Sorry, I mistakenly duplicated some pictures. How can I edit them out?
Here is a site with live streaming of WSM... twangy country music.
https://wsmonline.com/listen-live/

 

[anorlunda]

Sorry, I mistakenly duplicated some pictures. How can I edit them out?
Here is a site with live streaming of WSM... twangy country music.
https://wsmonline.com/listen-live/

No problem. I did it for you.

 

[AA6EG]

No problem. I did it for you.

Thanks.

 

[AA6EG]

I took pictures inside the matching house, and found interesting the stacked, spaced "pizza pan" capacitor plates. When a friend of mine and I got the insider's tour, first of all my friend was a history professor, and all this stuff looked like a Tesla science fiction laboratory of old to him and he did not fully understand it, and was a bit uncomfortable. Everett, cautioned us to just look, do not touch ANYTHING in the tuner house or get too close as WSM was on the air at 50KW AM (~ 250 KW peak to peak modulation. We all took a step back inside the tuner house. :-) Then, it became apparent to me that I/we could hear the faint but clear modulation of country music. I smiled and mentioned "what's that sound" Everett said the 50KW AM modulated power was vibrating the ~20" dia capacitor plates putting off clear but soft audio. This unnerved my friend who was driviing.. who said "Pat, I'm leaving." "I will be in the car, meet me there, when you are doing looking at this thing." :-)

Also a copper tubing coil, homebrew on a plywood wooden form. The coil had a twin that was interesting. Each turn of the copper was anchored by a sawtooth cut into the form, constraining movement, keeping spacing uniform.
One of the retired coils still on its plywood form, had nearly every turn flattened, as if a steam roller rolled them flat. Everett explained to me that a long ago lightning hit with possible, say, 10,000 amps or more current, produced so much turn to turn repulsive force that the forces crushed flat the tubing, which was about 1/2" dia.
The WSM TX visit was a waypoint memory so many years ago for me...

 

[Averagesupernova]

Reminds me of touring an AM radio transmitter site while in school. One of my classmates heard audio and started looking around for the source expecting to find a receiver plugged in left on and forgotten about. I told him he wouldn't find it and explained it was likely capacitor plates, coil windings or something else that was able to move under the influence of a magnetic field caused by high RF current. He flat out wouldn't believe me. I hadn't yet learned it wasn't my responsibility to educate people if they don't ask for it.

 

[dlgoff]

Extensive history of Clear Channel covered here:
https://en.wikipedia.org/wiki/Clear-channel_station

On that list of Clear Channel stations is Arkansas's KAAY. I was surprised to find it now airs religious programming. I remember listening to Beaker Street in the 1960s; in particular, this, not politically correct, Legend of the USS Titanic comes to mind.