Sunday, August 22, 2010

Mediumwave Oddities - Towers

Today, we move on to tower oddities in the mediumwave broadcast band. Broadcast towers have always been an interesting subject to me. Did you see the news earlier this month on the WWVA-1170 towers collapse, due to severe storms in the Wheeling, WV area? The photo at left shows how they looked in the 1940s. WWVA-1170 started broadcasting in 1926.

Like last time, we will use data from current FCC records dated August 7, 2010. There are 4784 licensed stations in this survey. Previous posts in this series were Mediumwave Oddities - Transmitter Power and Mediumwave Oddities - Geography. Information has been gathered using the Radio Data MW program.

Again, daytime and nighttime data will differ. I will be explicit on which is which when the statistics are presented.


Many mediumwave broadcast stations use multiple tower arrays to direct their signals towards intended markets, or away from other stations which they may cause inteference to. There are thousands of towers out there.

If we total all the towers of all the mediumwave stations transmitting in the daytime, how many would there be?

Answer: 7164. That's a lot of metal in the air.

Out of the 4784 licensed stations, how many stations have only one broadcast tower?

3569 stations have only one broadcast tower, which is 75% of all broadcast facilities. Surprisingly, the remaining 25% actually have more towers total than the single-towered 75%. Multi-towered stations average 2.95 towers per facility.

Essentially, a station with one broadcast tower has an omni-directional signal pattern, in other words, the station broadcasts with equal signal strength in all compass directions. Multiple tower arrays use phasing techniques to form skewed patterns of radiation, often in figure-eight or cardioid shapes. The lobes, or strong points of the pattern are directed towards market areas of interest. The nulls, or weak points of the pattern, are positioned toward areas to lessen interference with other stations or unwanted markets.

Back to tower counts, how many stations have two broadcast towers?

515, or 10.7 percent of the total have two towers.

How about three towers?

384, or 8 percent of the whole. The percentage drops dramatically off from there of course.

Which station has the most broadcast towers in use during daytime hours?

That would be KNTH-1070, Houston, Texas, with 11 towers. They are arranged in an odd grouping of three parallel rows of 3 each, with towers #10 and #11 jammed between the two rows. The rows head generally in a south to north direction. KNTH-1070 uses two less towers (9) during nighttime hours.

Are there more towers in daytime use east of St. Louis, Missouri, or west of St. Louis?

East of St. Louis, there are 4240 towers in use during daytime hours. 2924 towers are used west of St. Louis.

Radio station signal patterns are sometimes further modified at the tower site by what are called "augmentations". Augmentations are modifications to the standard broadcast signal pattern, usually to further null the signal strength in a certain direction to bring it into FCC "signal contour" compliance. An interesting discussion can be found on augmentation over at The Virtual Engineer AM forum.

Stations can have up to 28 augmentations to their signal pattern. This must be a technical nightmare for the broadcast engineer.

Which station has the most augmentations?

Four stations have 28 augmentations in use during daytime hours.


Again, like we saw in Mediumwave Oddities - Transmitter Power, nighttime seems to be the more interesting as it has more odd variety.

How many total towers are used for broadcasting during nighttime hours?

Answer: 7877 towers. 713 more towers are used at night than during the day.

Being a mediumwave DXer, you know that signals travel much farther at night than during the day, commonly upwards of 1,000 miles and more. Stations often must follow different signal pattern guidelines at night to prevent interference to distant stations. This generally requires use of either a different tower arrangement or different number of towers, or both. Stations also may operate at reduced power at night.

Which station has the most broadcast towers in use during nighttime hours?

That would be KFXR-1190, Dallas, Texas, with 12 towers. They are arranged in two parallel rows of 6 each, heading from southeast to northwest. KFXR uses only 4 towers during daytime hours. Texas holds the record for stations with the most broadcast towers for both nighttime and daytime hours. They always do things in a big way in Texas.

Are there more towers in nighttime use east of St. Louis, Missouri, or west of St. Louis?

East of St. Louis, there are 4427 towers in use during nighttime hours. 3450 towers are used west of St. Louis.

Which station has the most augmentations?

Nine stations have 28 augmentations. Among them again is KFXR-1190, Dallas, Texas. I think KFXR-1190 should be considered for the record here. It is the station with the most towers (12) and tied with 8 others with the most augmentations.

I was thinking about ground radials this morning, which are the (generally) buried wires that are placed under broadcast towers to create the artificial ground that they operate over. Usually broadcast towers must have at least 90 quarter-wave length wires, and as many as 120 or more. Seeing as how we know the number of broadcast towers used in nighttime operation, the following question occurred to me:

If we assume 120 radials under each tower, how many total ground radials are in use?

Answer: 945,240. Almost a million radials.

Using an average length of 245 feet for each radial (a quarter wave length at 1000 KHz), how may feet of radials lie under mediumwave broadcast towers in the US?

231,583,800 feet. That is 43,860 miles of wire, enough to encircle the world almost two times. Time to buy stock in copper.

Hope you have enjoyed this series. For more mediumwave oddities on RADIO-TIMETRAVELLER, see the Tower Talk article.

The WWVA-1170 towers after the August 4th, 2010 storm.


Stephen said...

I wonder if you've checked out KFBK-1530 Sacramento, CA's towers? I think what they use is quite interesting.

Here near San Diego, CA, I've been able to hear them at midday in winter on a few occasions.

Radio-Timetraveller said...

Hi Stephen,

Had a look at the article. Interesting setup. Thanks for posting.

Radio-Timetraveller (Bill)