Station Tower Efficiency

[SomeRadioGuy]

I'm back home safely from a long road trip, and I had to notice, particularly on the AM band, but FM too, that some stations seem terribly inefficient for their given location (on the dial), wattage, ground conductivity, etc. I learned in engineering that matching the tower to the wavelength (or a factor of that wavelength, like 1/4) is crucial for a clear signal. So what I'm wondering, is what stations do you feel are out of whack in your area? And also may I get some opinions on why it ends up like this?

My FM example:
KAZY and KRRR share the exact same tower and has very similar wattage, but KAZY made it farther than KRRR, and was more consistent.

Now for AM:
810 KBHB (25kw) does not make it as far as 920 (5kw) or 1380 KOTA (5kw). 650 KGAB (8.5kw) blew the socks off by being practically in the clear in Chadron, Nebraska, meanwhile KZMX (AM, 2.3kw) wasn't even audible in Hot Springs (come to think of it, they were probably off air, despite the FM side repeating the liner that they were on 580). Finally, 870 KJMP (1.2kw) is barely perceptible even within Fort Collins and Cheyenne.

I suspect 570 WNAX is even more efficient than 550 KFYR as they are both 5kw, and they both reach Rapid City. (KFYR was readable into Northern Nebraska near Crawford).

Oops, I got a bit on the long side, sorry.
Thank you for your responses!

Complete opposite when i lived in Laramie.. 104.9 KRRR made it to Laramie almost daily, 93.7 rarely ever did.

810 KBHB was audible quite well and way more often than 920 or 1380...1380 was kinda common, 920 was rare and 810 was daily in the winter.

8kw non directional on 650 or whatever the power is will carry far. 870 is highly directional.

 

[ZantennaG1]

Complete opposite when i lived in Laramie.. 104.9 KRRR made it to Laramie almost daily, 93.7 rarely ever did.

810 KBHB was audible quite well and way more often than 920 or 1380...1380 was kinda common, 920 was rare and 810 was daily in the winter.

8kw non directional on 650 or whatever the power is will carry far. 870 is highly directional.

That's different that KRRR was the strong one for you; I can also attest to KBHB popping up strong on winter mornings before skywave fades. Currently KGAB runs 8.5kw days. Since you were in Laramie for some time, how many of the Cheyenne stations made it in frequently?

 

[SomeRadioGuy]

That's different that KRRR was the strong one for you; I can also attest to KBHB popping up strong on winter mornings before skywave fades. Currently KGAB runs 8.5kw days. Since you were in Laramie for some time, how many of the Cheyenne stations made it in frequently?

Most of them over two years.. i also had a yagi antenna on a table in my yard and a good sony radio in the house

 

[Schroedingers Cat]

I thought what the Blaw Knox did was change the current distribution, which might make vertical radiation nulls shallower, and also makes the bandwidth greater, like a 3D structure vs. an infinitely thin wire, and like a 3D waveguide vs. transmission line. I didn't think it had any significant effect on horizontal radiation.

 

[rfry]

I didn't think it (Blaw-Knox tower) had any significant effect on horizontal radiation.

Below from the FCC CDBS are the groundwave fields at 1 km for the 50 kW transmit power at the feedpoints of those two tower configurations ...

WSM (190° Blaw-Knox): 2800 mV /m
WGN (195° Uniform Cross-Section): 2856 mV/m

Other things equal, the taller tower of WGN would result in higher radiation resistance and greater system radiation efficiency for it when compared to the 5° shorter tower of WSM.

PS: Blaw-Knox type towers fell out of favor as AM broadcast band radiators starting in the 1930s, when it was learned that they had more high-angle radiation for a given height than uniform cross-section towers — which worsened the nighttime self-interference produced by those towers.

 

[DavidEduardo]

PS: Blaw-Knox type towers fell out of favor as AM broadcast band radiators starting in the 1930s, when it was learned that they had more high-angle radiation for a given height than uniform cross-section towers — which worsened the nighttime self-interference produced by those towers.

Then why would a station like WSM, which considerably depended on skywave reception as well as coverage in the rural areas nearer to Nashville, not have moved to a (far less pretty) uniform cross section vertical radiator?

Am I correct in believing that WHO in Des Moines had a Blaw Knox and replaced it for that reason?

 

[rfry]

Then why would a station like WSM, which considerably depended on skywave reception as well as coverage in the rural areas nearer to Nashville, not have moved to a (far less pretty) uniform cross section vertical radiator?

The self interference occurs when the nighttime skywave signal and the groundwave signal are roughly equal to each other in a "zone" centered at various distances usually within ~80 to ~130 miles from the broadcast tower, depending on the operating frequency and Earth conductivity. Uniform cross-section 1/2-wave (and less) radiators are nearly immune from this effect, 195° radiators have ~negligible issues with it; Blaw-Knox Towers less so.

Am I correct in believing that WHO in Des Moines had a Blaw Knox and replaced it for that reason?

I don't know about that myself, but maybe S.C. has learned something about it from his great knowledge of the FCC History Cards for AM stations.

WHO has been using a Franklin-type, sectionalized, uniform cross-section tower antenna system for many years now. One big reason they would do that is to improve the field intensity of their ground wave, without greatly increasing their high-angle radiation to cause an objectionable amount of self-interference.

 

[Schroedingers Cat]

Here's a good source. The best answer is that WHO USED to have a Blaw Knox Tower, then half of a Blaw Knox Tower, and now a uniform cross section tower. I don't know if the Hx Cards have that information about WHO or not, specifically what type of tower. I'd have to look it up.

Blaw-Knox Diamond Radio Towers (Jim Hawkins Radio and Broadcast Technology Page)

www.j-hawkins.com

I think there was a certain amount of folklore about coverage with certain types of towers. I wonder if you did a bunch of actual FI measurements, and could study it at certain specific repeatable RLs, over a long period a time, and made meticulous records of it, you could get a more definitive answer. That would have to be objective, not subjective, and follow the Scientific Method, which is talked about a lot, but not always rigorously followed.

I heard a funny story about a Proof of Perfomance for an AM DA, for which a friend of mine was an assistant engineer, and was told, seriously, to "walk around with the field strength meter until you get this reading". Not exactly rigorous or objective. His boss was well known and well respected. That's all I'll say.

And the owner engineer of a Directional FM in a medium sized market told me that "you don't want to watch FM DA Proofing, as is a lot like watching sausage making".

 

[rfry]

I think there was a certain amount of folklore about coverage with certain types of towers.

No doubt, but the clips below show a "benchmark" exception to that with respect to Blaw-Knox towers ...

 

[DavidEduardo]

The best part of that portion of Brown's writing is this:



As an aside, do you know of any source of Brown's book, either on loan or to purchase or a scan. I'd very much like to have it to place on the website.

 

[DavidEduardo]

I heard a funny story about a Proof of Perfomance for an AM DA, for which a friend of mine was an assistant engineer, and was told, seriously, to "walk around with the field strength meter until you get this reading". Not exactly rigorous or objective. His boss was well known and well respected. That's all I'll say.

When I was rebuilding WQII (formerly WJIT and WITA) in San Juan, we redesigned the directional system with a very, very well known engineering consultant. We had a critical null towards Richmond, VA. On the key radial, we had very restricted power... about 600 watts equivalent. We had a monitoring point used in the original proof, but at that intersection there were now metal poles supporting street lights and a power line. We walked around till we found the lowest signal within about a 2 block radius, and that was our monitoring point in the new filing, complete with photos of the old point and its "heavy metal". That allowed us to toss about twice the power in that direction, all quite legal... and nobody ever complained as the protected contour for Richmond was out in the Atlantic Northeast of the Bahamas.

As you know, I know just about enough engineering to know how to hire a really good engineer for the heavy lifting. But I was pretty good aligning the heads on cart machines.

 

[rfry]

The best part of that portion of Brown's writing is this:

View attachment 2190

Totally agree with that, especially in this day and age.

As an aside, do you know of any source of Brown's book, either on loan or to purchase or a scan. I'd very much like to have it to place on the website.

I suspect it could be available from a decent-sized public library through the inter-library loan network they use.

 

[Schroedingers Cat]

Another thing you have to think about is the seasonal variations in both ground wave and skywave intensities. This makes the distance to the areas of maximum fading change with time and particularly the time of year.

Rich, I have recently been downstate and I took the FI meter. I was usually too busy to get outside until the beginning of Critical Hours. I was also amazed at the variation in FI of "local" stations at this time of day. One in particular, WJR 760 kHz, is about 35 miles away, and the FI varied on fades from 21 mV/m to 25 mV/m, meaning there had to be a 2 mV/m skywave. This surprised me, since you would think that this is too high an angle of departure to reflect much, or go right through. A ham on 160 meters in the Fenton area, Tyrone Township, US 23 and Center Rd., on 1875 kHz A3, varied from 200 uV/m to 60 uV/m until he signed off for the evening. This is a similar distance away, close to 40 miles.

 

[ZantennaG1]

Another thing you have to think about is the seasonal variations in both ground wave and skywave intensities. This makes the distance to the areas of maximum fading change with time and particularly the time of year.

I heard that winter is the best time for groundwave, is that right?

 

[Schroedingers Cat]

Certainly Winter is best in my experience at 41 to 46 Degree Latitudes. I've heard there is not nearly as much variation down South, where it is Semi Tropical.

 

[Radig]

I thought what the Blaw Knox did was change the current distribution, which might make vertical radiation nulls shallower

Here is an excerpt from page 79 Radio Antenna Engineering by Edmund Laport:

 

[DavidEduardo]

Certainly Winter is best in my experience at 41 to 46 Degree Latitudes. I've heard there is not nearly as much variation down South, where it is Semi Tropical.

When in Ecuador, I had a signal meter in my car radio (prior to the AGC of course) and noted changes in signal strength at fringe signal differences between the rainy season and the dry season. This was all within a hundred or so miles of the Equator.

Since seasonal temperature variations were minimal (a Quito on the Equator, about 30° F to 75° F on any given day) the only variable was ground level "wetness".

 

[ZantennaG1]

This thread has exploded in size! @DavidEduardo On a related note, how does approximately 12 hrs of Day and Night affect skywave down in Ecuador?

 

[rfry]

Below is a performance comparison of the 1930-era Blaw-Knox tower of WCAU vs. a uniform cross-section radiator, showing what Dr. Brown was describing in his autobiography about his/RCA's solution for the nighttime self-interference WCAU was having back then.