Saturday, September 18, 2010

Signal Patterns

Stephen from San Diego, CA, a reader of Mediumwave Oddities - Transmitter Power, wrote in with the following question:

"What stations actually are authorized with a HIGHER power at night than they are allowed in the daytime? I'll mention one - 760 KFMB San Diego, CA, about 7 miles from me. In the daytime they're only allowed a paltry 5KW omnidirectional, but at night they step up to 50KW."

An interesting question. And you mentioned signal patterns, all-important in the differences between daytime and nighttime mediumwave coverage.

I tweaked my database program Radio Data MW and came up with the following results:

73 US stations run higher power at night than during the day.

KFMB-760, your San Diego station, is the only one which runs 50KW at night and has a lower daytime power. KFMB uses one omnidirectional tower for daytime broadcasting. Power is set at 5KW during daytime broadcasting hours and 50KW at night. KFMB uses three towers at night to get that big 50KW signal out. But the pattern this time is pointed right out into the Pacific Ocean, to the southwest. San Diego's KFMB cardioid pattern has a deep, broad null at 60 degrees, nicely positioned for protection of Detroit Michigan's WJR-760 (50KW) and its western coverage area. Equally important, big gun KKZN-760, Thornton, CO (also 50KW), near Denver at 49 degrees is also protected, as well as a host of smaller US and Mexican stations.

KBRT-740 "K-Brite", a 10KW station in Avalon, on Santa Catalina Island is only a mere 85 miles to the northwest of San Diego's KFMB. It has a three tower array in use both day and night, pointing its signal due east to cover the Los Angeles and Long Beach areas, a scant 26 miles away. KBRT drops its power to a gasping 113 watts at night. Being nearly "co-channel" with KFMB at only 20 KHz removed, inspecting KFMB's pattern plot reveals that that the broad cardoid lobe to the northwest pumps an astonishing effective radiated power of 95KW up a 294 degree vector right at KBRT on Catalina. With a salt water path the whole way, that should be a lot of signal.

The next station with a big signal at night and a smaller one during the day is WLIB-1190 running 10KW daytime and 30KW nighttime, in New York, NY. WLIB uses three towers for daytime broadcasting and four towers at night. Both patterns are generally towards the east, with the nighttime pattern more focused and skewed a little more to the southeast. During the day it competes with WBIS-1190, Annapolis, MD (10KW), and WSDE-1190, Cobleskill, NY (1KW). WLIB's daytime pattern exactly bisects these two challengers on a line right out into the Atlantic Ocean. At night, WLIB only has to contend with New Yorker WSDE, as the Annapolis station is off the air.

The station with the greatest power differential, day to night, is WLAN-1390, Lancaster, PA. They run a mere 18 watts daytime and 1100 watts nighttime, both to a single, omnidirectional tower. Their nighttime power is a remarkable 61.1 times more than their daytime power!

WLAN's main competitor is WZHF-1390 (5KW), in Arlington, VA, a mere 92 miles away to the south-southwest. How does an 1100 watt station co-exist with a 5KW station at night when they are only 92 miles apart? Examining the tower pattern plots, you will find that WLAN in Lancaster, PA is exactly in a deep notch of WZHF's nighttime pattern.

Shown are nighttime pattern plots for San Diego's KFMB-760 and Avalon's KBRT-740, generated by Radio Data MW.


gkinsman said...

Hi Bill,

KBRT is on 740 kHz (same as KCBS in San Francisco). At night I can only hear KCBS from Simi Valley, which is about 40 miles NW of LA.

In one radio database app I use, it says KBRT drops from 10 kW daytime to 0.113 kW nighttime.


Stephen said...

That's quite interesting. Another thing I bet you didn't know is that there's actually a highway (CA-52 - speed limit 65 (55 for trucks), and the Mast Blvd on/offramps are right there) cutting right between two of the three self-supporting towers. You can see this if you zoom in on Google Maps.
I remember a few years ago on occasion hearing KFMB at 50kW within a couple hours of noon (radio sunrise here is generally almost never after 7am local standard time, even in winter). I could tell because they were splattering as wide as they normally do at night on my Panasonic RQ-SW20, which I still have. Even so, KBRT still sounded fine, with only a hint of occasional chirps from KFMB.

Also, while KBRT-740 is licensed for 113 watts at night, I usually hear them sign off at sunset, complete with an elaborate announcement of ownership, transmitter power & location, etc. I do sometimes faintly hear them in El Cajon after 6am before sunrise, mixing with KCBS, almost as if they use their night pattern as a PSRA. BTW, in the daytime here KBRT usually comes in about 45-48dBu on my PL-380. In a few places in the house, KNX-1070 is stronger, for example 51dBu in one spot, even eclipsing KFI-640 at around 45dBu, even though the former is 12 miles farther away and both are 50kW omni-directional.

Also, that's interesting that you mentioned the two relatively close nighttime co-channel stations. I also suggest looking at KBLA Santa Monica, CA, and KMIK Tempe, AZ, both on 1580kHz, and both authorized for 50kW. Where I am, near 32 45 40 n 116 56 50 w (which is on a nearby elementary school campus), KBLA is faintly heard in the daytime, I think under 24dBu or so. At night, KMIK is probably my strongest skywave-only(*1) blowtorch, regularly hitting 54-56dBu barefoot. I've even seen it hit 62dBu on occasion! (KBLA can also sometimes be heard under it.)
*1 - When conditions are right in the dead of winter, I've heard KMIK on a couple occasions in the middle of the day, as well as KFBK-1530 Sacramento, CA.

I wonder if it'd be possible, with the coordinates given above, to calculate the power KFMB (and KCBQ-1170, 50kW daytime, my strongest daytime local) send in my direction, as well as an approximate field strength? I know both are very strong here, as I can make both of them overload my PL-380 to where the audio on-channel is actually noticeably distorted, by coupling it and a Select-A-Tenna to a power pole. Also, I can do the same with KECR-910 (5kW, with whom KCBQ diplexes) at night. Also, it would be interesting if you also had the data for KCBQ when their 50kW/1.5kW transmitter was in Santee, about 5 miles from me. It was stronger considerably in the daytime than KFMB was at night. I suspect if I had my PL-380 then, it would have had 49,00 desense several channels off. As it is now, KCBQ generally desenses about 43-45 +/-15kHz, with KFMB around 41dBu or so. I suspect KCBQ's desense is stronger due to KSDO-1130 contributing to that, even though KFMB seems to actually be stronger.
For KFMB, with that setup I get the 50,00 desense all the way from 153kHz up to somewhere around 3.2-3.4MHz. Also, the harmonics up to somewhere around the 10th to 15th regularly peg the meter at 63dBu. I guess that will prevent me from hearing that German station on 756 kHz, though...

Radio-Timetraveller said...

Hi Stephen,

I plugged your latitude and longitude coordinates into the program and got some figures for you. I need to get a little technical on you here in order to explain some of these calculations, so please forgive me.

Unfortunately I don't have field strength calculated in mV/m. A more useful figure I have found is to calculate received power in dBm, which is dB below 1 milliwatt, and in so doing, attempt to use a real-world path loss variable in the equation (approx. 46 dB) instead of the usual free space value of 20dB which gives pie-in-the-sky signal strength values which are not realistic.

Sensitivity of modern higher grade receivers is usually measured in dBm. For instance, the Eton E1, a premium grade portable, has a sensitivity threshold at about -114dBm on the mediumwave band under optimum conditions. The cheap Chinese ULRs that we use usually give a value in mV/m, something on the order of 20 mV/m. I think most of the time they pull a figure out of the air just to have something to publish, and I don't consider these figures to be necessarily accurate or even remotely close.

You might be surprised to know that neither of the two stations you mentioned is the strongest. KSDO-1130 just edges out KFMB-760 by a hair.

Distances per your latitude-longitude coordinates given:

KSDO 7.047 miles
KFMB 7.82 miles
KCBQ 10.092 miles

Strength order is from top to bottom. The closer to zero (0 dBm) you get, the stronger the field intensity.


KSDO-1130 -43.13 dBm ant gain=6.53dB (45KW ERP)
KFMB-760 -43.77 dBm ant gain=0.93dB (61.9KW ERP)
KCBQ-1170 -54.63 dBm 7.61dB (16.7KW ERP)


KSDO-1130 -45.47 dBm ant gain=4.19dB (26.2KW ERP)
KCBQ-1170 -46.44 dBm ant gain=3.42dB (109.9KW ERP!)
KFMB-760 -54.76 dBm ant gain=0.0dB (5KW ERP)

KSDO comes out a hair stronger because of the shorter distance to the latitude longitude coordinates you gave me. Remember, power density (and thus received electric field intensity) is inversely proportional to the distance according to the inverse square law. It diminishes very rapidly with distance. 7 miles versus 10 miles makes a huge difference.

I'll try to get to some of your other comments this week if I can. Unfortunately, I'm up against a time problem here as I'm headed cross country to Arizona in about 5 days and I won't have much time the rest of the week and until mid-October when I get set up. Ultimately, some of this conversation might be easier to do via e-mail, if you'd care to reply with an e-mail address.

Take care, and catch you from AZ if I don't sooner.

Radio-Timetraveller (Bill)

Radio-Timetraveller said...

Hi Gary,

Thanks for the heads up on KBRT-740. I made some changes to the blog post.

Best to you,


Stephen said...

Hi Bill...

I looked around on this site for a way to send you an email, but was unable to find it. Looks like you are doing a pretty good job of hiding it from the spam bots, or I'm just looking in the wrong place.

I see that the distances you gave are a fair bit different than the actual distances to my location. As for my measurements, it's about 6.34mi, 350.28° to KSDO, 7.29mi, 320.85° to KFMB, and 9.35mi, 7.29° to KCBQ. If I could send you an email, I could give more precise coordinates. If you want to launch one in my direction (although I do realize you're probably about to embark on the cross-country trip), you could send one to my gmail address - username is pianoplayer88key. (Hopefully that lets you figure it out, without the spam bots getting ahold of it).

I also would be interested in knowing what KMIK's 10% and 50% nighttime skywave would be in my direction. They're about 299.67mi, 79.39° from here, according to FCC search.
Also, if I remember correctly, you said your eton e1 is quite resistant to overload, and very sensitive and selective. I wonder if it might be possible to hear KSL-1160 during the day from a place like this?
(substitute a much larger loop antenna for the puny little Select-A-Tenna that I'm using)
It would be hopeless there on my PL-380. In fact, I was having trouble with KECR-910 (unless I tuned the SAT to 910), and that's their transmitter site!
Or, is even the e1 not totally immune to overload, like the PL-380 below (was photoshopped though, and chances are I didn't scale it correctly) also wouldn't be immune? :)

Radio-Timetraveller said...

Hi Stephen,

KSL-1160 might be impossible, even with the Eton E1. Though 50KW, they are omnidirectional with a single antenna. I'll have a check when I get out there.

KMIK-1580 out of Tempe, AZ must have a huge signal to you at night (6 towers), as you are dead center in their main lobe. 4.65dB gain, some 146KW ERP right at you. Daytime is different, only a single tower at 50KW.

The E1 is pretty immune to overload even here in NY, 4.9 miles away from 20KW WYSL. It's a great receiver.

Radio-Timetraveller Bill)

Stephen said...
This comment has been removed by the author.
Stephen said...

Yeah, I figure KSL might not be easy without tip-top equipment from where I am, as it's 9.3mi and 7.3° to 50kW KCBQ-1170. With what I have, it's totally impossible.

You bet KMIK has a huge signal here. Have a look at the RSSI values on my PL-380 in the linked picture - that was BAREFOOT!
By comparison, KECR-910, 5kW diplexing with KCBQ-1170, 9.3 mi, 7.3° from me, is weaker in the daytime. KLSD-1360, 1kW, 8.3 mi, and XEWW-690, 50kW, 32mi, are both slightly weaker at night, as well. The only local stations that are stronger are KOGO-600 24/7, KFMB-760 24/7, KECR-910 night (5kW, shifts pattern), KSDO-1130 24/7, KCBQ-1170 day (50kW), and KLSD-1360 day (5kW).

Yeah... I wish I could afford something like an E1... but then I'm also looking for something to put in my pocket, or at least attach to my belt or something and carry it hands free while I'm doing other things and listening to the radio at the same time. Many of the stations I'd like to be able to hear in the daytime go completely undetected on what I have now.

I think I may be able to find a way to overload your E1, though...

As probably noted earlier, I'm 7.3 miles from KFMB-760, 320° from me to them, and at night when I do this (ignore the fact that it's daytime in the pic, and the SAT is tuned to 1170)...
it sounds like this on my PL-380...

Stephen said...

Uh oh... looks like KFMB is slated to get dethroned...

Radio-Timetraveller said...

That makes two! Looks like this station is 20KW daytime/50KW nighttime. Weird.

Thanks, Stephen.