overload

[bobdavcav]

1. What is the weirdest overload catch you've heard? Mine would be the time after the DTV transition, that I got 92.5 audio along with some other stations on the audio frequency for TV channel 8. This was on a radio that before the transition could get the TV channels. I've also heard KZOK 102.5 in that same general area of the dial on a different radio that seems really prone to overloading, even when most radios do not overload.
2. What is the worst area for overloading you've been in? I'd have to nominate Rapid City SD. You are about 2 miles from several big C1 sticks, which overload the radio. Because of this, the rimshots which are supposed to be strong by the coverage maps, are hard to get. Second place goes to Yakima WA, 8 miles from the 3 big sticks. Fortunately for Yakima, they only have 3 big sticks which cause overload. KATS 94.5 is a C1, but doesn't cause overload, it's just pretty strong. It's the 3 big class C sticks that do. Honorable mention goes to Portland OR. Blank frequencies are pretty overloaded, but somehow DX is fine from even the lowest quality of radios. In Yakima, I couldn't even pull in KPQ-FM on my BN, big signal from 58 miles away. In Rapid City, KSQY was weak even though it is supposed to be strong. What other areas are really bad? I wonder how Eugene would be with a low quality radio, since the towers are so close.

 

[boiseengineer]

Get out your calculator.
92.5 x 2 = 185. That's Channel 8.
180 to 186 MHz.
Since you hear multiple FMs there your TV or antenna pre-amp is overloading.

Google "FM Trap for TV" to fix that.

 

[bobdavcav]

Oh ok. The only TV frequency I know is channel 6, 87.75. How's the overload there in Boizee? I've only been there once about 10 years ago and didn't even get to listen to the locals.

 

[mofocat]

Seems like a strange topic but here goes: I began DXing a looong time ago as a 10 yr old stuck on a ranch 25 miles from the nearest AM stick, save for a 1kw religious station 6 miles away- (and that was directional away from me) AND the one major 'pest' that was 50kw, 15 miles away. The closest neighbor was 1/4 mile away.
Nowadays I realize that location was heaven for DXing.
Today my location is swamped with big time overload on both AM and FM. It could be worse- I am 1 to 3 miles away from the closest offenders- but still 100s of kw are banging my antennas from pretty close. One of the worst offenders tho is KMJ/580's 50kw all concentrated into a beam, straight at me from 25 miles East of me. I'm guessing about a 250,000 ERP directed right at me from KMJ alone. Could someone with the engineering smarts jump in and say just what kind of signal that KMJ array would send over Fresno- in ERP/ kw terms?

 

[Schroedingers Cat]

The maximum augmented inverse field, also the standard pattern inverse field, for KMJ, is 6494.46 mV/m @ 1 km. The minimum efficiency of Class B stations is 282 mV/m @ 1 km for 1 kW, and plenty of stations are at minimum efficiency from short towers, or inefficient phasors. KMJ uses four 178 degree towers, which also nearly doubles the "ERP" over minimum efficiency. Some would say they use 300 mV/m @ 1 km for 1 kW, and some say they would use the theoretical pattern, not the standard pattern or augmented pattern. But using the standard pattern and 282 minimum efficiency gives an ERP of (6494.46/282)^2X1000 or about 530000 watts at the 270 degree maximum. At 580 kHz, and 8 mS/m conductivity, at 10 miles, you get about 80% of the inverse value, or (4035.6/10)X0.8, or about 323 mV/m. I didn't look at the M-3 map, so you may want to check that. 323 mV/m will cause a lot of overload problems on today's radios and signal into other electronics that don't filter it out. At the minimum 80 dB down, the harmonic on 1160 would be 32.3 uV/m.

 

[Schroedingers Cat]

I don't know where Wikipedia gets 250 kW. Even at half wave efficiency or minimum Class A efficiency (362 mV/m @ 1 km for 1 kW), 6494.46 mV/m gets in the neighborhood of 300 kW. iERP, as satellites use, would be 1,400,000 watts. If you do the math, 173.2 mV/m is the isotropic value at 1 km for 1 kW radiated into a sphere 1 km in diameter (222 kV/m at 1 km is the dipole value on which FM and TV ERP is based). at (6494.46/173.2)^2X1000~1,400,000 watts iERP. I refer you to Reitz and Milford, or Corson and Lorrain, to check this if you doubt this calculation. It works for spherical chickens in a vacuum, and of course, uric acid in wet soil increases ground conductivity. See WAIT Elmhurst site discussion.

I do agree that at 26 miles, predicted field strength would approach 100 mV/m, as is shown in Wikipedia as between 50 and 100 mV/m. My rough calculation was 84 mV/m for 8 mS/m.

iERP based on a radiation into a half sphere would still be 700,000 watts, but a half sphere is not what isotropic is, it is into a sphere.

 

[Schroedingers Cat]

222 kV/m should read 222 mV/m.

 

[Schroedingers Cat]

Diameter should read radius.

 

[mimo]

I get pretty severe overload where I live on both bands as well. I have 3 FM's that have their sticks within sight of my hallway window. One broadcasts at 95.7 and runs 4 kw. Another at 97.9 and apparently runs only 800 watts but it is the second worst offender in terms of overload. A third is at 98.5 and runs 12 kw. All use the same stick, and both 98.5 and 97.9 can make life impossible for the average bo0m box (and even car radios) across the entire dial. On my cc plus with the antenna up they blow 6 higher power stations completely off the dial, and can be heard between the remaining stations. On a lesser quality radio it can be impossible to hear any thing between 90.7 and 105.3. They have no effect on the g8. On AM 1310 sends 50 kw right at me, I'm not sure what the distance is, but the signal meter on the cc plus is beyond maxed except during the summer and on the g8 it is anywhere from 73 to 77 dbu's during the day and 76 to 80 at night. The pattern change sends even more power right at me. On the corner where my office is, they max out the meter at 88 dbu's 24/7. The g8 desenses at home and stations at 1280 and 134 fade to almost muted. Local 1350 loses about 10dbu's at home compared to closer to what it is just a 15 minute walk north. 1200 sends a lot of juice our way as well, but is never better than 70 to 72. During the summer the heat reduces the strength of both. 1310 drops to 68 dbu's and 1200 down to 66. It's a go0d thing I actually like 1310. I have no use at all for the 3 fm's.

 

[ftballfan]

Speaking of strange catches, I once got WVCY (125 miles from me on 107.7) on 86.2 or something like that on my Grundig S350DL

 

[w9wi]

Speaking of strange catches, I once got WVCY (125 miles from me on 107.7) on 86.2 or something like that on my Grundig S350DL

Actually, there's a good reason you heard WVCY on 86.2.. lengthy technical explanation follows

Your Grundig (and all FM radios) uses Edwin Armstrong's "superheterodyne" circuit. In this circuit, the incoming signals are sent to a "mixer" circuit, where they are mixed with a "dead air" signal from a "local oscillator" circuit. When you send two frequencies to a "mixer", you get four outputs:

- Signal A
- Signal B
- Their sum A+B
- Their difference A-B

When you tune to 107.7, the "local oscillator" signal is set to 118.4. You get out of the mixer:

- 107.7 (WVCY)
- 118.4 (the "dead air" local oscillator signal)
- 118.4+107.7=226.1
- 118.4-107.7=10.7

That signal is fed to a narrowly-tuned amplifier which passes 10.7 and rejects the other three signals. The program is then extracted from that amplified 10.7 signal.

If you tune instead to 103.7 to listen to WXSS, the local oscillator is re-set to 114.4. You now get out of the mixer:

- 103.7 (WXSS)
- 114.4 (local oscillator)
- 114.4+103.7=218.1
- 114.4-103.7=10.7

So the only thing you have to do to change stations is to retune the local oscillator.

Now, let's say you tune to 86.2. The local oscillator is re-set to 96.9. You now get out of the mixer:

- 86.2 (the station you're trying to listen to)
- 96.9 (local oscillator)
- 96.9+86.2=183.1
- 96.9-86.2=10.7

And since there is, in fact, no station on 86.2, actually the only thing you get out of the mixer is the local oscillator. That's too far from 10.7 to get amplified, and you hear nothing (but noise)

But... let's say there is also a signal at 107.7 getting into the mixer. Let's look at the math there, without retuning the local oscillator:

- 107.7 (WVCY)
- 96.9 (local oscillator)
- 107.7+96.9=204.6
- 107.7-96.9=10.8

And the latter is close enough to 10.7 to be amplified. You hear WVCY 107.7 while tuned to 86.2.

Higher quality radios use a "pre-selector" circuit between the antenna and the mixer, to prevent anything above maybe 88MHz from getting into the mixer when the radio is tuned to 86.2. But this circuit is a bit expensive to implement, so most inexpensive portables don't have it. And as a result, these radios will often receive a station 21.4MHz above the selected frequency just as well as they receive stations *on* the selected frequency. Keep that magic 21.4 number in mind; you're going to see it again!

 

[ftballfan]

Very nice!! My Tecsun PL-390 goes all the way down to 64mHz, so I could probably get most stations 21.4 mHz below their actual frequency.

 

[rfry]

... One of the worst offenders tho is KMJ/580's 50kw all concentrated into a beam, straight at me from 25 miles East of me. I'm guessing about a 250,000 ERP directed right at me from KMJ alone. Could someone with the engineering smarts jump in and say just what kind of signal that KMJ array would send over Fresno- in ERP/ kw terms?

Downtown Fresno is about 25 miles from KMJ's array, on a bearing of 285 degrees true. According to FCC data, the inverse distance field (IDF) for the RMS value of their augmented pattern is 2719 mV/m.

The IDF at 1 km for 50 kW radiated by an omni, 1/4-wave radiator is about 2163 mV/m.

Per FCC data, the KMJ pattern has an IDF of 5808 mV/m at 1 km on a bearing of 285 degrees.

So in comparison with the radiation produced when 50 kW is applied to a 1/4-wave tower, KMJ produces 5808/2163 = 2.69X the field. The ERP change needed to do that, then, is 2.692 = 7.24 X.

Therefore KMJ is radiating about 7.24 x 50 kW = 361 kW on that bearing of 285 degrees.

The FCC M3 chart of ground conductivity shows about 15 mS/m for the groundwave path from KMJ to Fresno.

Plugging all these values into a computer program containing the digitized FCC groundwave propagation curves for the AM broadcast band gives these results:

Frequency = 580 kHz
Applied Power = 361 kW
IDF at 1 mile = 3609 mV/m
IDF at 1 km = 5808 mV/m
Conductivity = 15 mS/m

Field Intensity (mV/m) Distance (miles)
50 47.2
100 27.9
150 19.8
200 15.4
250 12.7
300 10.8

So downtown Fresno should have a field from KMJ of a little over 100 mV/m less any losses from nearby buildings, overhead wires etc-- at least the way I figure it.

Comments/corrections are welcome.

 

[Schroedingers Cat]

KMJ uses 178.3 degree towers. That also contributes to the concentration of the power in the horizontal, and increases the "ERP".

A 90 degree tower has an efficiency of about 310 mV/m @ 1 km @ 1 kW. This is another common, though not generally agreed upon, reference for "ERP" of an AM station. The Agreement Between The United States and Canada prescribes 300 mV/m @ 1 km @ 1 kW as the assumed efficiency when no other information is available. A reference of 282, 300, 310, or 362 (Class A minimum) all seem to be used in various discussions of AM "ERP", but keep in mind that the FCC doesn't define it. The isotropic ERP reference (173.2) makes the ERP sound impressive, but is rarely used in discussions of AM signals, since AM BCB signals radiate into a hemisphere and not a sphere.

 

[rfry]

KMJ uses 178.3 degree towers. That also contributes to the concentration of the power in the horizontal, and increases the "ERP" ...

Kindly note that the FCC 5808 mV/m groundwave IDF shown for the KMJ array at a distance of 1 km along their 285 degree h-plane azimuth radial already includes the "concentration of power in the horizontal" plane resulting from the electrical heights of the radiators in their array.

Therefore, shouldn't the ERP and groundwave field calculations for KMJ along that h-plane radial at an azimuth of 285 degrees as shown in my earlier post in this thread be quite close to correct?

 

[pianoplayer88key]

When I calculate the ERP of an AM station, I base it on the RMS IDF as specified in that record in the FCC's database. For AM stations, I think of the ERP in a particular azimuth as being the TPO necessary to have the same field if operating omnidirectionally with one of the same towers / the same antenna efficiency.

Using KMJ as an example...

39.22 km (24.37 miles) distant from 36° 44' 48.64" N Lat 119° 46' 21.31" W Lon, at a bearing of 104.26° using the great circle method of calculating distance. (Reverse bearing 284.52°.)

RMS Augmented: 2719.10 mV/m at 1 kilometer
RMS Theoretical: 2586.80 mV/m at 1 kilometer

Azimuth Etheoretical Estandard Eaugmented
285.0 5531.57 5808.19 5808.19

Calculating Theoretical ERP "50000*(5531.57/2586.80)^2" yields 228,634.163 watts.
Calculating Augmented ERP "50000*(5808.19/2719.10)^2" yields 228,139.914 watts.
I didn't try calculating Standard ERP this time. How do you determine "standard" RMS if it's not specified? I've heard multiply the theoretical by 1.05, is that true? Also what about augmented RMS if it's not specified, but azimuth data exists?

I'd like to be able to calculate field intensities for very near field situations. As a couple examples, what may be the field intensity of...

600 kHz measured at the sole/pad/underside of the right toe on the bird's right foot? (That's one of KOGO's towers, although I don't remember which one (guessing west) and don't remember how high up the bird was.)
770 kHz measured at the soles of the young man's shoes, his armpit, or his gloves? (That is, if KKOB was on the air - I understand they had shut off their transmitter.)

Last, but not least, what field intensity would be required for R. Fry's PL-310 to exhibit the behavior demonstrated near the end of this video of my PL-606, SAT & utility groundwire near 1170 KCBQ's TX site, if its internal ferrite antenna was removed? Also why did the signal briefly partially fade out as I was walking TOWARD the antenna? Was there some localized pattern null due to interactions between the towers or something?
I'd also like to be able to calculate the gain of that SAT+groundwire antenna setup. The previous video was taken 300 feet from the nearest tower right in their main lobe, and this one was 9.2 miles S (187-deg azimuth), with a calculated field intensity assuming 8 mS/m conductivity of about 130 mV/m. Also here's a video showing increase in 1550 XEBG's signal. I think there was some desense going on so the gain is actually considerably more than ~55 dB. Subsequent tests (not recorded) briefly showed 98 dBu on XEBG from the same spot.)

I have some more overload-demonstrating audio and video clips, but I'll save them for another post.
One of my main methods of determining if a radio is being overloaded is tune to a weak (or carrier-only) first-adjacent station around sunset or sunrise. If there's ANY/ difference in intelligibility of the desired station when the pest's transmitter briefly cuts out for pattern change, then yes, the radio is being front-end overloaded.
What is the type of overload called, though, when the radio exhibits behaviors like in the above KCBQ clips?

 

[Schroedingers Cat]

I still don't know how Wikipedia comes up with 250,000 watts, unless it is referenced to the Class A minimum efficiency of 362 mV/m at 1 km for 1 kW at the 285 degree off maximum azimuth. There's no Class A station in Fresno. The only Class As in California are KFI, KGO, KNBR, KNX, KFBK, and KNZR. An "ERP" would be higher for a half wave than a quarter wave, just like an FM ERP is higher for a two bay than a one bay antenna, for a given power. I guess that since there is no ERP reference given, it's just like FM, TV, and Amateur Radio antenna gain claims. Too bad no one agrees on a standard for AM "ERP".

 

[pianoplayer88key]

S-Cat, as I stated above, I consider AM ERP to be the TPO necessary to have the same field when operating omnidirectionally with the same antenna. I calculate it referenced from the specified RMS IDF. My calculation yielded just below 230 kW. I also just now used the method I think you mentioned Wikipedia may have done, and yielded a little over 230 kW.

When talking about increased radiation due to better antenna configurations, I prefer thinking in terms of antenna efficiency. For example, I understand a 1/4 wave AM tower is about 306 mV/m at 1 km, a 1/2 wave is about 381 mV/m, and a Franklin (two 1/2-waves, stacked, insulated, fed at center) is about 512 mV/m, all at 1 km.

Here's a couple Wikipedia articles I've come across...
http://en.wikipedia.org/wiki/Fading_reducing_aerial#Anti-fading_antennas
http://en.wikipedia.org/wiki/Charle...h-efficiency_Medium-wave_Transmitting_Antenna
The second one mentions 1km IDF for a few sizes of antennas.

What would be the efficiencies, in mV/m at 1 km in the horizontal plane, for a few examples below?

** 15.219-compliant antenna with no ground system at 540, 830, 1120, 1410 and 1700 kHz
** 15.219-compliant antenna with a ground made of 8 equal-spaced radials, each 10 feet in length (or whatever would be a good compromise ground when space is very limited)
** 15.219-compliant antenna with the same ground system that a full-powered station would use.
Assume the above 15.219 situations are mounted at the surface of the earth, OR, if elevated (including the radials), have a non-conducting path to earth. Also what difference would be made by using a single base-fed 2.96-meter radiator, vs. two insulated 1.48-meter sections, fed at the center, like a mini-Franklin type?
A few more...
** A 3-, 4-, 6-, 8-, 12- or 16-section Franklin, assuming each section is 180 degrees
** A circular array antenna, as is mentioned in the first Wikipedia link above. Also how would these be configured - phasing, spacing, orientation, and antenna height? And would it be possible to combine it with Franklins?
** various typical / popular FM and TV transmitting antennas

Also, note to a mod: my above post has a formatting error, and I'm past the edit window. Near the last paragraph, at "If there's ANY difference in intelligibility", only "ANY" should be italicized (and underlined), and the "/" needs to be omitted. Could this be corrected?

 

[rfry]

... I guess that since there is no ERP reference given, it's just like FM, TV, and Amateur Radio antenna gain claims. Too bad no one agrees on a standard for AM "ERP."

FM and TV broadcast ERP is stated relative to that of a perfect, center-fed, 1/2-wave dipole in free space, which is a power multiplier of 1. Such a dipole radiates a maximum of 1 kW ERP when 1 kW of Z-matched power is present across its input terminals. The peak field intensity produced by this antenna at a distance of 1 km when radiating 1 kW ERP is about 221.7 mVm.

The net ERP of a perfect, 1/4-wave, base-fed monopole driven against a perfect ground plane is 3.01 dB greater, or twice that of the 1/2-wave dipole in free space, due to a 100% reflection from the ground plane (all of the radiation from the monopole is confined to one hemisphere). This value includes the fact that for any given input power, the r-f current flowing on the 1/4-wave monopole is twice that flowing on each arm of the dipole in free space -- because the radiation resistance of a perfect 1/4-wave monopole is exactly half that of that of the 1/2-wave dipole. The peak, inverse-distance field intensity produced by this monopole at a distance of 1 km when radiating 1 kW ERP then is 221.7 mVm x SQRT(2) = 313.6 mV/m, approximately.

Following the convention used for calculating the ERP from FM/TV broadcast antennas, 50 kW of Z-matched power at the input terminals of a perfect, 1/4-wave monopole system would produce a peak, net ERP of 100 kW, and an IDF at 1 km of SQRT(50) x 313.6 mVm = 2217 mV/m.

The FCC IDF from the KMJ array toward Fresno on a bearing of 285 degrees is 5808 mV/m at 1 km. Therefore, peak ERP in that direction must be (5808/2217)2 * 100 kW = 686.3 kW, approx. So the KMJ array has a power multiplier of 6.863 times (or about 8.37 dB) in that direction, compared to the reference, perfect 1/4-wave monopole system. This ERP value includes the effects of the tower heights in this array as well as the tower field ratios, phasing, and geometric layout of the array.

This approach is not likely to be accepted by the FCC or probably anyone else for calculating the ERP of an AM broadcast station, but nonetheless it is technically accurate, and directly follows the convention used for calculating the ERP from FM/TV broadcast antennas.

 

[Schroedingers Cat]

I should have said there was a lot of disagreement about the gain of FM and TV RECEIVING antennas, not transmitting antennas.

One more point though. A LOT of AM stations with short towers operate at minimum Class B efficiency . If any other ERP standard other than 282 mV/m @ 1 km for 1 kW were used, their "ERP" would be LESS than the power input to the antenna. In other words, less than the 5000 watts or 1000 watts shown on many licenses. Even a lot of directionals are not at full efficiency.