Wavering field intensity

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kcbooboo
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Wavering field intensity

Post by kcbooboo » Fri Jun 08, 2018 8:15 am

1kw directional station, one monitoring point is at one of two pattern nulls about 6 miles west of the TX. The expected signal strength is around 1.5mV/m range, yet it seems to be slowly wavering up and down by several mV/m. The station is within 0.1 Hz of its assigned carrier frequency. The meter acts as if another station is beating with it.

I checked for other stations on the same frequency. There's a 10kw station to the NNW about 180km away, and a 1kw station to the ENE about 150km away. I didn't bother looking further than 200km. I had been to this monitoring point several years ago with multiple FIMs and they all were rock-solid at between 1.4 and 1.5mV/m.

The same FIM-41 is giving correct meter readings at other monitoring points and all the readings at the TX and phase monitor are correct and stable. Any ideas?

Bob M.

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RGORJANCE
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Re: Wavering field intensity

Post by RGORJANCE » Fri Jun 08, 2018 8:44 am

I ran into this for the firs.t time many years ago when doing FIM's. In my particular instance, I was still a bit of a rookie.

I had just taken a reading and closed the FIM, got back in the car to record the number.....and forgot what Isaw. Got back out and saw that it had changed (needle position). Well, after looking at the meter, I saw the needle very gradually decreasing then increasing to a peak. and back down again. So I recorded both the peak and valley with notes.

What I had observed was that in the late afternoon, I was seeing sky wave effects.

Sooooo the question is....what time of day are you seeing that change?

(Or did I just imagine this??? Could it have been early onset of old age?)

Fossil

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Deep Thought
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Re: Wavering field intensity

Post by Deep Thought » Fri Jun 08, 2018 10:37 am

Yeah, if it was after about 4 PM that low of a MP value can be affected by the onset of skywave propagation. I once did an antenna proof in November on a station on 830 KHz in northern California and had to quit taking readings on the backside of the pattern around 3:30 due to WCCO contaminating anything under about 5 mV/m. Just because your station is <0.1 Hz from perfect doesn't mean the incoming one is. It also doesn't mean the other guy's DA is "in".

Another thing that can cause wavering readings are power lines swaying in the breeze, especially on the upper part of the band, but that is usually more of a sudden change between different values.

This won't win you any friends at the station, but the easiest test is to go to the point and shut the station off for five seconds. If there is another one in there you'll hear it. Six miles is really too far from the station for a monitor point (the FCC usually wants them from 1-4 miles) but it is often impossible to find a clean, easily accessible one any closer.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
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Re: Wavering field intensity

Post by kcbooboo » Fri Jun 08, 2018 12:24 pm

Thanks. I'm not sure when the reading was taken but it was within the last two weeks and around here sunset is at least 8pm EDT now. Chances are it was done mid-afternoon.

The carrier could easily be dropped for 5 seconds to hear what, if anything is behind it. The 10kw station is pointed NNW so we should be in one of their nulls, which aren't that deep.

I agree, the monitor point is actually 4.11 miles away (my "6 miles" probably was "6 kilometers") but even still, it's pretty low. They could have found a point a bit closer in. Unfortunately the other null is on water, and the only other monitor points are on the major lobes.

I was going to stop at the monitor point this afternoon around 1:30 but never heard back from the CE so I didn't bother.

There are some power lines overhead but they've always been there. This is apparently the first time this condition has been experienced. When I was last there experimenting with multiple FIMs, it was noon in October; shouldn't have been much skywave at that point.

Bob M.
Last edited by kcbooboo on Mon Jun 18, 2018 4:44 am, edited 1 time in total.

RodeoJack
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Re: Wavering field intensity

Post by RodeoJack » Sun Jun 17, 2018 8:11 pm

Maybe a bit off-topic, but something to be aware of before you go out to do the monitor points.

You've probably seen the one with a freeway running between the towers. This is one of my client stations in Eastern Washington. 3 towers in this one and yes... the tracks were here first. Don't know why the former owner chose to locate this way. The site was engineered by one of the better consultants in these parts.
2018-06-13 16.47.06A.jpg

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Deep Thought
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Re: Wavering field intensity

Post by Deep Thought » Sun Jun 17, 2018 10:31 pm

RR tracks aren't really a big problem as long as the ground system is designed to accommodate it. I did a six tower inline here in the Chicago area with a spur running between towers 3 and 4. Trains never really upset things much, and this was a critical array (back when such things existed) with high-resolution monitoring.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: Wavering field intensity

Post by RodeoJack » Sun Jun 17, 2018 11:03 pm

That probably explains why this site has been an interesting topic around here, but has never really concerned anyone.

kcbooboo
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Re: Wavering field intensity

Post by kcbooboo » Mon Jun 18, 2018 4:52 am

There ARE railroad tracks to the southeast of the site but they've been there since the days of steam engines. At one time (1970s and earlier) they had to detune all of the RR structures within about a mile, but I don't think that system is in place any more. Since the 60s, a major interstate cropped up just about parallel to the RR tracks. Both of these are in one major lobe but there's always traffic and the trains run probably every half hour (major commuter line). A now-inactive landfill occupies some of the surrounding land; it grew to capacity in the 1980s. There have also been some pretty large advertising signs erected within a mile of the station, and possibly some have cellphone equipment within their structures. There's always road repair going on, plus a nearby exit got reworked.

Bob M.

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Re: Wavering field intensity

Post by radio_guru » Mon Jun 18, 2018 11:49 am

I know which station Mark speaks. I was part of that station's original tune up in 1991. What fun that was...

I've seen skywave occur any time of day. Though it's rare, mid-day skywave can occur when the sun is low in the horizon, such as December. I was doing a partial check-up for a station just about to go into a STA last December and two radials were fully contaminated by a co-channel station 300 miles away. Turned off the station for 30 seconds and confirmed that to be the case. They are an omni. So nothing to be done but call it a day.

The farther north you go, the more profound the seasonal skywave effect.

kcbooboo
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Re: Wavering field intensity

Post by kcbooboo » Mon Jul 23, 2018 1:22 pm

I met the station's CE and brought my FIM-41 with me. Both meters were very stable at 1.35mV/m in night pattern, about 5.6mV/m in day pattern. Doesn't seem like much of a null but they both said the same thing, around 1:35pm on a gloomy day. He checked some other points as well and they too were nice and steady.

Besides the weather and possible time of day, the station was running on the backup TX today. The main blew out its PA and it's remotely possible that the PA was in the process of failing during the previous meter reading attempt.

Bob M.

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