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5G versus commercial aircraft Radio Altimeters in the US, wtf?

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LaserSteve:
I attend a modest sized church with a multi-carrier cell tower on an adjacent property.

When the Digital Television switchover occurred,  our wireless microphones were now illegal. So my spectrum analyser attended church fairly often as we [actually I] decided  what band and type of modulation to switch to.


I've had really bad experiences with VHF wireless mics. 

900 Mhz was studded  with data bursts from gas and electric meters.

2.4 Ghz was out of the question with 150-200 cell phones attending service pinging for Bluetooth/ WiFi.  2.4 Ghz WISP was on the tower.

That left "T band" and up. We did not qualify for licensed spectrum and frequency allocation under the new rules.

I reassigned all our critical command and control to 5.6 ghz WIFI and the band's audio transport to hard wired ethernet.

Large amounts of whitespace WIFI was emanating from the high school across the street.


Four frequency agile decent spread spectrum wireless mics cost the church 2700$ due to spectrum auctions and unfunded mandates.  When the TV repack occurred, the cell companies already had their new gear on the tower. I was not planning on attending that Sunday, but got a call to bring the spec an as two mics went down that Sunday from jamming.

The frequency scanning software in the new receivers had a bad habit of placing the suggested  channel about 50 kHz
from the edge of the  nearest  ATSC spectral  mask it can find.. So manual channel selection is a must. We have the software to plot the mic receiver's scan on a pc. I am amazed by just how packed 450 to 950 Mhz is in Northern Ohio.

I have a WA5VJB Log Periodic on the A input of each wireless mic receiver  and the factory omni on the B input.
The LP does a good job of "looking under" the tower and most of the time the voter in the receiver prefers A as it looks at signal to noise ratios. Signal strength is about the same on A and B as I have the antennas about 60 feet from the alter. I have bandpass filters on everything now.

A chance meeting with a contract installer confirmed remotely controlled dynamic channel selection on nearly everything on the tower and auto adjustment of downtilt and other parameters. Out of band spurs are all over the place from the tower.

My point being when a modest church out in the country needs a spectral coordinator  due to a tower 350 feet away, things are a bit out of hand. I wonder how much they would have had to spend correcting all this if they did not have a well equipped Ham in the congregation. I at least gave leadership a one year heads up about the need for new
gear. I follow the FCC craziness religiously, as my Ham habit focuses on the microwave bands.

  It is far worse for touring concert sound technicians who can easily need to find sixty channels at each venue.

   I found hash and spurs all over the place from our neighbor. The tower tech mentioned corrosion can act as a non-linear mixer close in, resulting in  interesting effects in the very near field.

Couple that with gear without "Brick Wall" input filters and your bound to end up with problems.

Primary long distance search radar in the US, around the coasts runs around 1200-1300 mhz.

So I'm not surprised that there are a few locations where odd occurances happen.

  I'm an instrumentation technician for a research oriented
medium sized university. The spec an has had to visit campus a few times as well, for similar problems.

Steve








ejeffrey:

--- Quote from: coppercone2 on January 29, 2022, 12:08:55 am ---well keep in mind how real life microwave filters end up looking like on a transfer function.. there could be peaks, modeing in waveguide, etc. I see alot of graphs that look like a cutoff then it bounces like a ball falling down a hill. I bet microstrip filters don't help too much with that.

--- End quote ---

I will say it's possible that the problems noted in that document are from non-compliant cell towers, specifically those in the 2.1 GHz band generating a lot of 2nd harmonic distortion.  If so, obviously the non-compliant hardware should just be fixed or disabled.  I don't know how likely that is, although if it happened in the US and the FAA reported interference from a tower operating outside it's assigned band you can bet that there would be a response.

But if you take that report at face value there is no need to make excuses for them.  A radar altimeter that is screwed up by a LTE tower operating 2-3 GHz away is just hot garbage.  Yes, you *can* make a bandpass filter that has an unwanted transmission peak there.  That doesn't excuse putting it into a supposedly safety critical device.  There is no way that is acceptable, not now, not even in 1960.  It shouldn't be flying.


--- Quote from: LaserSteve ---When the Digital Television switchover occurred,  our wireless microphones were now illegal

--- End quote ---

Assuming you are talking about the 700 MHz band: that spectrum was allocated for UHF TV channels but little used in most areas.  It was never assigned for unlicensed use or licensed for wireless microphone use.  It is extremely debatable at best that the use was ever "legal" although it was certainly ignored as long as it didn't stomp on an active UHF channel at that specific location.  This is of course the whole reason why it was popular for wireless microphones: analog microphones are extremely susceptible to interference and since those bands were reserved for TV but unused they were largely clear of the interference that ISM bands see.  But the risk of squatting on spectrum that you don't own is that it can get reassigned from under you. Probably the FCC should have assigned some spectrum specifically for analog wireless microphones, but I guess they thought that 900 MHz was good enough?

LaserSteve:
The final decision from the FCC left roughly four Mhz for wireless mics in a cellular guard band. To obtain a license
and co-ordination you must need at least 64 devices for your
event.  Otherwise it is a free for all on picking channels.

The final rule making came down less then six months from the switch over.  The lawyers and the lobbyists were probably quite busy deciding who would purchase what and when. The entertainment industry was quite busy trying to band together and come up with money to lobby.  Rumors
of just where the proposed allocation would be were flying around the media.

Wireless mic manufacturers were offering rebates and trade ins in an attempt to get as much gear as possible out of 700.

Fcc is lead by lawyers, all of which come from the cellular industry. I note that a certain commissioner, when she first
joined FCC, was talking about all these Uhf and Vhf spectrum bands that were under utilized and should be auctioned off. I remember her "I know a band or two we should sell" quote quite well. She was hinting about selling UHF ham / biz band and the ~ 1 Ghz Navigation/Radar band.

Now she's the Chair of the Commission. She's calling for Congess to re-establish the in house,  independent,  non-partisan  science advising agency it disbanded twenty years ago, to prevent this sort of thing occurring again.  That is to her credit for figuring out she presides over quite a mess.

That is quite a turn around.  It took her what, four years to learn this?

There were jokes flowing around various email list-serves about what that spectrum was actually quietly used for by the military. It didn't take long for that offer to disappear.

Yeah, those Radalts should have never made it through approval.  However when the approval process is a self submitted packet, one wonders who checks it.

The point of this post is less lawyers, less lobbyists. More Engineers and planners are needed. More technical enforcement is needed.

NTIA advises Congress on spectrum. NTIA and FCC are not under the same roof, so to speak. Throw in the international allocation process every few years and you have considerable Chaos.

What woke me up this morning is realizing that four years ago I flew into Vilnius LT on a very similar aircraft with an IFR approach in marginal weather.

I'm a big fan of Capitalism. But there are a few things that should be carefully managed by independent agencies. . Spectrum is one of them. National standards are another.
Just think what a mess privately managed time and frequency standards would be.

Steve

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