C-band TI

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jantonuk
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C-band TI

Post by jantonuk » Wed May 09, 2018 3:13 pm

For the past week I've been getting intermittent Terrestrial Interference on my satellite dish.
There is a big quarterly air exercise going on from the local Eielson Air Force Base.

I suspect some kind of radar interference.
All radio and TV stations in Fairbanks are suffering TI to some extent.

I've contacted them but it's hard to get through to anybody that knows anything or can help.

Anyway, on my side, a 3700-4200 bandpass filter from Dawn did not eliminate the problem so it appears to be in-band.

If the suspected interference source was a point-to-point link, I'd think of terrain shielding.
But with it being airborne it's hard to not see it.
Oh did I mention that my dish is pointed to the southeast and the area of operations is also to the southeast. Murphy's law.

Here's something I thought of:
The dish only needs to "see" straight out to space. What if I built a shed/cover/garage over it and extending out 12-16 feet, with the dish pointing out the door?
It could be plain wood stud construction with plywood sheathing and then metal siding on the outside. (make the radar bounce off?)
My angle of elevation is only 9.65 deg here in Alaska.

It would be like blinders on the dish. Then the offending source would have to be directly in front of the dish to cause trouble.

Maybe better than nothing....

John

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kkiddkkidd
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Re: C-band TI

Post by kkiddkkidd » Wed May 09, 2018 3:39 pm

John,

If it is military radar, an "AWACS" Filter might fix your problem much easier. In general, the radar signals are outside the sat downlink frequencies BUT are so strong that they overload either the LNB or RX front end. These filters are placed between the scalar and the LNB and filter signals outside the downlink band down to a manageable level.

There are also filters for single transponders to be used in especially bad cases.

A spectrum analyzer would be very handy to diagnose the exact cause. Even an SDR RX connected thru a splitter to the LNB output would probably be handy.

Good luck,
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Kevin C. Kidd CSRE/AMD
WD4RAT
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http://www.amgroundsystems.com
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jantonuk
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Re: C-band TI

Post by jantonuk » Wed May 09, 2018 3:46 pm

I have installed a C-band passband filter between the LNB and rings as you've described.
This one is from Dawn Satellite and called a WiMax filter.

Is the "AWACS" filter you are referring to anything different?

My spec an is up on the hill at the ham repeater at this time....

Single channel not a good solution as I need three different transponders, 1, 17, 21

What do you think of the 'blinders' cover idea?

John

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kkiddkkidd
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Re: C-band TI

Post by kkiddkkidd » Wed May 09, 2018 4:24 pm

It certainly may help but if the interference generator is zipping around the sky around your site, it might be behind/beside the dish one minute and bore sighted the next.

Seeing what exactly is causing the interference is going to be important. Down here in TN we don't have too much AWACS interference but I do have a couple of customers with AWACS filters. As I recall, when they were ordered, they were specifically ordered as AWACS (way, way pre-WIMAX) but the WIMAX may be the same thing since it falls just outside the downlink frequencies.




jantonuk wrote:
Wed May 09, 2018 3:46 pm
I have installed a C-band passband filter between the LNB and rings as you've described.
This one is from Dawn Satellite and called a WiMax filter.

Is the "AWACS" filter you are referring to anything different?

My spec an is up on the hill at the ham repeater at this time....

Single channel not a good solution as I need three different transponders, 1, 17, 21

What do you think of the 'blinders' cover idea?

John
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

ChuckG
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Re: C-band TI

Post by ChuckG » Wed May 09, 2018 7:02 pm

jantonuk wrote:
Wed May 09, 2018 3:46 pm


Single channel not a good solution as I need three different transponders, 1, 17, 21
You can install a tuned filter to notch out in-band interference, as long as it doesn't land on top of one of your transponders.
http://www.microwavefilter.com/c-band-lp3.html
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TPT
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Re: C-band TI

Post by TPT » Thu May 10, 2018 5:33 am

Microwave filter makes a good AWACS filter. We've used hard ware cloth (looks like heavy window screen, made of metal) as shielding --but that was to protect against back scatter from a nearby MCI tower behind our dish. Hardware cloth is often used around gardens to keep rabbits out.

As I recall, the military radar is just above the top end of C-band, we saw some of it from a nearby ANG base, but then they switched to radar up near the KU band.

There is a proceeding going on at the FCC, presently an inquiry about sharing C band with terrestrial cellular uses. Comments due May 31. We will see a lot of nonsense on how the cell companies can grab spectrum yet still share with C-band receive dishes. Even mobile uses. Suggest you and your colleagues get in touch with the Alaska broadcasters association and have them add to the chorus of NO!

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kkiddkkidd
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Re: C-band TI

Post by kkiddkkidd » Thu May 10, 2018 7:46 am

TPT wrote:
Thu May 10, 2018 5:33 am

As I recall, the military radar is just above the top end of C-band, we saw some of it from a nearby ANG base, but then they switched to radar up near the KU band.
And just a reminder in looking for sat interference with a specan... The L-band output of the LNB is inverted in relation to the C-band input frequency wise. IE, the low end of the L-band is the high end of the C-band and vice-versa. To find out what C-band freq you are looking at, subtract the indicated freq on the specan from the LO of the LNB (5150mhz). IE, 5150-950=4200mhz (the top end of the sat spectrum) or 5150-1450=3700mhz (bottom end of sat spectrum). There are numerous charts and calculators available on the internet.

I have a funnel mount for an LNB that I have used to locate the general direction of interference sources. If the interference is strong enough, a bare LNB will receive it.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
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rftranstv
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Re: C-band TI

Post by rftranstv » Thu May 10, 2018 8:49 am

funny story...I was part of an exercise in northern michigan. A radar site was set up next to a busy highway. When the radar was active...cars that were on the highway when the radar rotated towards the road were shutting down, car alarms would go off, power windows would turn on and off,etc....they had to make it so the radar turned off when the dish rotated towards the road...it was crazy

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Re: C-band TI

Post by NECRAT » Thu May 10, 2018 10:44 am

kkiddkkidd wrote:
Thu May 10, 2018 7:46 am
To find out what C-band freq you are looking at, subtract the indicated freq on the specan from the LO of the LNB (5150mhz).
And hope the LNB is using a standard L.O. of 5150 and not one of those odd-balls. While 5150 is normal, I have seen some unusual ones on older dishes.
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kkiddkkidd
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Re: C-band TI

Post by kkiddkkidd » Thu May 10, 2018 7:06 pm

NECRAT wrote:
Thu May 10, 2018 10:44 am

And hope the LNB is using a standard L.O. of 5150 and not one of those odd-balls. While 5150 is normal, I have seen some unusual ones on older dishes.
Wow... That would cause a bit of consternation. I have never ran across a C-band LNB that used an odd LO but have worked around KU-band dishes that you had to be very careful to replace the LNB with the exact same part. I still have a NOS LNB KU around here somewhere that I ordered missing an important digit in the part number and it was a different freq range or something odd.

Later,
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Kevin C. Kidd CSRE/AMD
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rftranstv
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Re: C-band TI

Post by rftranstv » Fri May 11, 2018 8:27 am

NECRAT wrote:
Thu May 10, 2018 10:44 am
kkiddkkidd wrote:
Thu May 10, 2018 7:46 am
To find out what C-band freq you are looking at, subtract the indicated freq on the specan from the LO of the LNB (5150mhz).
And hope the LNB is using a standard L.O. of 5150 and not one of those odd-balls. While 5150 is normal, I have seen some unusual ones on older dishes.
70 MHZ I.F modems and LNBs are out there...seen them in the military but not in broadcast at all.

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Shane
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Re: C-band TI

Post by Shane » Sat May 12, 2018 12:23 am

In the early 80’s a Scientific Atlanta broadcast installation in Omaha used an LNA with a downconvert to 70 MHz after feedline entered the building. The downconverter was inside the receiver and was I think a double conversion to get down to 70. So there was some of that stuff in the broadcast realm back then.
Mike Shane, CBRE
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Deep Thought
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Re: C-band TI

Post by Deep Thought » Sat May 12, 2018 9:38 am

Our 1982-vintage UPI and NBC audio dish was 70 MHz second IF. Was kinda fun to tune around on the SCPC carriers with a comms receiver and listen in back then. :)
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kkiddkkidd
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Re: C-band TI

Post by kkiddkkidd » Sat May 12, 2018 2:12 pm

I had forgotten about the old SA and whatever the other SEDAT RX was. I have a station with at least one of those down converters still hanging on the wall.

Later,
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Kevin C. Kidd CSRE/AMD
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ChuckG
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Re: C-band TI

Post by ChuckG » Sat May 12, 2018 11:42 pm

The Microdyne receivers had a 70 Mhz IF as well.

Deep Thought wrote:
Sat May 12, 2018 9:38 am
Our 1982-vintage UPI and NBC audio dish was 70 MHz second IF. Was kinda fun to tune around on the SCPC carriers with a comms receiver and listen in back then. :)
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