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Author Topic: The QTH From Hell  (Read 821 times)
N1UBZ
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Posts: 6




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« on: September 21, 2008, 04:45:32 PM »

Well, I wish I could link photos in here, because I'd like to show you all the QRM source from Hell. Try this:

http://swaino.com/pictures/ca05/tower2.jpg

That photo looks to have been taken about as far away from my house as the tower is, too. It's spitting out about 200kW of FM and TV energy, so much that I can plug the antenna straight into my oscilloscope and measure the voltage! If I had any fillings I'd be hearing KFOG in my head. Wink Needless to say, there isn't a receiver made that will cope with that sort of RF overload gracefully, especially at VHF.

So I'm wondering if you guys have any suggestions for mitigating the INCREDIBLE intermod and raised noise floor I'm seeing from this noisy neighbor. I'm interested in software-defined radio (with the USRP) and my current project is decoding 1090MHz Mode S transmissions to track passing aircraft. So far the only antenna I've been able to put up that gets anything is a 1/4-wave loop with the sharp null pointed right at the tower! It's not exactly a super sensitive antenna, though.

Before I was thinking clearly I put up a Diamond discone, and it's as bad as you imagine. The problem is that I'd like to be able to receive 120MHz air band as well as local 2 meter and 440 stuff, so I'd like a wideband antenna. The other problem is that the #@$(*& UHF connector they put on the thing isn't worth a @&(* at 1090MHz, and it's not easily modified to accept an N connector.

Do you guys have any magic antenna recommendations, like an antenna with a single really sharp null, or should I just put up a 1/4-wave vertical for 1090MHz and live with the discone for everything below that? How would you filter out the 70db-over-noise-floor FM/TV I'm seeing? And should I filter at the antenna or the shack? What front end would you use to filter the interference as much as possible? How would I ground my coax to reduce the received noise as much as possible?

I was thinking about using resonant cavity filters for my VHF/UHF work, but I don't have any references for constructing one. Can you recommend a book or online resource describing the construction of a tunable cavity filter?
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K0IZ
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Posts: 737




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« Reply #1 on: September 21, 2008, 05:22:27 PM »

Remote station operation would seem the ideal solution, esp since you like SDR equipment.
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AA4PB
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Posts: 12639




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« Reply #2 on: September 21, 2008, 05:27:13 PM »

Notch filters tuned to the offending frequency would probably be the most helpful. You might start by trying 1/4 wave shorted stubs. Notch filters could reduce the level of the offending signal without limiting your abilitiy to receive other parts of the spectrum.

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N6AJR
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Posts: 9879




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« Reply #3 on: September 21, 2008, 06:40:36 PM »

checkm out ICE filters, they are the best

http://www.iceradioproducts.com/filtersrf.html

I use them inthe house to run 2 rigs at the same time. Sutro tower should be nuttin to notch out,  , or point your beams east and use the 147.24 machine out of Freemont
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K6AER
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« Reply #4 on: September 21, 2008, 07:07:44 PM »

The problem you describe is not the QTH from hell but the question begs to be ask why you chose a home under a commercial broadcast tower. I’ll bet many will ask that question.

You did not mention what kind of receiver you are working with but I can assure you that hams operate repeaters at locations much worse than you application every day with out front end overload or receiver intermod.

Several things san be done to mitigate your strong signal over load problems.

•   First you will need band pass filters for the bands of interest. These are placed in the antenna line to block out-of-band signals.

•   Second you will need receiver with high antenna port isolation from the outside world to the desired band of operation. This is generally a commercial receiver design. If the radio has a plastic cabinet don’t even think about it.

•   Third a directional antenna with a good front-to-back ratio will help.

•   Make sure your feed line has at least 100 dB of isolation. LMR-400 will do nicely.

•   A signal level of 70 dB over the noise floor is not much. Typical ham repeaters have to handle 170 dB or more of dynamic range from the receive noise floor.

Now if you have galvanic IMD from local loose tower hardware that is a separate problem that is handled with wrenches applied to lose hardware.

The bottom line is your problem is curable but the fix will cost money. You should be able to have a reasonable receiver noise floor for under $2000.
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N1UBZ
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« Reply #5 on: September 21, 2008, 10:22:03 PM »

I know, I know, why would I pick a place to live under Sutro Tower? Well, the YL had a say, too, and it was either this or a place with no garage. I'll live with the consequences. Wink

I already do run LMR-400, so that part's taken care of. Any tips on grounding (or not grounding) the coax shield to reduce pickup? Will grounding really make that much difference?

The receiver I'm running is basically a very fast A/D, and while it's in a nice metal box I'm sure it's picking up RF on the power supply leads. I can look at additional bypassing but somehow I think that's not the bulk of my problem. It's also, frankly, terrible with respect to filtering, mostly due to the fact that it's designed to receive with as wide a bandwidth as possible (16MHz at least). So pre-filtering is a must. I was thinking (dangerous, I know) that a sharp filter and an LNA with 15-20dB of gain would be good enough to get the worst of it out. Do you think I'll need a commercial receiver to use as a front end, or will filtering + LNA be enough?

K6AER, 170dB over a reasonable noise floor (-140dBm is pretty dang hot) gives, well, a 30dBm signal at the input! Can a repeater front end really handle a full watt on the input? That's a heck of an AGC!
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PD2R
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Posts: 131




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« Reply #6 on: September 22, 2008, 02:36:22 AM »

I second the ICE filters, they work great and are not all that expencive.
Good Luck with your station
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W8JI
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« Reply #7 on: September 22, 2008, 03:18:18 AM »

The idea is is coax shield thickness or anything like grounding is misplaced.

So long as the connectors are good then it's all about the antenna response.

You could use traps, or you could use filters. The trap would HAVE to be a 1/4 shorted stub at the desired frequency. You can read what contesters do for stubs. Or the trap would have to be a cavity or a serie-parallel tuned trap.

It also could be your mixers or any amplifiers before the selectivity are not hard enough. Direct conversion receivers without RF selectivity pretty much stink for wide spaced dynamic range. Where a nomal receiver might have 140 dB or more IM and blocking DR range at very wide spacings, even the very best direct conversion systems would be lucky to reach 100 dB wide spaced DR. We can't use them here in a contest environment.

73 Tom

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WA3SKN
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Posts: 5420




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« Reply #8 on: September 22, 2008, 04:47:37 AM »

Have you ever heard of a "Faraday Shield"?
Shielding, good grounding, and some wavetrap filters should work here.  Do you have access to a spectrum analyzer?  It would help to know the exact frequencies and power levels involved before trying to design any filters.  The shield can be designed to just shield the radio, or to shield the entire room!

-Mike.
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WT0A
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Posts: 922




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« Reply #9 on: September 22, 2008, 05:30:06 AM »

Ah, good ol' Mt. Sutro. there has to be 20 transmitters there. On the up side the analog TV channels will cease to xist soon. Of course th FMs will still bee there and
some digital tv may move there or incraese power.
Tough spot, good luck.
Glen WT0A
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AA4PB
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Posts: 12639




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« Reply #10 on: September 22, 2008, 06:42:07 AM »

The problem with ICE filters is that they are all (at least all on their web page) band-pass filters. When you are having problems with specific individual frequencies you are better off using tunable notch filters, especially when you want to receive a wide spectrum of other frequencies.

I'd start simple by finding their exact transmit frequency and cut a 1/4 wave shorted stub for it from a piece of coax. Connect it at your receiver with a "T" connector. You may find you can eliminate or at least reduce your problem with $1 worth of coax.
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W9OY
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« Reply #11 on: September 22, 2008, 06:49:27 AM »

how about adding a trap

http://www.eham.net/reviews/detail/6845

I used to make traps out of 300 ohm twin lead that worked pretty well.

73  W9OY
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K9YLI
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Posts: 846




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« Reply #12 on: September 22, 2008, 07:00:52 AM »

 Six guys can build a garage in a week end.

easier fix..
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W3LK
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Posts: 5644




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« Reply #13 on: September 22, 2008, 07:48:29 AM »

It's not the tower that makes it the QTH from Hell, it's all the identical houses sitting virtually on top of one another.

I can't imagine voluntarily moving into an area like that, wife and/or garage not withstanding.

73,

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
N1UBZ
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Posts: 6




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« Reply #14 on: September 22, 2008, 10:52:34 AM »

Glen: Digital TV is already up on Sutro, I can see the characteristic flat sidebands on several UHF channels already. The nice part is that ATSC doesn't appear to have a strong carrier signal, so the peak power is quite low. I'll be relieved when they turn off the NTSC transmitters.

The problem with running shorted stubs is that as mentioned above there are at least a dozen 10kW+ transmitters up there, and that's a heck of a lot of traps to put in. If it were just one or two, that'd be fine. Looks like I'm in the market for a narrow bandpass filter, then. I saw a Telonic tunable cavity filter go for $188 on eBay just the other day....

Lon, luckily I'm down the hill a little ways, and the neighborhood is mostly old houses (mine is 1920s vintage), and has quite a bit more variety and character than the apartments on top of Sutro. That said, it's still a city.

Is there an antenna design out there that provides "reasonable" omni pattern, except for a single sharp null? Would putting up a shield 1/2 wave away from the antenna in the direction of the tower do any good?
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