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Evaluating interference problems?

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Old 08-05-2005, 08:45 AM
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JasonS
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Default Evaluating interference problems?

Recently I have had a rash of interference problems, to the point of nearly losing two different planes. I am seeing the problems on four different receivers and two different channels. I have always had one area of the field where I got a momentary glitch, but now the problems seems to be in 3 distinct places- two of which are on final. The problem is compounded now because I am flying tail heavy 3d designs. What would be an almost un-noticable glitch with a sport plane is a near wreck with a tail heavy 3d ship. Some other 3d fliers have reported these issues as well. I am sure that the fact that 3d planes have such extreme throws makes them much more sensitive to glitches, thus the 3d guys are the first to notice these problems.
Last month, I experienced complete loss of control on a pattern ship that lasted for about 4 seconds. I spent some time with an Icom R2 and a Hobbico Frequency checker and I have some results I'd like some informed opinions about.
With the R2 and a rubber ducky tuned for C.B. use, I just happen to hear some chatter on the very two frequencies I fly. On one of the two channels, I can plainly hear a higher noise level with the R2 in FM mode. If I put the R2 in AM mode the noise is almost intelligable. If I put it in wide FM mode I can hear two radio stations, sometimes intelligibly. The primary offender transmits at 91.5 FM. The Hobbico checker shows nothing at the field, although I have yet to try an external antenna there.

The field is roughly 3 miles line of sight from the transmitter farm for 3 local T.V. stations and a couple of radio stations.
I live about one mile from these transmitters. At home, the Hobbico checker is seeing the interference one channel away from both of the channels I fly without ANY external antenna. With a 12 inch antenna it is seeing activity on about 1/2 the 72mhz RC band. With a 36 inch antenna the entire band lights up. My Icom "hears" the same radio station (in AM or WFM mode) on almost the entire RC band with the C.B. rubber ducky. I can set the squelch at the highest setting and it still hears the station in any mode but FM.
When flying a park flyer near my home I can only get about 100ft. of usable range as well.
I am flying JR ABC+W single conversion FM receivers. Dual conversion users seem to have minimal problems.
I have purchased a 770 pcm receiver in hopes of alleviating the problem, but I am planning on changing channels as well. How much should I read into the "interference" I seem to be seeing with the R2 and Hobbico checker? By evaluating which channels "trigger" on the hobbico checker with Varying antenna lengths could I determine which channels are the cleanest?
I have a pattern contest to attend in a week and I need an immediate solution. My last five practive sessions at this field have all been a loss due to the issue I'm having. Any advice is greatly appreciated!
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Old 08-05-2005, 12:08 PM
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Default RE: Evaluating interference problems?

Hard to say. The TV/radio stations could be putting out spurious signals on 72 MHz, but it also could be that the TV/radio stations are operating properly but overloading the front end of your R2, or that your R2 has image frequencies that fall onto the TV/radio stations assigned frequency. The fact that you say dual conversion RC receivers have minimal problems suggests to me it may be one of the latter.

It certainly warrants further investigation. I'd say ask around at your club or maybe visit the local ham radio club. There may be an electronics guru who has, or have access to a spectrum analyzer. That will provide a more reliable picture than the R2.

If the stations are putting out spurious signals 72 MHz then they have an obligation to fix their problem.
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Old 08-05-2005, 03:53 PM
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Default RE: Evaluating interference problems?

I've talked to a variety of club members and have heard enough conflicting opinions that I'm not sure what to take to heart. Supposedly, a local T.V. RF engineer found an issue with channel 31 that occurs only when a Corp of Engineers water level transmitter signals at the lake next to the field. Supposedly it was a 3rd order Intermod with one of the local T.V. stations and the Corps transmitter that blankets channel 31. Yet another engineer supposedly checked with an analyser and found no problems.
Part of the reason I acquired the Hobbico checker was to rule out an Image issue with the Icom.
One of the local members has a Spectrum Analyser at work that he has offered to bring out to check again. As he seemed to not understand my questions about "Image" issues with my Icom I'm not sure how knowledgable he is.
If he brings the scope out, what sort of general guidelines do I need to follow? I'm not very knowledgable about RF theory, although I'm a Video Technician/ self trained engineer of sorts who uses Waveform monitors and oscilliscopes daily.

As my somewhat unscientific observations are that dual conversion receivers seem to exibit fewer issues with whatever problem exists, I'm leaning toward moving into J.R.'s high end dual conversion PCM receivers. As much as they defend their filtering scheme on their cheaper receivers, obviously they must have a reason for offering dual conversion on their high-end receiver offerings.
Does this seem like a valid assumption?
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Old 08-06-2005, 09:43 AM
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Default RE: Evaluating interference problems?

If you could borrow a glitch counter and fly (older airframe of course) I would be interested in the results or number of glitches.

Are you flying different receivers with the same transmitter? Are you flying gas or glow?

I have always thought that the purpose of dual conversion was to filter out more of the extraneous or unwanted signal. I believe that PCM is even more discriminating.

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Old 08-06-2005, 10:02 AM
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Default RE: Evaluating interference problems?

Yes, intermod is another possiblity. Each of those transmitters may run many kilowatts, and there would certainly be opportunity for creating intermod. The intermod could arise at or near the transmitting site, say a corroded bolt on one of the towers or maybe even in the circuitry of your receiver?

The advantage of a spectrum analyzer is that it should be relatively clean of images. You can also set the bandwidth to be very narrow. This will allow you to tell whether or not another signal is really on your frequency. I don't think the R2 will let you set the bandwidth below about 10 kHz ( ?), so a signal located properly on a channel inbetween the RC channels may appear to be on an RC channel. The other thing that is helpful is that you can sweep the frequency and see what else is around that could be a problem.

Hams deal with this sort of thing alot. Some hams even enjoy tracking down RFI problems and consider it part of the hobby. They typically use directional antennas to locate the RFI source. It does take time and skill, and some specialized equipment.

Hopefully the problem will go away when you replace your single conversion receivers with dual conversions. Using a single conversion receiver in such a busy RF environment is possibly expecting too much.

Good luck!
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Old 08-11-2005, 08:44 PM
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Default RE: Evaluating interference problems?

I agree with the going to double conversion receivers with good frontends. Your receiver frontend may be getting swamped by the other strong local signals. Bummer when you can hear the interference, it's so bad. Good luck.
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Old 08-13-2005, 01:22 AM
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Default RE: Evaluating interference problems?

Certainly all receivers are susceptible to image frequencies and inter modulation distortion etc (including a spectrum analyzer) and susceptibility to those problems is much more likely with a wide band receiver such as the Icom R2 because of a lack of a front end filter. Also because it's a wide band receiver there tends to be more internal noise from multiple local oscillators causing even more undesired mixing. But then again it was designed to be a casual wide band receiver for listening to your local fire department, TV station or your neighbors cordless phone not to give accurate channel assessment on a tiny part of the RF spectrum. For those that use these Icoms or similar scanners for checking their 72MHz or 75 MHz channel status, adding a simple front end filter would do wonders for blocking out most of these out of band signals that can give them false readings. It could be built with bnc connectors and simply plug in between the scanner and the antenna. I can post a schematic if anyone is interested, or there are programs available for download that will calculate the component values for you when you punch in the center frequency and bandwidth you desire.
I suspect the Hobbico scanner also lacks a decent front end filter plus it is most likely a single conversion receiver making it suseptible to image frequency problems. For the price it's awesome, but seems to give some false readings particularly in dirty RF environments.
Now for the sales pitch. The RC Scan 7200 ( http://www.desert-wolfe.com/rcscan ) was specifically designed to give accurate channel assessment in the 72MHz and 75MHz band. It has a good front end filter, high quality narrow band IF filter and is a double conversion receiver. It can give you glitch counts assuming your using a standard PPM transmitter and now has a new mode of operation that allows it to scan in between channels to shows if there is interference present just outside the channel or to show if a transmitter is off frequency. This new mode isn't reflected on the website yet but will be shortly. Also, we will be standardizing on the Dual Band (72MHz & 75MHz) unit and selling it for the price of the single band units - $250.00. It also scans the band significantly faster than the Hobbico unit.
It's more expensive than the Hobbico by a fare amount, but will give you much more capability and more confidence in the readings.

Sorry, this got a little wordy.
Brian
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