The Chelsea repeater (WD8IEL), since this trustee assumed its maintenance responsibilities in 2022, has demonstrated an attribute of performance suggesting desensitization. That is, the background RF noise has a magnitude greater than the receiver’s sensitivity. The effect is that the repeater can be heard at a much greater distance than it can be worked.
BACKGROUND ELECTRO-MAGNETIC FIELD QUANTIFIED
The collective background electromagnetic field for the area measured from the parking lot of the Taco Bell restaurant was -66dBi based on a 50 Ohm impedance. This translates into 12u Volts which represents a very great limitation for any radio receiver in its vicinity.
APPLICABILITY TO THE CHELSEA REPEATER
The Chelsea Amateur Radio repeater (WD8IEL) is located in the water tower behind McDonalds and the Coney Island restaurant on M-52 which is in the immediate vicinity of this measured RF background noise. Because the repeater receiver is specified to have a sensitivity of 0.2u Volts, this background EM noise represents a very great limitation for the repeater. Any RF signals reaching the repeater having frequencies of 144.850 MHz (the repeater input) must have magnitudes greater than whatever the background EM radiation is.
Are you beginning to see the dilemma? Only very strong incoming signals will be able to be recognized by the repeater receiver. However, this does not affect how far away the repeater may be heard. The overall effect, therefore, is that the repeater may be heard at distances far greater than it may be worked.
A POSSIBLE SOURCE OF THE EM BACKGROUND RADIATION
On June 17th, with the help of George Byrkit (K9TRV), I was able to measure the EM background radiation present in the parking lot of the Taco Bell restaurant on M-52 in Chelsea. This location is situated directly across from where the Chelsea Water Tower is which houses the Chelsea Repeater operating on an output frequency of 145.450 MHz. As noted above, we saw -66 dBi representing 12 u Volts. If that is all that we knew about the EM radiation, then we would be toast with no alternatives other than to find another site for the repeater if the intent is to optimize the amateur experience.
WHAT ABOUT THE DTE UTILITY POLE IN FRONT OF CONEY ISLAND?
With some directional AM equipment, we were able to identify a broadband EM noise signal emanating from the power pole in front of the Coney Island restaurant on M-52. The offending power pole can be seen in the background in the picture shown on the right. A close-up of its transformer for ID is shown below that.
Such background radiation is not normal for a transformer and indicates that it is in need of attention. The nature of the EM radiation is broadband indicating arcing within the transformer.
It is particularly important that the responsible utility company address this issue since the FCC regulates such EM as excessive.
WHAT IS BEING DONE
The responsible utility company is being advised of this anomaly and is expected to address the matter in a timely fashion. It appears that the jurisdiction for the telephone pole is Consumers Energy (www.ConsumersEnergy.com, Lansing, Michigan). Calling them, I got nowhere with no suggestions offered on what alternatives may exist to get a remedy.
ALTERNATIVES FOR THE REPEATER
If the offending background EM radiation cannot be remedied, the only option is to find another site for the repeater, assuming optimal performance is required. There are many opportunities for alternative sites to choose from.
We met on Saturday morning, 17 June at about 10AM. Taco Bell was a good choice as they were right across the street from the water tower, hence repeater, and the offending power pole that I had identified the day before. Unfortunately, I neglected to have the MFJ-852-based MFJ-856 unit (an 852 meter with a 3 element hand-held yagi.) This unit is advertised as working at approximately 135 MHz AM mode. The manual and circuits are available online. So as a result, Wesley had to take my word for what I had observed the day before using this unit. I also observed a male voice audio, seeming to come from the McDonald’s restaurant. This location is notable in that it is at the base of the water tower where the repeater is located. Proximity magnifies small noise sources. Inverse square law and all that.
I also did not have an adapter for the N connectors on my Rigol DSA-815 unit. I was expecting BNC. Shame on me for not checking beforehand! We used that 1.5 GHz spectrum analyzer, running of a BioEnno LiFePo battery pack with inverter/charger. I did not have a yagi that I could connect to the Rigol DSA-815, to determine directionality of what we observed on the spectrum. I left that at home, too. I centered the Spectrum on 120 MHz, with a bandwidth of 200 MHz, so 20 MHz to 220 MHz, roughly. We used a ‘random wire’ (Wesley had some old cable TV coax in the back of his SUV, I stripped off the runner used to hang the coax, discovered it was copper-covered steel, and stuck it in to the center pin of the input N connector on the spectrum analyzer. It was set with a reference level of 0 dB, and no attenuator. We could have used a cellphone to take a picture of the spectrum being displayed, but we did not. We could have used the laptop I also brought, on which we could have also run Touchstone Pro with RF Viewer hardware. There was so much we could do, but ended up not being properly equipped to do. These are lessons learned for the next visit.
What we didn’t do was go away from that site, to a nice, quiet, farmland location, and replicate the background spectrum reading. The difference between that reading, and the reading at Taco Bell, would have established the increase in background noise level at the repeater location. We also have not controlled for noise coming from the inverter, nor for other close noise sources that we didn’t identify.
Don’t get me wrong; this location has two easily observable noise issues. One seems to be power line equipment related, and needs to be reported to the relevant power company (it could be either Consumers Power or DTE.) Once the power company has remediated what seems to be their issue, additional measurements, or more appropriately, ‘observations’, which allows for some ‘qualitative’ aspect to the report (perceptions, what one hears.) This is still likely a good location for a repeater transmitter. Perhaps it can be fortified by some remote receivers in quieter, more appropriate locations, that possibly link to the tower via 70cm or higher.
I did some testing at home over the weekend. I also repaired my discone antenna for use as an omni data gathering antenna.
I also soldered the ‘random wire’ that was about 95cm long into an N connector, so it makes better, more consistent contact.
First test, I hooked up the Rigol DSA-815 to mains power, and made some readings. A nice flat line around -68 dbm, with a spike at 91.9 MHz for WUOM, just up the road at Peach Mountain. I have a nice photo of this trace.
Second test, replace the mains power with the BioEnno battery pack/inverter that we used on Saturday. Oh, my! I have another photo that shows all the noise on that, considerable, almost as strong as WUOM-FM, and between roughly 5MHz and 100 MHz. This looks very much like the stuff we saw but did not photograph on Saturday.
Third test: use a TrippLite UPS, not connected to the mains, rather than the BioEnno battery/inverter pack. result: much more like the image when running from AC mains. A photo is also available.
Conclusion: the BioEnno battery/inverter box is very noisy, and we won’t be using that in the future. Also have not found a location where a reading less than about -70 is shown on the spectrum analyzer. Perhaps we cannot discern small signals with this unit. every instrument has its limitations.