Tower light, photo eye, and 2M repeater desense

[n4dbm]

I had an experience today that I thought might be helpful to share here to possibly eliminate someone else's grief when troubleshooting a repeater with serious desense or mixing at a tower site with a cheap dusk/dawn sensor and a steady marker light.

The details of the site are such as this. The structure is a water tower at a good HAAT in central NC. The water tower has a steady marker light at the top which is apparently no longer required since the construction of a 400-foot free stander right beside it. The "no longer required" portion is hear-say, so please don't hold me to that rule. The bulb has been burned out for years. The photoelectric eye is a cheap box store type of device with a twist-lock plug. It is located at the base of the water tower, and powered by a separate breaker from the nearby outdoor panel box.

A fellow repeater operator installed a machine consisting of two GM300's and a nice 6-cavity duplexer at the site several years ago. The frequency, although probably not relevant, is 146.805 transmit and 146.205 receive. He has noticed anywhere from 6 to 10 dB of what seemed to be transmitter to receiver desense. The repeater was upgraded to an MTR2000, with the power level at 80 watts. 62 watts leaving the duplexer resulted in about 50 watts to a clean and properly installed omni antenna with 1/2" Heliax type cable. The new repeater, along with different duplexers, filters, and arrays of all sorts never helped the desense. The "sound" of the desense also hinted with grossly distorted power line buzz, but here again, was only prevalent when the repeater was transmitting.

I went to the site today thinking I would possibly find the duplexer out of alignment or perhaps some obvious cause that might be easy to spot. The duplexer was aligned close enough that it should not have affected the performance. Moreover, the repeater did NOT desense into a dummy load, so this pointed to possibly a mixing issue on top of the tank. I remembered the marker light, and switched the breaker OFF thinking it should not make any difference, but it did. Turning the power OFF to the photo-eye completely eliminated ALL mixing and desense when the repeater transmitter was keyed. Apparently, RF was following the lead wires down the tower to the photo-eye and mixing with the SCR. The lamp load in the device is connected directly to the SCR, not through a relay. A good solution to this problem is to have the SCR drive a relay so that the SCR is completely isolated from the wiring leading up the tower (which is in PVC conduit, NOT shielded conduit). For now, the breaker has been turned off resulting in no power to the photo-eye device, and all is well.

So, if anyone has a VHF repeater that has nasty desense with a hint of intermod and mixing that sounds like AC line noise hum, check for SCR's or cheap photo-eyes on the tower. It was certainly the culprit at this particular site.

Best regards,
Derek - N4DBM/WRMD298

 

[mmckenna]

Photocells are so 1989….

GPS Timers Home

gpslightlock.com

But, good info. You never know where the source is until you start isolating things.

 

[AK9R]

Photocells may be out of date, but they are cheap and fairly reliable.

I know repeater VHF operators around Indianapolis who were really scratching their heads when the tower owner installed LED lighting on the tower. The interference really disrupted the repeater.

 

[mmckenna]

Photocells may be out of date, but they are cheap and fairly reliable.

They are.
However, I was doing a parking light changeout for the church and the location is in heavy forest, so photocells don't work well. Timers were a problem since power was unstable.
The GPSLightLock units were about $70 each and were a direct replacement for the photocells.
The system had been set up with half the lights on a photocell only, and the other half a photocell and a timer (on at sundown, off a midnight).
I ordered two GPS units:
1 was local sundown to local sunup
other was local sundown to 12 midnight.
Nice thing about GPS is it knows when local sundown is, it knows when local midnight is, and it knows when local sunup is, even if it is under the trees. If the power goes out, the reset themselves automatically. And no more adjusting the timer for daylight saving time.

Between that and swapping out for LEDs, I paid off the entire investment in less than two years, and they save almost $1000 a year in electric costs.

But, off topic….

 

[n4dbm]

I have a DB201 ground plane at a site that has been fully restored and cut for 52.5 MHz. It's out the top of the tower, on a candelabra, but just a few feet away from the beacon. This is a Flash Technologies system with a high voltage tube. It makes plenty of noise on 6M during the day, but is even more hideous at night. At night time, the system switches in series resistors with the beacon, and pulses the flash tube at 20 cycles per second to simulate a "soft" night-time on/off bulb. The system is scheduled to be replaced this year with a Drake Lighting LED outfit. All of the LED drivers are inside the building with shielded cable going up the tower. I hope this cure the noise, as I would really like to try a 6M repeater at this site, but until the noise is gone, it's useless.