2018-08-09

8/8 Update

It's the 30th anniversary of the first night game in the Friendly Confines of Wrigley Field in Chicago.  The game was called due to rain and was replayed on 8/9.  Many remember the "8/8/88" thing but forget that it was a rain-out.

The SCARS Tech Committee was almost rained out, or more accurately, lightning-ed out on their trip to the repeater site today.  However, the electrically charged clouds moved out and even the rain clouds were largely gone before the task force arrived at the site.

Unfortunately, the changes we made a couple months ago had no effect on the interference we've been suffering on the VHF system (147.06 MHz).  So today, the loaner duplexers (thanks to N5MS) were removed and replaced by the club's WACOM duplexer.  While there, the NORMAN APRS digipeater was also placed back in service.

We had also removed the Arcom RC-210 repeater controller (the device that gives us the auxiliary functions) as part of the diagnostic process.  While it was out, we had it updated by the vendor with their now-standard more capable CPU.  Embedded CPUs in this type of situation have built-in RAM and the new unit has a lot more of that (I don't have numbers) meaning more possible functions - the software author had run out of space in the old CPU's memory.  The real-time clock module was also "flashed" (new firmware) which hopefully means that the clock (and therefore the scheduling features) will work correctly now - the clock was a poor time-keeper in the past which is why it had been shut off.

Unfortunately, we were unable to communicate with the controller over its serial console which means we couldn't pull the config that's in there (to make sure it was correct and as we expected) nor could we properly set the clock - you'll probably note that we've moved to the Pacific Time Zone.  We'll get back out there in the near future and see if we can't beat it into submission.

In the meantime, the interference is still a problem.

2018-05-03

Still Fighting ...

But is it a good fight?  Ahhh ...

The fight with VHF repeater continues.  On April 11, 2018,  the RC-210 controller was removed from the repeater (that's why there is no courtesy tone).  There are actually two auxiliary receivers connected to that controller and while it was unlikely that this was injecting the interfering signal into the system, it was easy enough to remove it to test.  Removing the controller was not expected to magically fix everything, and it didn't, but it did give us some more data - in the interval between a mobile unit unkeying and the repeater's transmitter dropping, the "other" signal was apparent.  That's the good news.  The bad news is that the signal is probably the repeater's transmitter on 147.0600 MHz.

The duplexer (not diplexer) is the filter that is supposed to keep the repeater's receiver from hearing its own transmitter.  This is no small trick given that there is a 25-W signal on 147.0600 MHz and the repeater is trying to listen for signals 600 kHz away (147.6600 MHz) that are in the microvolt range ... at the same time ... on the same feedline and antenna.

Miraculously (or maybe not if you know N5MS), Mike had a duplexer "in stock".  The unit was tuned up on "66/06" (that's how hams 40 years ago described a repeater - input/output) and placed in service today by N5HZR and N5UWY.  If this "cures" the problem then the next steps are pretty clear.  Note that in the repeater rehabilitation work that the Technical Committee has done over the past several years, the duplexer was the only component not replaced.

Unfortunately, in order to install the temporary duplexer, the NORMAN digipeater (on the APRS frequency of 144.3900 MHz) was removed from service.  The digipeater shares the VHF repeater's antenna and feedline by use of a multi-coupler (similar to a duplexer - a tight filter that keeps 147.0600 MHz RF away from 144.3900 MHz).  This is unfortunate for a couple reasons - 1) APRS users are deprived of a high-site digipeater and 2) it's possible that the digipeater itself was an issue (it's not at all clear that it was).

Time will tell if this is the "cure."

2018-01-27

Repeater Site Expedition Report - 2018-01-27

A number of us visited the county yard on Friday the 26th (N5UWY, KD5UGO, N5HZR, W5HLG, WE5Z, WB5ULK, AG5DB, AG5LB, KB5LSB).  We looked only at the VHF system and not the UHF system.

A couple of numbers for posterity:

  • Squelch opens at 0.2 μV.
  • Full quieting is reached at 1.0 μV
  • Modulation  bandwidth is 9 kHz
  • Deviation of repeated stations is 3.5-4 kHz
  • Transmitter frequency error +30 Hz
  • Power output is 20 W
  • 70 dB reject on transmit.

KD5UGO provided several tons of test equipment and knowledge to operate it.  We looked at the parameters quantified above.

We listened to the input frequency through the duplexers as well as straight from the antenna and we were looking at it on the spectrum analyzer at the same time; No unusual signals were heard or seen, only the signal of the HT (or the service monitor) we were using to key the repeater. 

We "swept" the Bp (Tx) and Br (Rx) sides of the duplexer and found no anomalies.  Yes, we wiggled wires and tapped on connectors while watching signals, none of which had any effect.  We checked their tuning and were able to coerce another 2 dB of rejection from the receive side (I do not have the initial or resulting values, only the 2 dB difference).  We saw no evidence that the transmitter on 147.06 was being heard by the receiver on 147.66 via the antenna system.

Bottom line:  No Trouble Found.

Clearly, there is a signal from somewhere mixing with the signal being received from stations trying to use the repeater.  None of the tests we did yesterday revealed the source of that signal. In fact, we saw no evidence of another signal. But it's there because we heard it on repeated stations within an hour of our visit.  It's real, we just have yet to find it.