[SMCARA] Repeater

[Rob Hoyt]

 Very nice email Tom, excellent explanation! Thanks for the work on the repeater, it sounded great during the net this week. My signal may be a little rough, I live North of the Hollywood fire department and have a hill in my way to the South. I use a 4 element yagi on a short mast on my patio and feed it with 65 watts, I could work on getting the antenna higher. 
73, Rob - N2OMC 
  
    On Thursday, July 18, 2019, 12:12:57 PM EDT, Clarke, Frederic <Tom.Clarke at us.kbr.com> wrote:  
 
 Hi Folks,

Last Tuesday during the Net, I was at the repeater site to see if I could sort out some of the anomalies we have seen lately. My observations are, in no order:

1.  The repeater ran a solid 40 watts output for the entire net.  This was measured using a Bird in-line wattmeter. After 20 minutes of continuous net operation, the repeater was barely warm. The equipment fans and hut air conditioning were working! So the Tx is OK.
2.  No one spoke long enough to time-out the machine!  I plan on reprogramming the timer to 10 minutes to keep it from timing out during the net. The 3 minute "alligator catcher" will be enabled again after the net.
3.  Most stations were solid enough into the repeater to keep the CTCSS active and the repeater up. This is based on observing the controller status lights.  The tone decoder is sensitive enough that it will bring up the repeater, but there may not be enough recovered voice audio to understand the transmission. We had one station that was so weak and audio so low, that he was not copiable. Desense might have been the problem here also.
4.  The operators using handheld radios in the Lex Park area were all full quieting and easily copied.  The repeater is designed to give handheld coverage in the greater LP area and 25 watt mobile/base stations out to 25 miles.
5.  There was an active thunderstorm in the Ridge area, about 3 miles south of the site, which caused some "static" that could be heard on the repeater output. Sitting directly under a 150 foot " lightning rod", didn't seem to attract any lightning bolts!
6.  Some stations on the fringe were slightly noisy, but mostly readable. This is expected.
7.  I did hear what might be desense* on KB1YZ (on the fringe) and N2OMC.  This will take further investigation.

So, I concluded that the repeater is working as designed, but a little "TLC" might be in order to check the desense.

A couple of things to keep in mind regarding the repeater:

1.  Using a handheld in your basement or outside of the LP area is asking a lot from the machine!  You may have to walk around or go upstairs to find the "sweet spot" to get a good signal into the machine.
2.  Using a base station with a good outside antenna with good coax, ( i.e., RG-213 or LMR 400) is the secret to success. Higher is better, when it comes to antennas.
3.  Avoid using non stock microphones or having a scanner going while you are transmitting.  The 750 millisecond time delay in the controller will drive you slightly crazy!

Have fun, and don't hesitate t ask questions!

* Desense. Those of you who understand what I am talking about may go get a cup of coffee!  For the others, here is an explanation: One of the bits of magic that makes a repeater work while having a transmitter and receiver on simultaneously, is the Duplexer". The duplexer uses tuned cavities (Hi Q tuned circuits) to keep the transmitted signal out of the receiver. It does this by notching out the transmitted signal in the receiver path and by notching out the receive frequency in the transmitter path. There is also a band pass-band reject type of duplexer available, but ours is the notching type. This device combined with good shielding of the Rx and Tx permits us to use one antenna for both, without the transmitter overloading the receiver. 

Now, on to what is desense.  When a little bit of the Tx gets into the repeater receiver, it tends to drive down the sensitivity of the receiver, making weak signals weaker. Desense often presents as a noise or crackling on weak signals.  Sometimes the duplexer will change tuning slightly and that will cause problems.  Our duplexer has been spot on for 30 years, so we are probably OK there. It was factory tuned and is also temperature compensated, Another cause of desense can be from the cables running between the repeater and the duplexer and antenna. Often corrosion gets into the connectors and will cause an RF leak.  Occasional cleaning of the connectors, applying a little DeOxit, and tightening up the connector will fix it. Our connectors are PL-259s, the so called "UHF" connectors, rather than the preferred type "N" connectors, so we need a bit more attention to keeping them clean and tight.  If you want to get really geeky, monitoring the limiter current on the receiver will help spot desense.

Here endeth the lesson!
73 de Tom/W4OKW


Tom Clarke
Triton NATOPS Support  | Engineering Business Unit
KBR  |  Government Solutions – US 
22309 Exploration Drive | Lexington Park, MD 20653 | USA
Office: 301-863-4418  |  Mobile: 301-904-2053 
tom.clarke at US.KBR.com [Pri] | frederic.t.clarke.ctr at navy.mil 
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