[App_rpt-users] GE MASTR II Over Modulation?

Bob Pyke k6ecm1 at gmail.com
Thu Oct 12 23:26:22 EDT 2017


Makes sense. I'll try using the receiver at varying distances from the MASTR II.

Thanks,
Bob
k6ecm

Sent from my iPhone

> On Oct 12, 2017, at 7:39 PM, Mike - W5JR <w5jr.lists at gmail.com> wrote:
> 
> I'd be surprised that your Yaesu can deal with that strong a signal within the passband of its receiver front end. At best, it is 4 MHz wide, and probably no more that 6 dB down at two to three times that. 
> 
> A test to try is have the nearby repeater key up, the open the squelch on the Yaesu. My guess is the S-meter will show a "signal", possibly near full scale. I also expect the open squelch noise level to drop. This test is done without the distant repeater keyed. 
> 
> There will be zero amount of filtering applied to the nearby repeater transmitter that affects the issue because the problem is the RF coming into the Yaesu itself on the repeater's actual frequency. You can put a (several) bandpass cavity in front of the Yaesu, but then you wouldn't be able to transmit to the distant repeater as the bandpass cavity would significantly attenuate your transmitter. 
> 
> You could also install a notch filter in front of the Yaesu tuned to notch the local repeater frequency. A single notch cavity may not be enough. This will allow you to transmit to the distant repeater. However, the notch from a single cavity is likely to be wide enough to reduce the sensitivity some of the Yaesu at 200 KHz. 
> 
> My analogy to folks is that it's like shining a bright flashlight in one eye while trying to see a candle at one mile. Pretty impossible to overcome the overload. 
> 
> tnx
> Mike / W5JR
> Alpharetta GA
> 
> sent via my HP95LX 
> 
> 
>> On Oct 12, 2017, at 8:16 PM, Jesse Lloyd <ve7lyd at gmail.com> wrote:
>> 
>> Ya so if you put a bandpass filter on it, set to 3.2 dB insertion loss you'll get about 22 dB of reject 250 Khz off.  If you use two cans set each to 1.1 dB insertion loss each (totaling 2.2 dB) you'll see about 27 dB of reject. You could also put a notch filter on it for that one receive frequency, but you should check first to make sure you aren't getting into other gear at the site (if there is any). At 250 KHz I don't think you'd be able to take a duplexer notch / pass can and tune it to get any significant rejection. Sinclair says the Q201 (6 can duplexer) min freq spacing is 300 KHz. You'd might be able to get 30 dB out of 3 notch/pass cans series'd together, not sure.
>> 
>> To get an additional 30 dB of isolation (totaling 70 dB)  between the antennas you'll need to space them 3000 ft apart. http://www.repeater-builder.com/antenna/images/horizsep.jpg
>> Vertical separation is better, you need about 100 ft to get 70 dB, maybe a little more. http://www.repeater-builder.com/antenna/images/vertsep.jpg
>> 
>> 
>> +49 dBm (79W) less 2 dB of duplexer loss, less 2.5 for cable loss plus 3 dB for repeater antenna, less 70 dB of isolation plus 3 dB for rx antenna gives about -19.5 dBm.  This probably still isn't enough, you'll probably still see some desense, but might be good enough to work, it'll just make it a little deaf.
>> 
>> Really you need to get it further then 240 Khz off freq. then you can get away with only one or two extra cans.
>> 
>> Jesse
>> 
>> 
>> 
>> 
>> 
>>> On Thu, Oct 12, 2017 at 5:35 PM, Bob Pyke <k6ecm1 at gmail.com> wrote:
>>> I'm guessing 75-100 watts on the repeater PA. I can hear intelligible audio (sounds like it is in the bottom of a well) when listening to the other repeater, 240 kHz separation, on my Yeasu. I'm sure the modulation is correct for GE, established with the Allstar setup, but can check with my service monitor. I looked at it on my SDR receiver, and it looks like a lot of rf drive... perhaps just too close at about 150 ft.
>>> 
>>> Thanks,
>>> Bob
>>> k6ecm
>>> 73
>>> Sent from my iPhone
>>> 
>>>> On Oct 12, 2017, at 1:40 PM, Jesse Lloyd <ve7lyd at gmail.com> wrote:
>>>> 
>>>> Bleeds over as in desensitizes the receivers or you can actually hear audio?
>>>> 
>>>> Most modern receiver front-ends have minimal filtering, just a wide pass-band filter and the IF filters which reject the adjacent channels more then channels further off frequency.  If you have more than -30 dBm showing up on any antenna regardless of frequency spacing it'll probably cause problems.  150 ft is only ~45 dB of isolation, so if you're transmitting with +45 dBm (32W) less 2 dB of duplexer loss, less 2.5 for cable loss plus 3 dB for repeater antenna, less 45 dB of isolation plus 3 dB for rx antenna gives about +1.5 dBm which is definitely going to mess up a receiver unless more filtering is used.  You can play with filters to see what works, but I suspect you'll need two notch filters to get that down to something that doesn't effect your receiver.
>>>> 
>>>> You need to hook the system to a service analyzer and see if it's actually over modulating, it's doubtful unless you've injected audio after any limiter circuits. Running the math it's probably front-end overload, you'd be best to notch out the transmitter on the effected receivers, you may have to put a pass filter (and/or notch filter) on the transmitter too to remove some sideband noise as well, maybe not if it's a clean transmitter. It's usually a good idea to have balanced reject filtering on both your transmitter and receiver.
>>>> 
>>>> Jesse
>>>> 
>>>>  
>>>> 
>>>>> On Thu, Oct 12, 2017 at 1:13 PM, Bob Pyke <k6ecm1 at gmail.com> wrote:
>>>>> Our RPi controlled radio has an rf spectrum so wide that it bleeds over in my Yeasu (150 feet between antennas) when I'm working a distant repeater that is more that 200 kHz away from our tx freq. Any suggestions where I can look for info on how to deal with this? Our duplexer is the notch type.
>>>>> 
>>>>> Thanks,
>>>>> Bob
>>>>> k6ecm
>>>>> 73
>>>>> Sent from my iPhone
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