[App_rpt-users] GE MASTR II Over Modulation?

Bob Pyke k6ecm1 at gmail.com
Fri Oct 13 08:17:22 EDT 2017


Thanks David, See responses interspersed below.

Sent from iPad




Sent from iPad

> On Oct 13, 2017, at 4:05 AM, David McGough <kb4fxc at inttek.net> wrote:
> 
> 
> Bob,
> 
> I caught up on this thread, this morning. Is the Mastr II VHF HI or UHF?  
> If VHF HIGH, do you know if it is a PLL-type direct FM exciter or the
> earlier phasing-type, crystal multiplier variety?
Local repeater is on 147.000, and the distant repeater is on 146.760. It's been awhile since I looked at the schematic, but I believe the latter. We have a Yeasu repeater (DR-1X on analog) offered to us, but I need to find notes on interfacing to Allstar.

> Also, what type of sound device are you using for interfacing the RPi--a 
> URI, RIM or homebrew board?
URIx

> And, what type of Yaesu radio?  What types of antennas?? And, any 
> differences in vertical height between the antennas?
FT-8900. Antennas are pretty much at co-elevation, about 150 ft apart, both setup as omni-directional (147.000 stacked dipoles, ~9 dbi, Yeasu 5/8 vertical). I just assembled a yagi for the Yeasu, which would point away from the 147.000, but I don't believe it will have enough rejection on the back side.

> When you're hearing the Mastr II 200+ khz from the TX frequency, is it 
> only at certain, distinct spots or completely broadband?
I've not checked that. When I use my SDR receiver, the 147.000 signal really blooms, which I'd expect even if properly modulated, and being in too high a field strength.

This all came about when I began looking for a quick solution to chicken link the two repeaters to see if folks would want them linked... inexpensive, and easy to do for a short term experiment (Ha!). I'm now expanding my considered solutions; one, a remote repeater already on Allstar owned by a friend, that I help maintain, and the other repeater is rf linked to the 146.760, which is reported to have a microwave link (Ubiquiti) that I may be able to link in an RPi Allstar node on-site (that will take some discussion with the owner). Allstar provides us with the ability to easily disconnect the two systems when necessary, and on a regular basis, for separate events. We (both repeater owner groups) are working to get coverage over a larger mountainous region.

Thanks,
Bob
> 
> 73, David KB4FXC
> 
> 
>> On Thu, 12 Oct 2017, Bob Pyke wrote:
>> 
>> Understand. Good info. Learning a lot here from all.
>> 
>> Thanks,
>> Bob
>> 
>> Sent from my iPhone
>> 
>>> On Oct 12, 2017, at 6:16 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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