[App_rpt-users] GE MASTR II Over Modulation?

[Jesse Lloyd]

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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