[NLRS] 915 MHz ISM Band and Digital Modes

[Doug Reed]

Jerry's comments are right on. I believe most of the interference is
from frequency hopping spread spectrum Part 15 equipment. Most vendors
choose FHSS because the rules let them use 1 watt RF power. I don't
know how often you'll find Direct Sequence spread spectrum because
there is ALWAYS a signal somewhere in the band. And single frequency
Part 15 devices are seldom found because they must reduce RF output
depending on duty cycle.

With all the hash in the band, the only usable weak signal spectrum is
at the lower end of the band. 902.100 is well placed to avoid the hash
because the manufacturers don't want to get too close to the band edge
since none of their equipment has a high accuracy oscillator..... FM
repeaters are starting to use the lower end of the band for repeater
inputs for the same reason. Hopefully most repeater coordinators are
aware that 902.100 is weak signal and they should *not* coordinate
repeaters +- 25KHz from there. OTOH, given the lack of FM repeater
activity in the 902 band in and around Minnesota, it may never be a
problem. But if you are near Chicago you may have trouble....

The upper end of the band is where most of the pagers are, mostly 928
to 930+ with voice applications above that.

Ubiquiti (ubnt.com) does make 900MHz WiFi equipment. They use DSS
style dishes for the directional radios. Or if you want to play
around, you can probably still find bits of the old Metricom Ricochet
system laying around and on Ebay. The Ricochet modems were basically a
64Kbaud FHSS 1 watt data modem. The nifty thing is they used
Hayes-style modem commands and you could have one unit "dial" another
unit within range using just the ID code on the bottom. The TwinsLAN
group was looking at using some of the Ricochet equipment for data
links, but one of the first sites had a 400 watt 928MHz pager on the
same roof...... There used to be a lot of info online, but I'm not
sure how much still exists....

73, Doug Reed, N0NAS.



On 7/11/16, Russ Ramirez <russ.ramirez at gmail.com> wrote:
>
>
> Thanks Jerry. As I do not have a 'real' antenna up yet for 33 cm, I am not
> very familiar with the congestion or QRM in general yet, but that will be a
> good step 1 once I do and can get a look at/feel for the spectrum.
>
> 73, Russ, K0WFS
>
> On Mon, Jul 11, 2016 at 11:17 AM, Dr. Gerald N. Johnson
> <geraldj at netins.net>
> wrote:
>
>>
>>
>> I believe there are lots of signals across the 902 MHz band. We
>> concentrate mostly at 902.1 for repeater inputs and narrow band because
>> the
>> broad band users have to avoid the band edges, but fill the rest solidly
>> and some are not low power, like pagers. I was consulted years ago about
>> a
>> bank 915 link between a branch and the main office in Boone Iowa that was
>> messed up by a pager located between the two locations. While good
>> directional gain antenna can help the desired received signal strength it
>> can also increase the undesired if that undesired is in the same
>> direction.
>> 915 with power and antenna gain should work but there will be lots of QRM
>> from the unlicensed equipment spread all over the territory.
>>
>> 73, Jerry, K0CQ
>>
>>
>> On 7/11/2016 10:48 AM, Russ Ramirez wrote:
>>
>>>
>>>
>>> Since David Palm asked a 902 MHz related question, I thought I would ask
>>> a
>>> another question about 33 cm.
>>>
>>> Are any of you doing any work with digital messaging or beacons up at
>>> 915
>>> MHz?
>>>
>>> Since as amateurs we are able to use the ISM band in ways that
>>> unlicensed
>>> makers cannot, this has been an area of recent interest for me as a
>>> number
>>> of microcontroller designs utilizing the RFM69HCW -
>>> http://www.hoperf.com/rf_transceiver/modules/RFM69HCW.html have appeared
>>> lately.
>>>
>>> For general wireless communication between microcontrollers, 915 MHz
>>> works
>>> far better than 2.4 GHz WiFi, BLE, or ZigBee do around your home/office,
>>> and 915 devices are much easier on battery/solar power.
>>>
>>> For the amateur who might already have 33 cm loopers and amplifiers, the
>>> use of these small devices opens up some interesting possibilities for
>>> very
>>> little investment.
>>>
>>> Thoughts?
>>>
>>> Russ, K0WFS
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