[MIham] article

[Frank]

D-STAR Repeaters Can Cause Interference Too!

By Ray Abraczinskas, W8HVG



Digital communications activity utilizing ham radio repeaters keeps 
expanding. The lure of the many capabilities in connecting repeaters through 
the Internet keeps growing. Modes such as Echo Link?, IRLP?, VOIP, Wires?, 
and D-STAR, etc. are all present day technology means of connecting radios 
and repeaters to the Internet allowing talking with hams all over the world. 
Some people say that it isn't really ham radio, but truly it is, and it will 
continue to grow and affect every amateur radio operator's future both for 
the good and bad.




How are all these advanced technology digital voice modes being implemented 
and provided for? Successes are being touted all over the country. Hams are 
jumping on the digital bandwagon everywhere. What are they learning? There 
are ham nets growing everywhere on the Internet but what if the Internet 
capability goes down? What real purpose do nets on the Internet serve? Will 
the only ham repeater stations operating in the future be reflectors and 
gateways? Cell phones are everywhere and affecting ham radio utility. Ebay? 
and the price of gas are affecting hamfest attendance and if people reduce 
their driving habits, more than likely the role of repeaters will become 
affected also. Will we still need repeaters, repeater networks, and link 
repeater systems? Commercial FM broadcast stations are discovering 
"interference effects" from digital transmissions in-between the analog FM 
channels. Digital TV is coming in February 2009 with its associated 
weather-related antenna reception problems (pixel fadeouts) that will raise 
ire while watching TV. As hams are we all heading for that inevitable 
question someday (God forbid), "Do we need ham radio at all?" Will it too 
eventually follow the way of the 8-track and VHS cassettes?




Most every ham has some knowledge of Icom? and D-STAR. Simply, it's a 
versatile digital communications mode for voice and data. It can connect 
D-STAR users to other D-STAR radios all over the world through the Internet. 
It's growing everywhere but not all hams are jumping into the fray for 
various reasons, i.e., knowledge, compatibility, expense, naivety, and band 
plan constraints.




The Michigan Area Repeater Council (MARC) started addressing making 
provisions for D-STAR repeater frequency coordination in latter 2006. It was 
a challenge because most 2M-repeater frequencies in the lower-Michigan 20 
kHz band plan are fully assigned with analog repeaters. As a provisional 
means, the MARC created several channels in the 145 and 147 MHz bands to 
allow 2M D-STAR repeaters to exist in-between the coordinated analog 
repeaters frequencies. The resultant 10 kHz spacing necessitated focusing on 
distance separation for adjacent channel repeaters. Initially it was 20 
miles and after further deliberations in June 2007, it was made 50 miles 
(without any reference to TX power levels). However, due to variability's, 
further tests and observations may be required to adjust the 50-mile 
adjacent channel repeater separation factor even further and also consider 
including limits on D-STAR transmitter output power. As is, the burden falls 
mostly on all repeater owner/trustees (involved) to show that the newly 
assigned D-STAR repeater will be compatible. Realistically, that may take 
(?) years to prove because of all the variability's involved, e.g. equipment 
differences, repeater profiles, propagation variability (10 to 25 db band 
enhancements), D-STAR transmitter power levels, terrain, observation 
methods, and observation times, etc.




While D-STAR sellers and users tout a narrow operating bandwidth, supposedly 
allowing more operating channels, the fact remains that D-STAR repeaters 
must fit into the existing repeater spectrum band plan, which differs from 
state to state. In lower-Michigan in early 1984, by repeater owner/trustee 
agreement through the Michigan Repeater Council then, the 2M-repeater band 
plan was made 20 kHz, which took over four years to fully implement 
including "flipping" inputs and outputs in the 147 MHz band to conform to 
the new ARRL band plan. All the surrounding states including Michigan's 
Upper Peninsula and Ontario, Canada, opted or stayed on a 15 kHz band plan 
for their 2M-repeaters giving them 15 more repeater frequencies but 
complicating (somewhat) the coordinating process (then).




Another factor sometimes ignored, is that all ham radios commercially 
manufactured and sold (for ham repeater use) have different receive 
selectivity factors among other things including FM discriminator response. 
This becomes an important factor in rejecting adjacent channel interference 
5 or 10 kHz away. It's called "capture effect" response. That is, in 
receiving an FM signal using a discriminator circuit, it will generally lock 
to the strongest signal in its capture range. It can even capture on strong 
signals 10 kHz away and hence, with a strong D-STAR carrier, it would swamp 
out the desired signal and sound just like noise. That is called 
interference at the user receiver, and even though some people suggest it, 
using a tone encoded squelch will not solve the problem. Only by reducing 
receive signal levels or by controlling transmit power and distance 
separation will the problem be solved. With a 10 kHz band plan, this becomes 
a huge burden for everyone involved. NOTE: Some coordinating bodies used to 
consider 5 kHz and 10 kHz spaced channels as "co-channel" in their 
coordinating processes.




Some D-STAR experts are suggesting the user needs a more selective radio, 
but why did Michigan go on to the 20 kHz band plan in the first place? 
Answer: to easily manage adjacent channel interference problems both 
internally and with the surrounding states and Canada. It was based then on 
a typical receiver bandwidth spec of 13 kHz at 6 db and 30 kHz at 100 db. 
Placement of D-STAR repeaters in a 10 kHz channel spacing with analog 
repeaters using those receivers may allow the D-STAR repeater to work OK but 
it will probably cause heart burn for many repeater owners and their users 
(and the frequency coordinators). It can and will also raise the noise floor 
in many cases which affects overall analog repeater sensitivity.




Besides the Michigan peninsula VHF band enhancement phenomena typically 
occurring 8-months out of the year affecting the problem, there are other 
factors that can affect the described situation. One is called third order 
intermod resulting in interference that, usually, most hams are neither 
concerned about nor capable of checking for.




Let's describe a typical situation involving an analog repeater transmitter 
and a D-STAR repeater transmitting (10 kHz away) with a user in a mobile (or 
a base station) located between the two repeaters running reasonably high 
power and transmitting into either repeater. There are potential intermod 
frequency products that can be created in either the mobile transmitter 
(because it has no rejection filtering), which then could interfere with one 
of the repeaters, or there could be intermod products created in either 
repeater transmitter because the repeater duplexer cannot reject it from 
occurring (10 kHz away). Also, most repeater owners do not use isolators or 
even check for intermod effects. This problem then becomes a hellacious 
burden on the repeater owner to track down and eliminate, all because 
adjacent channel repeaters are placed too close in frequency and too close 
in distance relative to their users.




In July 2008 in southeastern Michigan, an enthusiastic D-STAR systop placed 
his new D-STAR repeater in operation supposedly on a frequency that was NOT 
recommended by the Michigan Area Repeater Council. Immediately, several hams 
using long time coordinated analog repeaters 10 kHz away, up to 45 miles 
around the area began noticing "cutouts" in reception blaming it on the 
analog repeater they were listening to. Even when some mobiles were 
operating close to the analog repeater, their mobile receiver was completely 
blanked by the strong D-STAR signal any time it was up.  It was very obvious 
that there was something out side the analog system affecting it. The 
involved analog repeater(s) trustees were informed of the problem with one 
being a Michigan statewide linked repeater system trustee and the other a 
knowledgeable former MARC Board member. Hours were then spent analyzing what 
was happening with frustrations building because nothing in either repeater 
system appeared to be wrong. Suggestions were made to those users observing 
the cutouts to check and see if a D-STAR repeater was involved, as it was 
known that the MARC had assigned "splinter channels" for D-STAR.




Bingo! A new D-STAR repeater was found operating on a frequency 10 kHz away 
from the analog repeaters (that the MARC supposedly did not assign). 
Apparently, the enthusiastic D-STAR systop jumped the gun! After several 
communications between all involved including the MARC, the systop turned 
the interfering D-STAR repeater off and stated that he will be moving it to 
a newly assigned frequency. He also expounded that the problem is not the 
DSTAR repeater, it's really the users FM receivers; they should use a more 
selective receiver! Like my 8-year old granddaughter frequently says these 
days, "Duh"!




In June 2008, there were four D-STAR 2M-repeaters assigned "splinter" 
frequencies listed in the MARC on-line directory with eight more assigned 
waiting to be added. The WX8GRR D-STAR repeater on 147.29 MHz south of Grand 
Rapids is not listed yet. D-STAR is an exciting mode and systops are saying 
it's much better than analog FM in many ways. Some say it takes much less 
transmit power with D-STAR to communicate as far as analog. Therefore it 
would make sense for the MARC to consider limiting the D-STAR repeaters 
power output in these provisional assignments (say five to ten watts 
maximum) for splinter frequency compatibility. D-STAR itself is not the 
problem; the MARC provisional "splinter" implementation plan may need 
refining.




This recent wide-area observation of D-STAR interference is probably one 
example of the potential problems yet to come facing the Michigan Area 
repeater Council and Michigan repeater owner/trustees. But then how many 
hams will know why their analog repeater reception is occasionally cutting 
out? Is it happening elsewhere now? Could it happen during an emergency 
communication? Who will investigate it, prove it, and then correct it? With 
the technologies involving digital communications apparently requiring 
placing more transmitters into the repeater bands, the Michigan Area 
Repeater Council will need everyone's support, cooperation, and patience in 
their volunteer efforts.




Maybe it's time to look at and consider "other approaches" to implement the 
2M D-STAR repeaters in Michigan; there are probably several. One is to look 
at going back to the 15 kHz band plan possibly gaining up to 15 more 
frequencies that could contain D-STAR repeaters. There would still be some 
complexities with the coordination process, but possibly not as onerous and 
burdensome as the "10 kHz provisional plan." It was done once but it took 
over four years.




Another approach would be to survey and promote repeater clubs and trustees 
as to their "spirit and willingness in cooperating" to implement a D-STAR 
repeater in place of their existing coordinated analog repeater on the 
existing 20 kHz band plan (or a revised 15 kHz band plan). After all, there 
are many individuals, clubs and cities with more than one analog repeater 
that virtually covers the same area. It would seem feasible then that one 
(or more) analog repeater(s) could be sacrificed in these areas for D-STAR 
implementation!!! With either plan, coordination of 2M repeaters in Michigan 
would still be under existing proven coordination standards, not an 
experimental observation provision subject to long term effects and changes.




A more ambitious plan might be to investigate the possibility of integrating 
an analog and a D-STAR repeater operating together on the same frequency at 
the same site in an "either / or fashion". It would seem technically 
feasible albeit there might be occasional "busy lockouts" to some users 
while either repeater is in-use. However, that's what ham radio is about, 
making things work and getting along with your fellow hams!




It should be clarified that the Michigan lower-Peninsula 2-meter 20 kHz band 
plan only differs from the surrounding states and Ontario plans in the 146 
and 147 MHz bands. That's where the extra 15 frequencies would come from if 
it were changed. The 145 MHz band is a 20 kHz band plan in all states around 
Michigan (including lower-Michigan).




The purpose of this article is to report an occurrence of typical repeater 
interference, which many hams may not know about or understand, and which 
was the fundamental reason why lower-Michigan operators selected the 20 kHz 
band plan in 1984. This information is also meant to create conversation 
among repeater owners and trustees in assisting the Michigan Area Repeater 
Council to accomplish their goals. Thanks for all their volunteer hard work. 
Comments are welcome and if I can be of further assistance to anyone please 
contact me at abra at i2k.com, or send them directly to the MARC at: 
mail at miarc.com.

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