D-STAR Repeaters Can Cause Interference Too

This is quite a well thought out treatise on potential interference problems between digital D-STAR repeaters and analog repeaters. It was written.by Ray “Abe” Abraczinskas, W8HVG. Abe is one of the founders and a Director of the Independent Repeater Association.

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 sysops 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@i2k.com, or send them directly to the MARC at: mail@miarc.com.

SOURCE: Ray “Abe” Abraczinskas, W8HVG