This page relates our experiences with installing a D-Star repeater at a very busy site, the problems encountered, and their resolutions.

Figure 1:
A partial view of the Farnsworth Peak complex.  The mast near the right of the picture - the one with the odd-looking "roto-tiller" antenna about 1/3 of the way down -  is the one containing the D-Star repeater antennas.  One of the D-Star antennas is next to this "roto-tiller" (a standby FM broadcast) antenna and the others are at/near the top.  These antennas - along with everything else on this site - are in a sea of RF energy at frequencies from VHF through microwave!
Click on the picture for a larger version.

Getting ready to install the D-Star stack:

The purpose of this was twofold:

• To verify that the antenna/cabling/duplexer itself were working properly and that the coverage was as-expected.
• To analyze site noise.
  

This latter point is very important because, as you might realize, trying to use D-Star gear - whether it be as a user or repeater - is a disaster when trying to diagnose link problems. Without any built-in tools (ones that could have easily been built into the software/firmware) diagnosing a problem of any sort is a significant challenge, typically requiring that the operator(s) switch to analog in an effort to "hear" what might be going wrong with the path - problems that could include interfering signal, multipath, or simply a weak signal. The repeater gear is arguably more difficult to use when diagnosing problems as, unlike the portable gear, one can't simply switch to an analog mode to "hear" what's going on!

What we learned was that the site noise raised the effective noise floor of the receiver by a mere 10-15dB.  This type of measurement may have been possible to ascertain with the D-Star gear, but measurement of this parameter is comparatively trivial when done using FM analog gear!  This amount of site noise might sound terrible, but in actual practice one can still achieve very good coverage considering that this site is at the top of a 5000-foot tower made of rock, so HT-power coverage from 80 miles away is still quite practical - if you are using a good mobile antenna.

Installing the D-Star stack - and a problem:

After several months of operation, the time and equipment became available and the D-Star stack was gradually installed in September-November 2009.

The VHF repeater didn't work.

Fortunately, John, K7JL, who was doing the install had the foresight to install RSSI and Discriminator test jacks on the VHF, UHF and 23cm gear. ANYONE who plans to put up a D-Star repeater on ANY band is well-advised to make these modifications and complain to Icom about their having been left off the gear in the first place!

These modifications are documented by N5UD at this link:

http://www.dvara.org/icom/DstarRptMods_NU5D.pdf     An alternate link may be found here.

In testing with an Iso-Tee, the service monitor being used (an IFR 1200 without the booster amp) could not output enough signal to register ANY quieting on the discriminator output, indicating an effective desense of at least 40dB.  Thinking that something was wrong, the cables were moved back to the Kenwood TK-740 and everything measured out fine.  When the D-Star repeater was connected directly to the service monitor to check sensitivity, it tested within specs.

Hmmm...

Figure 2:
John, K7JL, surrounded by an array of gear used while installing and testing the D-Star stack.  The bandpass cavities added to the 2-meter side are visible in the lower-right corner of the picture.
Click on the image for a larger version.

Well, we knew that the duplexer itself wasn't at fault owing to the fact that the Kenwood repeater seemed to work just fine, and even if the Icom was being overloaded by its own transmitter, that wouldn't explain the severity of the desense - especially since it was still deaf if the transmitter was disabled!

Fortunately, due to work on another amateur repeater project going on at the same time elsewhere on the site we had a 2-meter bandpass cavity (a 4" DB Products DB-4001) to try out, and that made all of the difference, reducing the desense from some immeasurably high value (at least with the service monitor's available output and the Iso-Tee's coupling) to something on the order of 20dB or so.  Because this cavity was in poor shape and had been pulled from service for rebuilding after having been used for 25 years or so - suffering heat, cold, moisture and lightning - it was quickly replaced on the next trip with a pair of 6" Sinclair bandpass cavities in series, each set for 1dB coupling.

It took BOTH of the 6" Sinclair cavities to reduce the desense of the Icom VHF repeater to the same level that the Kenwood analog experienced WITHOUT these two bandpass cavities - that is, down to the site's noise floor.

It should be noted that the Discriminator output - while a useful diagnostic tool - was only partly helpful in diagnosing and quantifying the problem:  An important clue came about by monitoring the RSSI output - which was noted to be nearly "pegged" when the repeater was on the duplexer alone, but the discriminator output just showed normal "noise".  Once the extra bandpass cavities were added the RSSI dropped to the 1.3 volt area with no signal, not too terribly far above the "no antenna" reading.  (Note that RSSI values vary from radio to radio and depend on the site noise.  Having said that, different radios should still be "sort of" similar in their responses.)

Something that you may not know about your duplexer:

One of the most important things to know is that most duplexers - even those marked or marketed as "Bandpass/Band-Reject" - are REALLY only Band-Reject once one moves very far away from the design (notch and pass) frequencies.  The duplexer used on the 2-meter D-Star repeater - a brand new, 4-cavity TX/RX brand "BP/BR" type costing >$2000 - was no exception!

If were to assume that "Bandpass/Band-Reject" automatically means that everything way off-frequency will be filtered out, you would be WRONG, WRONG, WRONG!  While it is true that SOME duplexers have honest-to-God bandpass responses - that is, a coupling loop on one side of the resonator and another loop on the other side - most do not!

The biggest clue to this was that putting a wattmeter on the RX coax (along with a dummy load where the receiver would have connected) showed 10's of milliwatts of RF coming down the pipe from the single-band Telewave VHF stick mounted at the 60 foot level, and this energy was the combined, intercepted power of the FM and TV broadcast transmitters on site.  Admittedly, it is a bit much to ask for any receiver to deal with, maybe,  1/10th of a watt of garbage coming down the coax - even if none of it is anywhere near 2 meters - but it was interesting to note that the lowly Kenwood TK-740 seemed to have no problem (and was not really helped by the addition of the bandpass cavities!) while the Icom was totally demolished by the same RF configuration.  One of the reasons for this is that portions of the Icom repeater itself essentially consists of modules from standard mobile radios:  As you are likely aware, many mobile radios are designed more for absolute sensitivity rather than strong-signal performance and, unfortunately, the Icom repeater's front end is no exception to this rule.

Actual responses of a typical duplexer:

For a graphic example of a typical response of a "Band-Pass/Band-Reject" duplexer, refer to the spectrum analyzer plots in Figure 3.  The duplexer being tested was a "6-can" Phelps-Dodge unit capable of over 95 dB of TX/RX isolation.  While this is not the same one as used at the repeater site, its response is very typical of such duplexers from different manufacturers - including the "4-can" TX/RX unit on-site. 

In each of these pictures, the plots are as follows:

• Yellow:  The amplitude response of a "6-cavity" (three "cans" per leg) "Band-Pass/Band-Reject" duplexer.
• Magenta:  The amplitude response of a single 4-inch bandpass cavity - the same DB4001 as mentioned above, now rebuilt - with the coupling set to 1.5dB.
• Cyan:  The amplitude response of the combination of the above duplexer and bandpass cavity in series.
  

"Close-in" response:

In the Top image of Figure 3 we see the response of various combinations in the range from 140 to 155 MHz.

If we look at the Yellow trace - the duplexer - we can clearly see the "high-side" notch.  Note that because of the setting of the resolution bandwidth on the analyzer, the true depth of the notch isn't visible (it was really more than 95dB) but the relationship between the two frequencies is very apparent, with the peak response (minimum loss) being at the "low-side" transmit frequency:  It is from this combination of deep notch and minimum loss at these two frequencies that the "Band-Pass/Band-Reject" designation arises.  It is our opinion, however, that this "Band-Pass/Band-Reject" designation is misleading - as we shall soon see!

Now, take a look at the Magenta (purple) trace.  This is the response of the single 4" bandpass cavity tuned to the "transmit" frequency.  As you can see, the insertion loss is minimal at center frequency and the loss increases as one moves away from that peak.  Its rejection at the "receive" frequency (where the notch is) is only on the order of 10dB -  not nearly enough to provide adequate TX/RX isolation of a typical system.

Finally, look at the Cyan (blue) trace.  This is the response of the "3-can" duplexer leg and the 4" bandpass cavity in series.  Around the transmit frequency, there is only slightly more loss than with the 3-can filter by itself - which is understandable.  You can also see that just within the span of this plot that the combination of the two types of filters greatly increases the off-frequency response:  Note in particular how the Yellow trace - the response of the 3-can duplexer alone - is increasing with frequency while, with the combination of the two, it's actually decreasing!  Finally, although it is not apparent from the plot, the notch depth is greater!  You would expect this, as the band-pass cavity alone has 10dB of rejection at the notch frequency, but the insertion loss of the two sets of filters cascaded actually measured out to being more than 15dB greater than that of the "3-can" duplexer alone owing to the added "magic" of the interconnecting coax cables and their impedance transformation properties:  Proper selection of these cables and their electrical length can allow the two filters to reinforce each other!

One important thing to notice is that if you were to locate your repeater on a site that has other VHF users - say, those in the 150-174 MHz region - or even another 2-meter repeater on-site, the 3-can duplexer alone will probably not help you when it comes to keeping those other transmitters out of your receiver!  The addition of a single bandpass cavity can reasonably provide at least another 35-40dB of isolation from those "other" transmitters.  Since we already know that the Icom's front end is quite fragile, this extra filtering is arguably more important!

Figure 3:
Refer to the text for an explanation of the above plots!
For each plot, the Yellow trace is that of the 3-cavity "Band-Pass/Band-Reject" duplexer, the Magenta trace is that of the bandpass cavity, and the Cyan trace is the combination of both.
Top:  Response of various cavity configurations from 140 MHz to 155 MHz.
Bottom:  Response of various cavity configurations from 30 MHz to 1 GHz.
Click on either image for a larger version.

"Wide-band" response:

Now, look at the Bottom image of Figure 3. This is a span that covers from 30 MHz at the low end to 1 GHz at the high end.

First, look at the Yellow trace - the response of the "3-can" duplexer leg.  Although not noted on the plot as such, the tops of the peaks on the yellow trace represent an insertion loss of only about 1-2dB while the analyzer's vertical scale is set at 10dB per division.

One alarming fact is that within much of the FM broadcast band (included in the first peak on the left edge) falls within a portion of the response curve where the rejection is less than 10dB!  What this means is that FM broadcast energy intercepted by your antenna will not be much-hindered by the duplexer!  If your repeater antenna is on the same tower as an FM broadcast station - or even if it is on a tower that is near an FM station - you could be intercepting quite a bit of power which, as we know, will absolutely demolish the Icom receiver!

You might also note that there are plenty of responses in the area of 450 MHz where there are often high-power paging systems as well as throughout much of the UHF TV band.  In the case of Farnsworth peak, we were also getting contribution from those transmitters as well!

Also note that that throughout the range shown on the Yellow trace you'll see that for most frequencies, the 3-can duplexer leg does not offer much isolation!  As you might expect, there are many spurious notches scattered throughout the frequency range (again, owing to the resolution bandwidth of the analyzer, the true depth of the notches is not apparent) the 3-can assembly is, for the most part "wide open!"

Now look at the Magenta trace - the single 4" bandpass cavity.  As you can see there is a strong, narrow peak at the 2 meter frequency for which it tuned (the first peak of the Magenta trace at the far left) but there are a number of spurious peaks as well.  As it turns out, a 1/4 wave bandpass cavity has a natural tendency to respond to odd-order harmonics as well as the fundamental frequency - that is, if the cavity is tuned to, say, 146 MHz, it will also have peaks at approximately 438, 730 and 1022 MHz.  In spite of these peaks you can see that, for the most part, the signals are attenuated across the band.

Finally, look at the Cyan trace.  As with the upper image in Figure 3 this is a combination of the 3-can duplexer leg and the single 4" bandpass cavity.  You'll notice that compared to the Magenta trace (the single bandpass cavity) the insertion loss is higher-still for most frequencies.  If you compare the Cyan to the Yellow trace you'll see that there is a tremendous difference in what passes through and gets to the receiver!

As with the bandpass cavity alone, the 3-can duplexer leg also has responses at its odd-order harmonics - in addition to lots of other places - which is why we still see those frequencies coming through.  Practically speaking, we can eliminate those "harmonic" responses very easily by installing a low-pass filter - such as that from a junked 2-meter transmitter.  If we were to do that, there would be essentially nothing getting through above 200 MHz - or anywhere else!

How about adding more cavities?

Many amateur 2-meter repeater installation use "4-can" duplexers (2 cavities per leg) and they exhibit much the same response - although the "off-frequency" responses may have even less attenuation than that of the "6-can" duplexer - but only by a couple of dB overall.  This reinforces the fact that if you have a "4-can" duplexer and have interference, a "6-can" duplexer will probably not help - unless the problem is really too-little TX/RX isolation!

As shown, if you do have a "4-can" duplexer, you can improve its performance with the addition of a bandpass cavity:  You would not only get vastly superior "off-frequency" rejection, but your TX/RX isolation would improve as well.  The caveat in adding a bandpass cavity (or cavities) is that sometimes there can be an interaction between the notches and the bandpass cavity response, so you'll want to readjust both of them to achieve both minimum insertion loss and greatest notch depth - and be prepared to experiment with different-length interconnect cables between the duplexer and the bandpass cavity, trying cables that are multiples of both 1/4 and 1/2 electrical wavelength long and seeing which one works best.  If you do this, also be prepared to check and re-tune the cavities as necessary to maxinimize notch depth/minmize bandpass loss as the tuning may change slightly with these different configurations.

One important measure of performance of any duplexer is the amount of isolation between the Transmit and Receive ports:  Too little isolation, and your own transmitter will deafen your receiver! As it turns out, for 2-meter ham use with a 600 kHz spacing, a good-quality 4-cavity duplexer will have plenty of isolation for a typical repeater - assuming it provides 85dB or more TX/RX isolation.  There are a few reasons why one might need more isolation than this, however, including:

• Using high power.  Clearly, if you have high RF power (100 or more watts, for example) than the "stock" repeater, you'll probably need more isolation.
• A "noisy" transmitter.  Some transmitters - including some synthesized units - produce a higher amounts of low-level noise than others.  This can be a result of phase noise from the radio's synthesizer, noise contributions from a power amplifier, or even from a switching power supply feeding low-level garbage into the radio's power supply line which can get modulated on to the transmitted signal.
• High VSWR on the feedline.  Normally, the "return loss" (which equates to VSWR) of a properly-working antenna system means that well under 10% of your transmit power is reflected back to the duplexer.  If, for some reason, the VSWR is very high - such as due to feedline or antenna problems - that increase of reflected power could effectively reduce the amount of transmit/receive isolation.  Of course, if this is happening, you have other problems!

• If we had replaced our "4-can" duplexer with a "6-can" duplexer, we would not have solved our problem simply because TX/RX isolation wasn't the issue!  Remember, the plots above are for "6-can" duplexer and as you can see, their "off-frequency" response is rather abysmal.  Again, the problem was the fact that other, off-frequency transmitters' energy was finding its way through the duplexer - which had, by design, rather poor isolation to far-removed frequencies.  It was only through adding a strong band pass response to the system that we limited our receiver's antenna port to "seeing" only a narrow bandwidth near the repeater's input frequency.


• Re-radiation of RF.  On busy sites it's not uncommon for your transmitter's energy to get into something else's transmitter, mixed, and be re-radiated.  Often, this is due to another user's transmitter not being properly equipped with bandpass filtering and an isolator to prevent your transmitter's energy from being mixed within that "other" transmitter's finals and being re-radiated.  It could also be due to severe overload of a preamp on a receiver somewhere and its subsequent re-radiation, or even due to two pieces of metal (usually rusty) that are acting as non-linear junctions and causing mixing problems.  Finally, we have even seen cases where non-radio equipment on-site - often in the form of devices that are not well-shielded an/or with long interconnect cables with inadequate RF susceptibility measures taken - that will intercept RF and re-radiate garbage.  Since this is happening outside your transmit/receive and antenna system, the only way to fix this particular problem is to locate the source of this re-radiation (which can be difficult!) and/or move the repeater's antennas farther away from the suspected source.
  

• It is likely that we could have "fixed" the problem by using a strong helical bandpass filter in the front end of the receiver instead of using a bandpass cavity.  While such a filter would have added some  loss to the receive signals, this added attenuation would have not affected overall system sensitivity owing to the elevated noise floor of the site.  Had we taken this route we would not have used a filter with a preamp as not only would this have not improved system sensitivity - which was limited by the site noise anyway - but that extra gain could re-amplify some of the off-frequency signals and contribute to overload - either in the receiver or the preamplifier itself!
  

We aren't done yet...

Because we ran out of time (and bandpass cavities) we have yet to do one more important thing:  Put a bandpass cavity on the TX port. While we have a 2-stage isolator there now, it is worth remembering that a high-band VHF isolator isn't going to work very well at keeping the FM-band or UHF DTV signal that might be 10's to 100's of MHz away, so one has to "pre-filter" before applying the TX antenna to the isolator.

In our testing, we have found that the UHF Icom D-Star repeater isn't being demolished by the extra signals coming through the duplexer, but we consider that to be mostly a matter of luck:  This receiver will also sport at least one bandpass cavity when we get the opportunity to do so.

With the onset of winter, it probably won't be until May or June 2010 before we'll be able to drive up to the site again and make additional changes/improvements.

Figure 4:
The 4-cavity "Band-Pass, Band-Reject" duplexer used on the 2 meter side of the D-Star stack.  While this duplexer provides more than enough TX/RX isolation, it does NOT provide much attenuation to signals that are outside the immediate vicinity of the TX/RX frequencies - we had to add additional "bandpass" cavities for that!
Click on the image for a larger version.

Adding "test equipment" to the D-Star stack:

In light of all of this, we are considering adding another piece of test equipment to the D-Star repeater:  An analog repeater.

This may sound odd or even sacreligious, but it makes sense in a way:  On a shared site such as this there are a lot of things that can go wrong once you rule out a problem with the gear itself. For example, a malfunctioning FM broadcast transmitter can throw lots of garbage across the spectrum that is often transient in nature. Unfortunately, the very nature of D-Star complicates using the tried-and-true diagnostics previously used for track on-site QRM (like "hearing" what the QRM sounds like) and especially on an all-digital system, determining the cause of system degradation can be particularly difficult - especially if it is intermittent and occurring only when it is inconvenient to drop everything and race to the top of the mountain, hoping that it is still happening by the time that you get there!

The plan would be to have the analog repeater normally commanded off, using a T/R relay to switch between the two transmitters. Because of the site noise we have the option of simply using a splitter on the receiver and ignoring the 3-4dB hit on insertion loss - which would make no difference in receive sensitivity, anyway.  If problems show up that appear to be RF-related, we'll simply command the analog repeater online and do "normal" tests to determine the possible cause.  Fortunately, we have enough rack space and gear around to pull this off.

Another thought was to simply install a GE Exciter and an NHRC-4 repeater control (and a simple squelch circuit) inside the repeater's box to accomplish the same thing, using the Icom's discriminator output as the audio source - which would be more representative of the system's receive state, anyway:  The 200mW or so of TX output from the exciter board would be more than enough for testing.

In our brainstorming we have also thought about having an analog repeater in which one takes the discriminator outputs of the various D-Star repeater's receivers and selecting among them, providing a way to remotely "hear" what the D-Star receivers are hearing to see what sort of QRM may be present - a valuable diagnostic tool especially if the QRM is intermittent in nature.

Finally, one suggestion made on the Yahoo Group by Greg, N6LDJ, was to use a computer on-site with its sound card connected to the discriminator port of the D-Star repeater.  In this way the on-site audio can be piped (something possible using a number of available programs - or even via an on-site IRLP or Echolink node) back, via the internet to a monitoring point.  In this way one can monitor the "analog" world that the D-Star repeater sees and if some sort of interference appears, there's some hope in discerning its origin!

A bit more about the tools and techniques used:

Using the RSSI indication:

As was mentioned, the RSSI (Received Signal Strength Indicator) and discriminator output (audio) signals from the D-Star's receivers were brought out via added rear-panel BNC connectors.  The RSSI provides a voltage that is more-or-less proportional to the input signal level in dB.

As a baseline, one would measure the RSSI voltage without an input signal (but with the receiver connected to a dummy load) and then, using an unmodulated carrier, measure the resulting voltage as one increased signals and note the readings.  This would provide a ready-made "calibration" chart for that radio (remember - they will all be a bit different!) that one should keep on-hand - preferrably taped to the radio!

Having this information is very useful for a number of reasons:

• One can get an approximate measurement of the "site noise" level as seen by the receiver.

• Diagnosing overload problems.

Because digital transmitters are becoming increasingly common in FM, TV and even land-mobile use, it is increasingly likely that these types of interfering signals - which are, in essense, just bandwidth-limited white noise - will show up only as noise in a receiver.  This is in contrast to the interference that one might expect from analog TV and FM signals in which there may be tell-tale "sync buzz" or some identifiable audio component!

Using conventional test gear to evaluate and test D-Star systems

Disclaimers:

• The above recommendations are based on experience, analysis, and the testing described.  They also take into account current Utah frequency coordination policies, which are based on previous and ongoing experience and geographical considerations.

• The above recommendations should not be applied in other areas  of the world without due consideration of local operating practices, needs, and conditions to determine if they are appropriate.

Other Utah VHF Society links related to D-Star:

• Using conventional test gear to evaluate and test D-Star systems - This page covers some aspects of D-Star and analog signals and related test equipment that may make it easier to evaluate the performance of D-Star systems and links.

• Observations of the codec used for D-Star - How does the codec used for D-Star respond if subjected to sounds other than those of the human voice?  We decided to find out.

• D-Star Channel Spacing recommendations - Recommendations of channel for D-Star analog channels.

The following are FAQ's provided by the Utah VHF society.  Note that these may topically overlap the links above:

• FAQ:  A brief overview of D-Star

• FAQ:  D-Star and sharing with other D-Star and analog users

• FAQ:  Direction-finding and D-Star signals

• FAQ:  D-Star vs Analog audio quality

• FAQ:  Utah channel spacing recommendations for D-Star and Analog signals

Questions, updates, or comments pertaining to this web page may be directed to the frequency coordinator.

Return to the  Utah VHF Society home page.

Updated 20100422