The system as it stands right now consists of two sites Fremont Peak and Presson Hill. Both operate on the same temporary frequency pair.
147.405(output) 144.940 (input) PL 94.8
This week was historic for the Salinas Valley Repeater Group, Thursday 9/24/2026 marking the day we put out the first simulcast system on the air! Over the last 7 years, I’ve been wanting to convert SVRG’s linked system to simulcast. But why simulcast you ask? These systems are hard to build, maintain, and are complex to design. Well let me list out some of the advantages and disadvantages of a simulcast system and you’ll see why I’ve wanted to bring an open VHF simulcast system to the amateur radio community.
Seamless coverage – users experience better coverage as they move between repeater sites, since all sites transmit the same signal at the same time
Single frequency – no need to program or scan multiple frequencies/channels for the same system, simplifying radio programming and user operation
Simpler user experience – users just talk; there’s no need to select a specific repeater, figure out what channel to be on and understand which site they’re nearest
Efficient spectrum use– Allows SVRG to reduce its spectral footprint and free up repeater pairs in the already frequency congested greater bay area.
That being said there are some Trade-offs worth knowing:
Simulcast requires precise engineering (GPS timing, phase alignment studies, link reliability) and is more complex/costly to design and maintain
Linked repeaters (voted or non-simulcast linked) systems are generally simpler and cheaper to deploy, more forgiving
Linked systems can be easier to expand incrementally site-by-site
All that said, the goal was set. Simulcast was the path forward for SVRG, but how would it be built? Our first experiments were with the Motorola DSM-II T1 based simulcast system developed in the 90’s. After building a test system of 3 sites and mulling over the installation requirements, rackspace constraints, T1 circuit emulation TDMoP, 48v DC power systems. It became quickly apparent that a DSM-II based simulcast system would not be feasible for the system we wanted to build.
So where do we go from here?
DSM-II is off the table, so I started to look into the Motorola MLC-8000 and Micro-Node RTCM. The MLC-8000 requires a 5Mhz 1PPS composite signal that the TRAK-9100 produces (very costly) and the Micro-Node RTCM requires hardware modification to use for simulcast as it requires a 9.6MHz non standard clock, so that’s out too.
Our solution was to design our own simulcast hardware! Dub’d the SV-180, our simulcast board that connects to ASL3’s linking and voting platform. More to come on that in another post.
What are our future plans for SVRG and its simulcast system?
As of right now our plan is to convert all VHF systems to simulcast on 1 frequency pair (excluding Meadow lakes VHF). This in and of itself is going to be a bigger challenge to work out than the engineering required to design our own simulcast controller and build out a system over multiple counties.
Due to the frequency congestion on the VHF band in our area and with our plan being very aggressive, coupled with the fact that there is no perfect frequency pair for us to land on. I ask you, the community, for understanding as we work through the next steps of the simulcast system we are going to buildfor you.
Putting several transmitters on one frequency on purpose means managing interference instead of avoiding it. This post touches on the science behind simulcast, and how coverage and timing are tuned in a real system.
Simulcast is controlled interference
Every mobile radio already deals with interference from its own signal. At a single site, the signal reaches the radio by a direct path and also by reflections off buildings and terrain. The reflected copies arrive a little later and at a different strength. This is ordinary multipath, and receivers handle it every day.
Simulcast adds more of the same. A second site on the same frequency is, from the radio’s point of view, one more source of “reflections”: its direct path and its own reflections all land on top of the first site’s. This is co-channel multipath interference. The job of simulcast design is to keep that interference under control, so that where two sites overlap the radio still recovers clean audio or data.
What the receiver does with it
A receiver amplifies what arrives at its antenna, mixes it down to an intermediate frequency, and demodulates it. When two or more signals are on, or very nearly on, the same frequency, they go through the mixer together and come out combined. Distortion is added along the way. What the radio finally produces depends on how its demodulator handles that combined signal. Everything else in simulcast design exists to make sure that combination still demodulates cleanly.
Carrier frequency and phase
A mixer produces the sum and the difference of the signals fed into it. Put two transmitters at 147.000000 MHz and 147.000001 MHz, only 1 Hz apart, and the receiver sees a 1 Hz difference product. That is heard as a rolling fade, sometimes called “mobile flutter”.
Being frequency-locked is not the same as being phase-locked. To a receiver, a small change in phase looks just like a change in frequency, so a slow phase drift between sites also shows up as a rolling fade, and there is little that can be done about it at the receiver.
Hear this your self
You can reproduce the effect on a bench with two audio signal generators. Set one to 440 Hz and the other to 441 Hz at the same level, and mix them into a single speaker. The combined tone swells and fades once per second, because the two waves drift in and out of step, adding together and then cancelling. The beat rate is simply the difference between the two frequencies. Two carriers at 147.000000 and 147.000001 MHz beat at the same 1 Hz as the two tones.
This analogy has a limit. In the radio the beat happens to the RF carrier, before the audio is recovered. An FM receiver ignores changes in signal level, so the listener doesn’t hear a smooth swell. They hear the audio flutter and break into noise each time the two carriers cancel. Try turning one generator down: the fade becomes shallower. That is the capture effect at work, as described below. When one signal is much stronger than the other, the weaker one can barely pull it down.
Note: the demo needs both tones summed into one speaker or one channel, not one tone in each ear.
Audio timing: the 30-degree rule
The decoded audio from each transmitter has to reach the radio within 30 degrees of phase of each other. Turning that into a time depends on the audio frequency. Using 1,000 Hz as the reference:
360° ÷ 30° = 12, so the budget is one twelfth of a cycle.
A 1,000 Hz tone has a 1 ms period.
1 ms ÷ 12 ≈ 83 µs.
Audio frequency
Period
30° timing budget
200 Hz
5 ms
420 µs
1,000 Hz
1 ms
83 µs
2,000 Hz
0.5 ms
42 µs
The higher the audio frequency, the tighter the budget.
The same effect applies to data. For digital modulations, the limit is expressed as delay spread, and refers to the bit-error-rate versus delay-spread curves in TIA TSB-88. These show the error rate climbing as the delay spread between sites increases, with some modulations tolerating much more spread than others.
Capture effect and overlap
An FM detector will capture the stronger of two signals when their levels are different enough. It locks onto the stronger one and largely ignores the weaker. The narrower the deviation, the less capture effect there is.
This defines where timing matters. An overlap area is a place where no single transmitter is strong enough to capture the receiver. Only there does the delay between sites need to be managed. The thing that matters is the delay difference inside the overlap areas, not the distance between the transmitters.
The speed of radio adds up
Radio travels at about 5.4 µs per mile. That sounds quick, but the miles add up: 10 miles is 54 µs and 20 miles is 108 µs. Compare that with the 83 µs budget at 1 kHz. A path-length difference of 20 miles is already too much if the receiver is in an overlap area.
What matters is the difference in arrival time at the radio, not the distance to either site.
Example
Path lengths
Arrival difference
Result
Two sites, mobile close to one
20 mi / 2 mi
97.2 µs
OK only if the near site captures the receiver
Two sites, mobile roughly equidistant
20 mi / 22 mi
10.8 µs
OK both in capture and in overlap
Three sites (C, A, B)
40 / 10 / 5 mi
189 µs (C–A), 27 µs (A–B)
A radio might be in overlap with all three. Avoid this by controlling coverage.
The three-site case is closer to the real world. A pair of nearby sites can be well matched while a distant site’s signal arrives far too late. The is answer is to control where each site’s coverage goes, so a radio isn’t left in overlap with a far-off site.
Design it on the computer first
A coverage-prediction program for simulcast has to calculate time-delay interference as well as signal level. Design criteria:
Modulation
Capture ratio
Maximum delay
Wideband analog FM
10 dB
80 µs
Narrowband analog FM
15 dB
80 µs
P25 C4FM (DAQ 3.4)
15 dB
30 µs
P25 LSM (DAQ 3.4)
15 dB
70 µs
With those two numbers, a prediction can mark each place as either captured by one site (more than 15 dB stronger than the others, so delay doesn’t matter) or in overlap (where the arrival-time difference has to be within the delay limit). The 2016 talk also compared delay-interference predictions for the same system at wideband and narrowband deviation. Because narrower deviation gives less capture, deviation is one of the inputs that moves the overlap areas.
Antenna downtilt: a trade-off
Tilting a site’s antenna down clearly raises signal levels close to the site. It reduces the areas of overlap interference, but at a cost. High sites lose some of their wide-area coverage, so talk-out is reduced in some places. Simulcast from high sites is often difficult, and much of the reason for choosing a high site goes away once its coverage has to be pulled in.
You can’t tune a simulcast system by driving around. You need to model it in software.
Software modeling: delay spread
Salinas Valley Repeater Group uses its own in house modeling software to calculate and map the delay spread for our simulcast system. A delay spread map lets us visualize the differential delay or ‘delay spread’ within the designed coverage area of our system.
The map pictured shows an area in Santa Cruz County that will be out of our phase budget so the resulting signal in this area will have distortion artifacts.
reference sources
Joe Blaschka Jr., PE, ADCOMM Engineering Company, “Simulcast – Theory or why we do the things we do,” Simulcast Forum XX, Las Vegas, 2016.
Joe Blaschka Jr., PE, ADCOMM Engineering Company, “Simulcast – Propagation and Tuning,” Simulcast Forum XXII, Orlando, 2018.
Both presentations are published by Simulcast Solutions in its Simulcast Forums / Case Studies collection: simulcastsolutions.com
