Insufficient RF filtering “strikes again”

Earlier this year, I had a look at a Capacity Plus system where the customer was complaining of the system being permanently busy. After a few minutes of looking through their configuration, the probable root cause was found: with the exception of one, all the transmit frequencies were spaced by 12,5kHz. Such a narrow spacing is a recipe for disaster in terms of performance and interference. Assuming, hypothetically, the system used a (multicoupler and) combiner. A good quality Hybrid Combiner will provide 50dB of isolation at around 300kHz. At 12,5kHz, this drops to around 2dB! This means that RF energy from adjoining transmitters is able to enter into any transmitter and produce intermodulation products. This is because the transmitter circuit has a non-linear impedance and therefore acts as a diode and hence frequency mixer. This will occur even if circulators are used. In any case, any combiner manufacturer worth their salt will red flag an order with such narrow frequency spacing. For interest sakes, a Cavity Combiner would only offer 50dB of isolation from around 3MHz. In the case of this system, each repeater had its own duplexer and antenna (i.e. 4 3 repeaters, 3 duplexers and 3 antennas). If this is the case, then RF from one transmitter will still enter another, just as it would in the above configuration. A duplexer (be it notch or pass-reject) offers no filtering to adjacent channel RF. Assuming the antennas are spaced by ½λ, the free space loss, between adjacent antennas, will only amount to 6dB or so. Again, the potential here for intermodulation is extremely high.
In traditional analogue radio systems, desense is in most cases, invisible - the repeater is just deaf to varying degrees. In DMR, the symptoms are different: instead of a lousier signal to noise ratio (i.e. more hiss), we loose bits and packets. The result of this is:
  • Call fails
  • System busy - even though there are idle channels
  • Transmit Interrupt does not work
  • Garbled audio
  • Call terminates unexpectedly
  • Missed calls
  • Extended late entry into calls
  • GPS not updating
  • Text messages failed
  • Poor coverage
Desense is more prevalent in Capacity Plus and Linked Capacity Plus systems because:
  • There is more than one repeater at the site and in the same band (and frequency range).
  • In an idle state, the rest channel is transmitting a beacon.
  • In high traffic conditions, more than one repeater (or all the repeaters) may be keyed up.
Here, the customer should have:
  • Applied for frequencies, where the transmit frequencies are separated by at least 300kHz.
  • On VHF, the transmit to receive separation should be at least 5MHz and on UHF at least 10MHz.
  • Included an adequate RF filtering system in the system proposal, despite the additional cost.

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