Modification to eliminate RF on LNA Wiring

A problem has surfaced that involves RF getting directly into the Microwave Radio LNAs by means of the LNA wiring harness. In tests on the LNA, it was found that the feed throughs on this LNA do not attenuate RF. This presents a problem, especially on the Ultrascan Antenna System. Stray RF can induce RF on the wiring harness of the LNA, and wiring to the LNA act as little antennas and couple RF directly into the amplifiers and GaAsFET switches.

Look at E3, E4, E5, and E6 in the picture below.   These are the feed-throughs on the inside of the LNA. No chokes! No RF bypass caps!    

At KABB-TV, RF from a 5 Megawatt UHF TV station entered the LNA housing here, and severely restricted their ENG coverage. The station was about a mile away.

ABOVE:  RF LNA power and switching wiring allow stray RF to enter key points in amplifier and GaAsFET switches.

BELOW: The "A" power lead from the outside goes to the horizontal amplifier circuit "E1". Stray RF can be transfered to Q3, an MMIC (Miniature Microwave Integrated Circuit) amplifier. Q3 is the first "Horizontal" LNA stage, and has a gain in excess of 20 db. This device appears to be identical to a $5 HP "INA" device that has the same gain, voltage, and noise figure. This stage is the "heart" of their LNA.

  1. Obtain parts kit from PHILLIPS MICROTECHNOLOGY at 1-888-828-8775
  2. After installing the High Selectivity RF Filters to protect the front end of the LNA, your LNA input is protected from broadcast, paging, and most two-way transmitters, but not "C" Block PCS. You must also replace the LNAs and add more filtering to have this system work within one mile of a "C Block" PCS Site. Unfortunately, the old LNA’s wiring harness still acts like an antenna.
  3. Lay out LNA as pictured.

ult-lna.jpg (25200 bytes)
Microwave Radioä LNA

  1. Starting with "A", remove wire and insert choke between wire and terminal.

    Before

  2. After - keep leads short! Add a bead of RTV between chasis and choke for support.
  3. Add RF shunt capacitor across feed-through
  4. Repeat for the remaining 6 terminals.

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