KJ1K Analysis

Background:

The Mount Greylock, 146.31/146.91, repeater located on Mt Greylock in Western Massachusetts supports public safety activities such as the recent search and rescue of passengers of an airplane that crashed in south Berkshire county.

Significant interference with the Mount Greylock repeater started during early spring of 2003.

The interfering signal is on 146.32 MHz, about 10 kHz from the nominal Greylock input frequency. It is our understanding that the 146.34/146.94 repeater on Mt Tom, in Holyoke Massachusetts, may also be experiencing similar interference.

Northern Berkshire Amateur Radio (NoBARC) members N1FGY and KE3HT used direction finding equipment to isolate the source of interference to a tower located on Hogback Mountain, in Marlboro Vermont (see

http://www.nobarc.org/hounds/realhunts/index.html

)

Picture 25 tower on Hogback Mountain

Investigation:

KJ1K and N1FGY brought an HP 8590B Spectrum analyzer, a bandpass filter, and a beam antenna, to Hogback Mountain to measure the amplitude of the interfering signal on Wednesday May 28, 2003. The filter was tuned to 146.31 MHz and has the response characteristic shown in picture 15. The equipment used is shown in pictures 17 and 18.

Picture 15 Bandpass Filter Characteristics

Picture 17 Bandpass Filter and Spectrum Analyzer

Picture 18 N1FGY with beam antenna

Data:

The BPF was used to verify that the interfering signal is real and not a result of mixing in the spectrum analyzer. The BPF was then removed and we observed that the interfering signal was always present when two signals (152.03 MHz and 157.74 MHz) are present. See picture 24

Picture 24 Spectrum analyzer view of Interfering Signal

Signals (Left to right)

Interfering signal at 146.32 MHz (-53 db below reference level)
Unknown signal (-50 db below reference level)
Transmitter at 152.03 MHz (-17 db below reference level)
Transmitter at 152.79 MHz (-20 db below reference level)
Unknown signal (-62 db below reference level)
Transmitter at 157.74 MHz (-23 db below reference level)

Analysis:

Note that Signal 3 minus (Signal 6 minus Signal 3) is equal to Signal 1

152.03 - ( 157.74 - 152.03) = 146.32

152.03 - 5.71 = 146.32

146.32 = 146.32

The interference is only about 35 db below the 152.03 MHz carrier (Should be more than 60 db below carrier)

Preliminary conclusion from data:

Mixing is occurring either inside the 152.03 MHz transmitter, the 157.74 MHz transmitter or both. From our direction finding effort we knew that the source of the interference is the tower on Hogback mountain. We knew that the tower has a 157.74 MHz transmitter, but we did not know, when we were on Hogback, that the tower also has a 152.03 MHz transmitter. Because the analysis shows that mixing has two 152.03 MHz terms in it, the 152.03 MHz transmitter is most suspect.

Telecom to Karl Rinker 15:40 EST 5/30/2003:

After consulting with W1BS, KJ1K called the licensee of the 152.03 MHz transmitter on Hogback and told him about our efforts and asked him what "protection", such as isolators or filters are on the subject transmitter. Karl checked his database and reported that the transmitter does not have an isolator and he instructed his assistant (Allen Wilde, N1IOE) to order a circulator while we were on the phone. He also said that he believed that the 157.74 transmitter probably already has a circulator on it because the licensee of that transmitter usually installs transmitters which have isolators as an integral part of their design. Allen said the circulator vendor, TxRx, has promised a 2-3 week delivery. Karl promised to have Allen call me when the circulator has been installed. I said that we plan to make a follow up trip to Hogback Mountain to re-measure the signal on 146.32 MHz after the circulator has been installed.

Recommendation:

Addition of the Isolator will likely "solve" the problem, but we need to follow up on the fix to make sure it is adequate.