Richard, Has it been particularly dry where you are this year? I recall from my military days a story about a particular antenna installation had problems when it was particularly dry. This was a circular antenna array of about 100 feet in diameter. When the ground got really dry, they had to go out and salt it with something to make it more conductive. When it was dry, the conductivity decreased and thus so did antenna performance. Maybe something like that is affecting the quality of the connection between your ground screen and the earth. It could be that the ground connections of the screen at the turntable end of the site are not as good as they used to be causing this problem. Just a far-fetched thought.
Scott [email protected] -----Original Message----- From: [email protected] [SMTP:[email protected]] Sent: Thursday, July 22, 1999 8:46 AM To: [email protected] Subject: RE: NSA Here is our present status. We added ground screen across the table and soldered it down. We now pass 3m horizontal by a fraction of a dB, but vertical is out over a dB. When we moved the receive antenna out to 10m we passed vertical! We are using tuned dipoles and have shown that they are balanced by rotating them 180 degrees. They were recently calibrated by the same lab we used last year. The sum of the antenna factors drifted by only about 0.5 dB, but our NSA has moved lower by several dB. So it appears that we can rule out antenna calibration and balance. Oh, and yes we have ferrites on the transmit cable and we have 10 dB pads at the antennas. The surface of our turntable is about 2 cm above the surface of the pad. Our brushes are mounted on the side of the table and brush against the top surface of the ring. I have noticed some rippling in our ground screen. Could these surface variations be a problem? As I indicated before, the site passed its tests over the last four years. Any other suggestions. ---------- From: WOODS, RICHARD Sent: Friday, July 16, 1999 5:20 PM To: 'emc-pstc' Subject: RE: NSA We have a 10m site, but because of high ambients, we calibrate and test at 3m. So, we have a very large ground screen for a 3m site. When we place the transmitter on the table, we fail NSA. When we move the transmitter onto the ground screen we pass. We can place the transmitter to the side of the table on in front of the table as pass. Therefore, I conclude that we are not experiencing reflections from objects near the OATS, nor do we have problems with the screen or its connection to earth. The antenna and cable setup also appears to be correct since we can pass off the table. It appears that our 1.5m table is the only unknown. We did find that the continuity between the ring and the screen (a bolted pressure connection) had deteriorated significantly. That is why we added the copper straps as a more permanent connection method. There are eight short, wide copper straps around the 1.5m table. ---------- From: [email protected] [SMTP:[email protected]] Sent: Friday, July 16, 1999 4:00 PM To: WOODS,RICHARD; "INTERNET[ <[email protected]>]"@anritsu.com Subject: re: NSA Richard, Can you detail your procedure to the conclution: "The problem appears to be the turn table" ? Barry ------------- Original Text From: "WOODS, RICHARD" <[email protected]>, on 7/16/99 12:28 PM: Argh! For the first time in five years, our NSA is out of spec on our 3 meter OATS at 30 MHz. The problem appears to be the turn table. We can move the equipment off of the table to one side or toward the front and comply, so it does not appear to be an off site reflection problem. We have replaced marginal brushes from the table to the ring and we have added wide copper ground strips from the ring (attached with screws) to the ground screen (soldered). Still won't pass. Suggestions? --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). D --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
