For a better pickup loop, solder a 50 ohm surface mount resistor in series with the loop at the end of the coax, and put a few ferrite RFI beads or clamps at random distances along the coax. This reduces pickup by the coax itself which can otherwise cause misleading measurements. Since the loop is now terminated you can also drive the loop safely with a low power signal generator (perhaps even a handheld on low power if the 50 ohms resistor is a 1 Watt type), and use this to find out where noise is getting into the receiver. 73, Mark VK3BYY
_____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Derek J. Lassen Sent: Monday, 1 September 2008 4:20 AM To: [email protected] Subject: Re: [Repeater-Builder] Wits End -- Desense Try putting a one inch dia, single turn loop on the end of a coax feed line to a receiver. Use it to sniff around inside the box to find the RF. 73 de KN6TD (s) Derek At 11:42 AM 8/31/2008 -0400, you wrote: Ive tried everything, it seems, and I still have desense!! Even when I connect only the repeater (Yaesu Musen FTR-1510) and a controller (needed to make the repeater transmit) and put a dummy load on the TX out, I get desense. Following up on Erics suggestion about holes leaking RF, I sealed the edges and holes in the TX and RX units inside the repeater, and I built a shield to enclose the back side of the TX connector that is on the back side of the repeater. That might have reduced the desense a little bit, but not much. I even ran the TX feedline to an outside wire-mesh chair in an effort to reduce any possible radiation getting from the dummy load to the receiver. There was still the same desense. The desense is at least 10 dB. It appears to me that the desense has to be occurring inside the repeater cabinet, but for the life of me I cant see how this can be. Everything seems to be well shielded. So, Id very much like to hear your theories and suggestions. Is there some way to find the source of the desense radiation? Is there some way that unshielded control lines, audio lines, and power lines can carry RF to the receiver? I have looked at the output of the repeater TX with a spectrum analyzer [tnx Tom N4ZPT] and it is clean. While I dont think the following is significant, for completeness I note that the repeater RX and TX both appear to be several kilohertz low in frequency. However, I do not have a frequency counter, and I am only checking the frequencies by the use of an HT and mobile, both modern transceivers by Yaesu. Your thoughts, suggestions, and sympathy will be appreciated. John AF4PD
