Brett, Some additional information will be helpful. What makes/models of equipment are in your repeater? Are all jumper cables and the antenna feedline double-shielded? Are any of the connectors nickel-plated? Are there any barrels or adapters in your jumpers? Is there an isolator/circulator following the transmitter? What antenna are you using, and how far above the repeater equipment is it located?
Try putting your attenuator right at the RX input connector, and repeat your IM test. Putting it at the antenna output is not a good idea, since the TX output power can cause it to overheat. Your description of the IM product suggests that it might be a spur generated within your PA, which could drift due to temperature changes. Have you verified that your TX carrier frequency is stable, and not drifting? 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of brett Sent: Saturday, September 04, 2010 4:27 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Interference on VHF repeater Hi all, I have come across an interesting problem which you may be able to shed some light on. I have an intermod issue where my TX sometimes opens up my RX. I have the distinctive hollow pipe sound. Both TX and RX have the same CTCSS tone. The intermod product is however not always present, and after looking at the RX output from the duplexer with a SA I see a comb of products that move slowly in time. When one of the products in the comb falls within the RX bandwidth the RX opens, until it moves on. This is not a busy site, and I have been able to power down everything on site except my repeater. Problem remains unchanged. I have also disconnected feeders from all other RF equipment on site - still no change. The fact that the IM product frequency changes with time (drift rate is roughly a few kHz's an hour) makes me think that there is either another unknown source of RF on site which has poor freq stability (pretty unlikley), or somehow my TX freq is involved in producing this freq. I have inserted a 6dB pad in the antenna port of the duplexer and found that the IM products drop 12dB, and also curiously, the frequency of the products change. Removing the pad reverses this effect. I have repeated this many times and the result was always the same. It appears that the frequency of the IM product is dependent on the strength of the radiated field from my antenna. This is my question: I have read that it is possible for a strong EM field to excite metal (eg tower member) such that re-radiation will occur at a frequency which is different from that which excited it. Can anyone confirm they have seen this, or can anyone point me to a reference that talks about this? I should also mention there are multiple solar panels and associated regulators on site. The regulators have been discounted as possible sources, but the panels (given they may have bypass/blocking diodes) may be a mixing location, however the source of the drifting tone is still unclear. Thanks, Brett VK2CBD.
