Eric, all cables are double shielded. No adaptors are used. There is no isolator on the TX side. Connectors are silver plated, with gold pins.
Antenna is a single folded dipole mounted about 15m above the equipment shelter. TX power into the antenna is approximately 15W. There are solar panels within 2m (below) of the anntenna. Putting the attenuator at the RX port of the duplexer reduces the IM products 1 for 1, ie 6dB of attenuation at the RX port reduces the IM by 6dB. BTW I used a 50W rated attenuator and Txed for short periods only so think overheating of the attenuator is not that likely. Yes TX freq is stable and not drifting. Also I have tried a second TX with smae results. Regards, Brett --- In [email protected], "Eric Lemmon" <wb6...@...> wrote: > > 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: [email protected] > [mailto:[email protected]] On Behalf Of brett > Sent: Saturday, September 04, 2010 4:27 AM > To: [email protected] > 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. >
