> Have you tried putting one or more pass cavities on the transmitter? It's > more of a longshot, but if there's something strong coming back down your > feedline it could be getting to your PA resulting in intermod. The problem > would come and go with the strong signal obviously, and only when your Tx > was keyed.
I have tried putting one of the 11" pass cavities on the transmitter. No change. There is a fairly low traffic APRS digipeater (144.39) on a tower 250 feet away but my noise problem does not come in packet-length bursts. It was also down for a while and I still had the problem. The only other VHF transmitter is 158.x on the tower that is only 60 feet away. However I have never yet found that transmitter to be up when my problem was occurring. It sees VERY little use. The only other sources of RF are cell towers several hundred feet away. > Yeah, they're coated with that semi-clear plasticy stuff usually. I > wouldn't consider that to be a "high risk" factor, but hard to say. How > close is the nearest guy wire attachment point on the tower to your antenna? Close, just a couple feet below the antenna at the present time. In general, moving the antenna closer to a set of guys does seem to make the problem worse. If I could somehow get to a point where I believe it's the guys, I would seriously consider replacing them with Phillystran, leaving a few feet of EHS at the anchor end. > Is your antenna currently side-mounted or top-mounted? If side-mounted, at > what height? Half and half at the moment, actually... the 20 foot stick is mounted on side arms at the 90 foot level of a 100 foot tower. Moving it to top mount does seem to decrease the desense by some 5 to 10 dB (hard to quantify due to variability) but I still have 20 to 30+ dB desense at times. When side mounted, spacing from the tower also makes some difference (wider spacing = less desense) but again not enough to be truly helpful. > What kind of mounting brackets/clamps for the antenna? Sinclair heavy galvanized mounting clamps on the antenna, side support arms are heavy galvanized angle stock. Galvanized U bolts to tower legs. Other mounting arrangements have been tried, including the use of insulating material for side arms. Some change (less desense with the insulated mount) but still not enough to be really helpful. > What else is on the tower? The only things on the tower are my 147.105 and 444.950 antennas. I tried completely removing the 440 antenna and feedline from the tower, but nothing changed. > How are the feedlines attached to the tower? They are both LDF5-50A up the inside of the tower (that was fun), attached by many heavy duty nylon cable ties. > Does the noise get any better or worse when it's raining out? There *seems* to be some tendency toward the noise being worse during the first hour or so of a rain or snow event. But it is not always so. The noise is sometimes there when it is raining, but not always. It sometimes there when it is dry, but not always. > How are the guy wires attached to the tower (looped around a leg, with or > without thimbles, torque triangle/arms, etc.)? The preforms are looped around a leg and through the Z braces of the tower. No thimbles at that end, there are some where the guys attach to the turnbuckles. > I'm guessing not, but is the tower lit? It is not. > When you changed out antennas and heliax, I presume you replaced any topside > jumpers as well? Topside jumper and the jumper between duplexer and feedline were replaced. > Where is your equipment with respect to the tower (i.e. how close to the > base)? About 30 feet away. > And how well-shielded is the equipment (enclosed metal cabinet > hopefully), and are you taking the usual precautions as far as cabling into, > and within, the cabinet to keep RF out? Hmm. The cabinet (rack) is not completely enclosed. Each piece of equipment is in a shielded enclosure. There are feedthrough capacitors and chokes/beads on every non-RF lead entry point to a piece of equipment. However, the 13.6V interconnecting cables and 117VAC power cords are not shielded. They do have the filtering wherever they enter equipment. Paul
