I've been warned about passive intermod, since I had a ranch with
several buildings with very poor condition galvanized roofs and a 50kw
BCB station at 150'ASL line of sight 4 miles away from my antenna at
1500' ASL. Of course a BCB filter was a must as were good line
filters. But I never heard PIM.
My first suggestion is to install a good BCB filter to be sure the
intermod isn't in the radio.
Grounding the coax top and bottom also, good practice for lightning
protection. Also a ground conductor around the "thrust" bearing if it
is metal on metal, i.e. ground the mast to the tower top.
Since the intermod changes with the wind, perhaps this is real PIM from
the tower connections. Depending on the crank up design perhaps a brass
wiper could be placed in the top of each section where there is
overlap. Hard brass would have enough spring to stay in contact and
perhaps a couple of SS hose clamps could hold it in place without
interfering with up/down tower motion. I'd do all three legs for
redundancy. I doubt any hard stop contact would work given that
temperature cycling moves the sections up and down a bit.
Let us know what you learn. AFAIK, my two crank ups are quiet in a
multi-multi contest with stations on both towers. I have no nearby BCB
transmitter now to worry about.
Grant KZ1W
On 10/19/2015 20:08 PM, Jim Brown wrote:
On Mon,10/19/2015 7:00 PM, Rich Hallman - N7TR wrote:
Has anyone had any problems with intermittent mixing from a close
broadcast station with a Crank-Up tower? I think I may have an issue
with one of my towers. Cranked up I hear mixing all over 80 meters
(Every 10 KC).....Cranked down its gone. I already eliminated the
antennas. Mixing gets very intermittent in wind as the tower moves
around.
I have no experience with crankups, so will leave most of this to
others. The fundamental cause though is less than ideal electrical
contact in a conductor.
Thoughts about grounding the top section to the bottom when fully
extended?
Definitely. Should be a short fat conductor from the bottom of the top
section the top of the lower section that it mates with.
Was thinking of grounding the coax at the top and bottom of the tower?
This is ALWAYS good practice, to prevent arc-overs between the tower
and the coax, which will fry the coax at the point where the arc occurs.
73, Jim K9YC
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