[TowerTalk] [Bulk] Re: RFI Broadcast Station Mixing in Crank-Up tower?

[Grant Saviers]

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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