Gary,
Thanks for the reminder on the importance of regular maintenance and the
explanation about of the lightning loop.
I saw a picture of this loop on Jim Hawkin's excellent site from WCBS/WFAN and
wondered what the loop was used for.
Looking back I see Jim included the explanation with the picture:
<http://j-hawkins.com/wcbs_wfan/cbsfan_twr14.jpg>
Jim also has pictures and describes diplexer operation.
Mike N2MS
> On 01/11/2023 1:15 PM Gary Peterson <kzerocx@rap.midco.net> wrote:
>
>
> I am reminded of two situations. When I was corporate engineer for a company
> that owned almost 50 stations, I received a call from one of our market
> engineers. He had two diplexed AM stations and an excessive third-order mix
> was being radiated. I told him to pick a time when he could shut both
> stations down and tighten everything up in the diplexer and antenna matching
> system. He did and the problem was gone. Temperature cycling and vibration
> from wind can loosen up hardware between inductors, capacitors, RF
> contactors, etc.
>
> A contract engineering friend was trying to determine why an AM station had
> excessive harmonics being radiated. He was poking and tapping components at
> the tower and antenna tuning unit, while monitoring the level of one of the
> harmonics on a Potomac field intensity meter. The poking and tapping was
> done with an old, dry broom handle. When he was rapping on the “lightning
> loop” between the ATU and the tower, the weld broke loose and started arcing.
> A lightning loop is a large, one turn loop in the copper tubing between the
> ATU and the tower. It represents a blob of inductance which, supposedly,
> encourages more of the lightning to choose the spark gap across the base
> insulator, rather than the ATU on its path to ground. Apparently, over time,
> the lightning loop’s brazed connection to the tower work hardened, due to
> jiggling slightly in the wind. The weld separated, microscopically, under
> the red tower paint. The only thing that was holding the copper tubing to
> the tower was several layers of hardened tower paint. Once the lightning
> loop was cleaned and re-brazed, the harmonics were down to normal levels.
>
> Both of these are good examples of entropy. Unless work is done on a system,
> it will proceed toward a state of maximum randomness. Perform regular
> maintenance on it or it will eventually fall apart.
>
> Gary
> K-zero-CX
>
>
>
> And you should not discount the probability that the 'mix' is caused by
> a corroded joint between two conductors somewhere.? The rusty joint
> could be any where, in the near field of the TX or near the RX. Anywhere
> there is enough RF to excite the joint and re-radiate.
>
> AL, K0VM
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