Mark Jessop Profile picture
Aug 9 9 tweets 3 min read
This is 5MU (1125 kHz), an AM broadcast station near Murray Bridge, South Australia. This particular transmitter site has been around for a long time (~1980s), and there's an interesting story about it and the newer tower in the foreground. 1/9 Image
Notice how there's two masts at this station? This is so that the radiation pattern of the station can be controlled through phasing, in this case to 'protect' (reduce interference with) another station interstate on the same frequency. 2/9 Image
In the early 2000s the tower in the foreground was built (used for telemetry relay and FM broadcast). After install it was noticed that the pattern of 5MU became skewed! Why? 3/9 Image
These towers are all fairly close together, and the new tower was tall enough to be somewhat resonant at 1125 kHz! This resulted in the significant pattern distortion (and other mismatch effects...) 4/9 Image
So what was done about this? Let's take a close look at the new tower - see those arms on the side? They hold the key to the solution, which is very ingenious... 5/9 Image
These arms hold wires that run down the side of the tower. How does this help? 6/9 ImageImage
It's actually turning the tower into a folded monopole! A capacitor at the bottom of the wire allows the entire tower structure to be 'de-tuned', so it appears electrically invisible to the broadcast station! 7/9 Image
The measured field pattern after the fix was installed shows just how well this fix worked, with the skew almost entirely removed. 8/9 Image
This was all designed by a local broadcast engineer and amateur radio operator, Paul Lawson VK5SL, who has been involved in the broadcast industry for many years. Thanks to Paul for the details for this thread! 9/9

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More from @vk5qi

Aug 14
So it turns out my dashcam captured my DoA Heads-Up-Display (…) pretty well during the hunt on Friday night! Here's us finding out that some cars are extremely noisy on the 2m band - watch the direction indication move as the taxi passes us!
... @AdelaideMetroSA buses are particularly noisy too!
Finally, a sped-up video of us navigating to the 70cm transmitter. You can see the signal strength (bar graph on the right of the HUD) increase as we get closer.
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