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Using a Cushcraft MA5B beam, when I apply high power from an amplifier, the SWR will go up and down intermittently. Something is breaking down at high power (200-500 W).

I have a new coax feedline, RG213 and checked grounds.

Could it be the matching box on the antenna?

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  • $\begingroup$ well, if my scratching-my-head calculation isn't totally off, then at at 500W you're dealing with RMS voltages of more than 150V over a 50Ω load, and that's 1.4 times less than the peak-to-peak voltage, not to mention mismatches leading to higher voltages. So, what does that box look like on the inside? Is it made for such voltages (or powers), given the SWR you see (at lower powers)? $\endgroup$ – Marcus Müller Jan 28 '18 at 17:58
  • $\begingroup$ So you are switching the amp in and out of series between the transmitter and the antenna? $\endgroup$ – AG5CI Jan 28 '18 at 18:35
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The MA5B is rated for 1,200 watts PEP. 200 watts is certainly well below that threshold. Things to check:

  1. Arcing, corrosion or moisture inside the match box
  2. A loose or corroded connection from the matchbox to the driven element
  3. Loose or corroded connections in the "X hats"
  4. A waterlogged, corroded or otherwise failed trap
  5. Loose or corroded connections in the telescoping joints in the elements
  6. Loose joints between the elements and boom
  7. Loose or corroded coax connections
  8. Overheating or voltage breakdown of a balun

I would also recommend substituting a dummy load in place of the antenna to eliminate problems with coax, tuners, SWR meter, amplifier, switches, etc.

If the problem is on a particular band, pay close attention to the applicable traps per point 4 above. The traps have a coil on each end. Water and bug ingresses is a common problem.

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