Great hard data!
It also shows the need for a low pass filter in front of your receiver on high level sites.
Gran K6RIF
At 09:22 PM 11/13/2003, you wrote:
The recent discussion regarding the suitability of pass cavities as
harmonic filters made me curious as to if the odd wavelength resonances
really do fall well away of the base 1/4 wave resonance (IOW would a 1/4
wave cavity tuned to 150 MHz pass the 3rd harmonic at 450 MHz).
I swept a Motorola TU255 400-470 MHz cavity with 2 dB loops from 25 MHz to
3 GHz & have attached a plot of the results. The figures above each peak
indicate the true minimum loss obtained via additional narrowband sweeps,
since the resonances were too sharp to be accurately measured over such a
large span with only 801 measurement points available.
In summary, here are the insertion losses in dB at each harmonic of 450 MHz:
x1: 1.9
x2: 87
x3: 50
x4: 73
x5: 78
x6: 61
Of course this is only 1 cavity, & it is the type where the loops are on
opposite sides of the cavity (as opposed to the "square" Motorola UHF
cavities), which may reduce spurious responses. But at least it's nice to
know that this particular model does make a good harmonic filter.
Bob NO6B
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