Jim, you are right, it was a 200 ft length for 432 MHz and 100 ft for
higher frequencies. This is a great application for old RG8 coax that has gone
bad.
200ft of new RG213 measures right at 4.5 dB/100ft at 432 MHz so 200ft is
almost 9 dB and a return loss of 18 db with short or open at the far end. I
have a couple of calibrated for loss 100 ft cables that I use for
attenuators for these higher frequencies. They allow power measurements using
lower
power Bird slugs which seem to be more accurate than the 2500 watt slugs I
have used, especially at 1296 MHz.
Of coarse the Bird meter is terminated with a smaller 200w load when used
this way and the return loss looking into the "attenuator" cable becomes
more like 30 to 40 dB.
73,
Gerald K5GW
In a message dated 2/4/2010 2:50:49 A.M. Central Standard Time,
jim@audiosystemsgroup.com writes:
On Wed, 3 Feb 2010 20:24:47 EST, TexasRF@aol.com wrote:
> For example, a 100ft length of RG213 has about 10 dB loss at 432 MHz
That would be some pretty awful coax -- Belden's RG8X (9258) is only
6.6dB/100 ft at 400 MHz, and a decent RG8-sized coax is on the order of
3dB/100 ft.
Yes, an infinitely long line looks like a resistor equal to its own Zo,
even with an open circuit as a load but it takes more like 1,000 ft to
approach that at 432 MHz.
73,
Jim K9YC
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