Noise bridge used to measure resonance of antennas

[Bill Coleman AA4LR]

On 11/20/99 13:24, Huttinger at Huttinger at FOOTHILL.NET wrote:

>I measured the
>SWR at both ends of the bands (and other points) this vertical covers using
>an ordinary SWR meter and noticed that the SWR minimum points were quite a
>bit different from what the noise bridge indicates.  When the manual speaks
>of coax "masking" the effects of the antenna from the noise bridge what
>exactly does that mean?

Simple, really. Coax has loss. The loss in a long piece of coax will tend
to "improve" the measured SWR. A 50 ohm dummy load has an SWR of 1:1, but
it is totally lossy. Same principle applies to the loss of the coax. The
more coax, the more you'll get closer to 1:1, but only because power
isn't getting to the antenna.

If you use a short length of coax to measure an antenna, you'll get
closer to the actual figures at the antenna connection itself. Problem
with connecting directly to the antenna is that your body may affect some
of the measurements on some antennas.

My limited experience with noise bridges are they aren't that sharp a
measuring device. I have had good experiences with inexpensive antenna
analysers made by the likes MFJ, Autek and AEA. While they aren't
lab-quality instruments, they can help you tune an antenna to resonance.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales

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