nuckolls.bob(at)aeroelect
Guest
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 Posted: Mon Aug 03, 2020 4:21 pm Post subject: SWR minima not at resonance? |
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Followup:
Earlier I wrote:
>Generally speaking, an narrow-band antenna can have
>a perfect or even low SWR at only one frequency.
>That 'dip' in SWR may or may not be at the
>antenna's resonant frequency.
A Lister wrote that he thought lowest swr was
the primary manifestation resonance. I told
him I would elaborate with a posting to the
List.
Here's an exemplar plot of characteristics
for one of several communications antennas
I evaluated for use on our local EMS/
LE/FIRE services:
Note that the SWR minima is at 493.6Mhz while
the reactive component swing (resonance) is
at 457Mhz. At lowest SWR, the impedance of
the antenna is 56.9 Ohms, resistance is 56.6
Ohms, reactance is 5.8 Ohms inductive.
The 56.9 Ohm impedance produces the not quite
perfect but still satisfactory value of 1.18:1.
One COULD craft a matching network that would
yield 1:1 somewhere . . . but it would still
rise either side of optimal. In the illustrated
case, the 1.5:1 bandwidth (markers 1 and 2) is
66Mhz. NOT centered on our frequencies of
interest in the 460 to 470Mhz but still quite
satisfactory for task at hand with SWR of much
less than 2:1 over the range.
This is the kind of data you can get from
a Vector Network Analyzer, a test tool discussed
here on the List a few weeks ago. Quite
illuminating when it comes to matters of
feedlines and antennas.
The antenna Joe is working with will hopefully
offer 2:1 or better over his range of interest,
118 to 132Mhz.
Bob . . .
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