Hi Ken, the statement "low swr does not indicate resonance" is a bit
narrow. Perhaps "does not unnecessarily indicate resonance" would be more
accurate.
If the feedline has a high loss, the swr would be low even with a highly
non-resonant load connected
If the feedline loss is low, then minimum swr would coincide with
resonance. Using your example of 30 ohms, at resonance the resulting 50 ohm
vswr
would be 1.67. Moving off frequency until there is 10 ohms +x results in a
vswr of 1.77 (ignoring any change in the 30 ohm value). Moving off frequency
until there is 10 ohms -x also results in vswr= 1.77 (also ignoring any
change to the 30 ohm value).
Minimum vswr then indicates resonance in that system. As you say, resonance
does not necessarily provide a low vswr. Resonance does provide minimum
vswr at the frequency in use.
73,
Gerald K5GW
In a message dated 12/22/2011 7:09:30 P.M. Central Standard Time,
wa8jxm@gmail.com writes:
A low SWR does not indicate resonance.
Resonance does not necessarily provide a low SWR.
A true quarter wave resonant vertical over a perfect ground plane has a
feed impedance of about 30 ohms and therefore a 1.6:1 SWR "at resonance"
(assuming 50 ohm coax).
Seems to me, IIRC, radials for a ground plane should be 5% longer than the
radiator (check the Antenna Book to be sure.)
Ken WA8JXM
On Dec 22, 2011, at 5:23 PM, Tom Osborne wrote:
> Hi All
>
> I just put up an elevated 40 meter vertical. The feedpoint is around 15
> feet or so up and I have 8 radials on it right now. I cut the radials
the
> same length of the vertical section. I have a coil of around 8 turns of
> coax for a choke.
>
> It works good, as the first day I put it up, I worked some Europeans
from
> here in Oregon.
>
> The thing that seems strange is that at the low end of 40, the SWR is
around
> 1.3:1. As I go up the band, it goes up to about 1.5:1, then when I get
to
> the top of the band, it goes back down to 1.2:1.
> Is that because some of the radials are resonating at different parts of
the
> band?? 73
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