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finn.usa(at)gmail.com
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nuckolls.bob(at)aeroelect
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 Posted: Thu Jul 16, 2020 9:41 am Post subject: DDRR as Comm antenna |
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At 09:28 AM 7/16/2020, you wrote:
Looking into the antenna's feedline connector, the
observed or measured resistance is mad up of three
components: (1) ohmic resistance, (2) radiation
resistance, and (3) reactance.
Ohmic or metallic resistance is that which emulates
an ordinary resistor . . . when it comes to resistors,
the can do only one thing . . . get warm. I.e. convert
electrical energy into heat. This is the analog to
'dummy loads'; devices that allow you to test a transmitter
while minimizing if not eliminating all radiation.
In antennas, ohmic resistance equates to dissipated
energy.
Radiation resistance is that component of the antenna
that launches energy into the environment. Suppose
your antenna shows 1:1 SWR (no reactance) at 50
ohms but 40 of those ohms are ohmic and only
10 ohms are radiation resistance. Then 80% of
energy at the end of the feedline goes up in heat;
20% gets radiated.
In the 'perfect' antenna, reactance is zero at
resonance and the resistive components add
up to 50 ohms; i.e. 1.0:1 SWR and ohmic resistance
is near zero (impossible dream).
But check out the two comparative plots of an
experimental antenna evaluated with a VNA Tiny
analyzer:
The thin spike antenna is a simple, 1/4 wave
ground plane with no attempt to achieve 50
ohms resistance at the feed point. Note
that SWR never gets down to 1:1 [3] because
it's about +6 ohms (Xs inductive) at resonance.
1.5:1 bandwidth [1]->[2] is 67Mhz
The blade antenna is also 1/4 wave long with
no special attempt at hitting a 50 ohm feedpoint
impedance. This antenna has two swr 'dips' and
a measured 1.5:1 bandwidth of 104Mhz. Here we
see better SWR conditions at two points
where we also see lower reactive components
at [3] and [4].
Tese studies only go to the antenna's
ability to accept energy squirting out the
end of the feedline. It says NOTHING about
efficiency or radiation pattern. But with
simple, 1/4-wave radiators on clear-space
ground planes there is little that needs
to be done in those departments.
If you're interested in pursuing the DDRR
approach, it would be easy to do on your workbench
with a VNA. The antenna is small and easy to
fabricate. You could explore ways to increase
bandwidth.
| Quote: | | I'm a hopeless novice on antennas. |
We all were in the beginning . . . No better
way forward than to get out the hammers-n-saws
and MEASURE stuff!
Bob . . .
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6p0_spike_over_ground_plane.pdf |
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