One thing to remember about a multi-band receive antenna is that it
need not be an efficient antenna on the lower bands. It only needs to be
efficient enough to receive the ambient noise level a few db above the
receiver noise floor. On the lower bands, the recovered signal-to-noise
is determined by the ambient noise level. Indeed, having an inefficient
antenna may result in less absolute signal amplitude and therefor a
greater SYSTEM dynamic range.
An in-efficient low band antenna would not transmit very well but
would likely hear anything that is to be heard above the ambient noise.
My multi-band 20-6m vertical still provides an ambient noise floor
>10db above the receiver noise floor on 80m, and hears anything I can
hear on my 80m antenna. ( YMMV ) It's a horrible 80m transmit antenna.
Al, K0VM
On 7/31/2012 9:54 PM, George Allen wrote:
Gerald provides a very interesting discussion of antennas in the FlexInsider
Issue 2; but, what is wrong with just paralleling antennas for different bands
together?
Lets suppose that we wish to monitor both 20 meters and 80 meters at the same
time. If we have two antennas that are resonant respectively on 20 and 80
meters and we parallel them together, the receiving SCU's should be happy.
Yes, even though there is a high impedance for the out-of-resonant antenna,
the receive losses will be low and we will get signals from both antennas. But,
nothing bad should happen.
At transmit time, if I transmit to the paralled antennas on 20 meters, the transmitter
will see a high impedance on the 80 meter antenna and llittle energy will go out on that
antenna. The 20 meter antenna being resonant, will accept the transmitted energy. The
reverse holds true if I transmit on 80 meters. This is the same principle that makes the
multi-band "fan dipole" work, where a single feedline feeds multiple resonant
dipoles. Perhaps decoupling stubs might be needed for each antenna in this case.
Am I all wet? Perhaps someone with a better knowledge of transmission lines
and antennas can challenge and clarify this.
George
K2CM
.
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[email protected]
http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz
This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for
posting topics related to SDR software development and experimentalist who are
using beta versions of the software.
