[hpsdr] Proposal for Loop Antenna Project

[brian at kf6c.com]

I tried to post these two message a few day ago but my outgoing emails
seam to go nowhere, dispite a report that all is well from outlook.

First Post ref Guido’s post


If the loop were to have the same Q at 30 MHz as 3 MHz the bandwidth would
also increase by 10 times. Tuning in 10 KHz steps would not be necessary.
The Q of the loop antenna is going to reduce significantly as the
frequency is increased for the same size loop further increasing the
bandwidth.


Taking a loop 1 meter square as an example it would (ignoring the Q of the
capacitor) theoretically  have a Q of about 40, giving just under 1 MHz
bandwidth. The same loop at 3 MHz would have a Q of about 2000 giving a
bandwidth just over 1 KHz.


Brian KF6C



Second post



It seams to me that the real target of the proposal is to build a module
that will tune a highly selective antenna automatically from the
receiver/transmitter.



The basic result of using a small antenna is that the Q will rise and
hence the bandwidth fall as the wavelength to size ratio increases.



There is the obvious advantage to this for the receiver that the
selectivity at its furthest forward point has increased. If the losses
that are encountered in achieving this higher Q are less than, the
difference between the atmospheric noise and the receiver noise figure,
the overall receive performance is better. This is not taking into account
any  antenna directionality.



>From a transmit point of view there are more disadvantages than
advantages. However the one of the antenna being smaller is for many an
overriding issue. The practical small antenna has resistive losses that
are often hard to reduce. The higher the Q involved the higher the
voltages developed, resulting in higher cost and more engineering effort.



The project is an excellent complement to the SDR as places selectivity in
the best possible position, lessening the advantages of analog processing
ahead of the DSP.



The big challenge is make the module versatile enough to interface with as
many forms of small high Q antenna as possible.







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