TopBand: Re: Loops and preamplifiers

[P&V Nesbit]

At 11:06 1/10/1998 -0400, Paul W5DM wrote:
>
> Is it practical, or desirable, to match
> the very low impedance of the loop to the coax?

Yes. Every dB of improved matching is 1 dB lower system noise figure. You
can compensate for poor matching to a certain degree, by adding more
preamplifier gain, but depending on the external noise level, it can be
impossible to achieve external noise limiting if the matching loss exceeds
a certain threshold. Since the output level of most loops is already quite
low, you don't want to add any more loss than you have to.

For a noise floor degradation of 1 dB, the system NF (including antenna)
must be at least 6 dB below the environmental NF, which can be calculated
from CCIR322. At a quiet site with a broadband antenna and moderate
mismatch, this can mandate a combined receiver and preamplifier NF as low
as 4-5 dB. If the mismatch loss is too high, you could theoretically need a
negative NF at the receiver/preamplifier, just to maintain external noise
limiting!

This is why high loss antennas like loops and active whips are rarely seen
at the highest performance receiving sites, at least for general reception.

As for preamplifiers, the extra effort involved in building one with
transformer feedback, rather than just a simple monolithic IC, will be
rewarded with a significantly lower NF and higher intercept point.

73,
Peter VK3APN
pnesbit@melbpc.org.au


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