[Elecraft] K-1 Sensitivity

[Bill Coleman]

On 3/14/02 9:44 PM, James R. Duffey at jamesd1@flash.net wrote:

>"What happens to this discussion if we consider using narrowband (eg
>High-Q) antennas?"
>
>As long as all the stages in a receiving system are linear, from the antenna
>to the earphone (loudspeaker), the frequency response of the entire system
>is the frequency response of each individual stage convoluted together.

Part of the problem is that every stage isn't linear. If every stage were 
linear, we wouldn't talk about blocking IMD, now would we?

>What
>this means in plain language is that the narrowest stage will determine the
>selectivity of he entire system and hence the bandwidth over which the noise
>is integrated.

OK.

>I use 300 kHz for my FL-3 setting on my K-1. For the antenna
>to play a role, it would have to have a bandwidth narrower than this.

My FL3 only goes as wide as about 2.5 kHz. Is there a special mod for 300 
kHz bandwidth? <grin>

>For
>the ham bands, most antennas have a significantly higher bandwidth than
>this. At 7 MHz, an antenna with a Q of 350 (corresponding to an antenna
>loaded with a very good low loss coil) has a bandwidth of 20 kHz, so the
>bandwidth is still determined by the receiver. At 160 M, the bandwidth is 5
>kHz, and the receiver still determines the bandpass over which the noise is
>integrated.

Understood. I was just reading your message about sensitivity, and it 
appeared that the Q of the antenna might make a difference.

As an example of how antenna (or input network) Q might affect receiver 
performance, consider the use of today's 2m radios. Many have wide-band 
front ends and often succumb to intermodulation products with nearby 
commercial services. Wheras 25 years ago, most 2m radios had rather 
narrow band front ends. My first 2m rig was an Icom IC-230, which had 5 
helical resonantors on the front end. Intermod was completely absent.

However, as far as sensitivity is concerned, it's not an issue. With 
sufficient bandwidth, the low HF bands are pretty much noise-limited 
rather than receiver-limited. At high HF or VHF (or higher), the picture 
becomes a lot different.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901