Topband: Re: Topband receiver dynamic range

[S56A]

160m RX dynamic range is a very narrow topic which is certainly not on the top
of the performance goals for hamradio rig manufacturers.

Mauri, I4JMY writes:

>I believe that a receiver using an high level balanced mixer (better if
>passive) and without a preamplifier has a decent noise figure for 160m (even
>using a beverage) and shouldn't suffer an overloading unless the antenna
>picks up much more than milliwatts by the interfering signal.

You don't have to believe, just add 6 dB diode mixer loss, 6 dB for a good 160m
input filter and IF stage with modest 3 dB noise figure and you end up with 15
dB RX NF.  This is at least 30 dB more then required on 160m band polluted with
atmospheric and man-made noise.

Modern solid-state HF RX are all over-sensitive with NF as low as 5 dB which is
good enough even for 2 m band.  Heavy price is subsequently paid on third order
intermodulation immunity.

Cheap FT-840 with reasonable -129 dBm "lowest" sensitivity becomes a clear IMD3
ARRL winner!  I am intentionally skipping the subject of VCO noise as I prefer
my rig to be noise-limited.  Unfortunate SSB higher order TX intermodulation
products bellow 2 MHz in wideband PA shouldn't affect close 160m operation that
much as near field drops with cube of distance.  Good DX low angle receiving
antenna certainly helps...

Otherwise IMD3 at ranges closer then 20 kHz has been Measure-d by RSGB (English
magazine!) and DARC for ages...  There is much more QRM in EU!

73 de Mario, S56A, N1YU




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