I have the DX Engineering preamp that I use with the DX Engineering portable
flag and can confirm the 4.5 dB noise figure. Using the methods I described
here in an earlier post, I measured the noise figure as between 4 to 5 dB.
Also I measured the gain (S21) from 0.1 to 30 MHz with my VNA-2180. It
comes out to be 32 dB at 1.8 MHz. Overall the gain response is essentially
the same as what WD8DSB measured on a prototype of the preamp and reported
on his Web page (https://sites.google.com/site/portableflagantenna/). There
is a broad dip in gain of about 1 dB in the middle of the HF range, but it's
inconsequential.
Finally I measured the input impedance (S11) of the DX Engineering preamp.
It varies a bit with frequency but when reported as SWR relative to 50 ohms,
it ranges between 1.2:1 and 1.5:1 over the HF range. It will be a fine
match for the flag which has a 50 ohm feedpoint impedance.
Overall I think these are very good numbers for a relatively inexpensive
preamp that is portable and battery powered. (Disclaimer: I have no
affiliation with or commercial interest in DX Engineering).
73, John W1FV
-----Original Message-----
From: Topband
[mailto:topband-bounces+john.kaufmann=verizon.net@contesting.com] On Behalf
Of Don Kirk
Sent: Tuesday, March 9, 2021 6:22 PM
To: topband
Subject: Re: Topband: Portable Flag follow up (DX Engineering preamp noise
figure update)
I should have said the so the 4.5 dB noise figure is certainly not
unreasonable.
Sorry for the typo.
73,
Don (wd8dsb)
On Tue, Mar 9, 2021 at 6:20 PM Don Kirk <wd8dsb@gmail.com> wrote:
> A couple of weeks ago there were a lot of postings about my portable flag
> for radio direction finding (for tracking down RFI), and someone asked
> about the DX Engineering preamp that was designed for use with my portable
> flag and specifically what the noise figure was for this preamp. Tim
> (K3LR) said DX Engineering would measure it, and today he reported it
> measured 4.5 dB.
>
> Tim mentioned that there were trade offs in the design such as low current
> draw and high gain, so the 4.5 dB noise figure is certainly unreasonable.
> It really is the only preamp I now use with my portable flag, and very
> pleased with how it works.
>
> Just FYI,
> Don (wd8dsb)
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