RadioBanter - View Single Post - Sangean ATS-909 external antenna impedance??

Lostgallifreyan · December 20th 09, 09:02 PM posted to rec.radio.amateur.antenna

(Dave Platt) wrote in :

What limits your ability to receive, under these conditions, is band
noise and other spurious signals, which "drown out" the desired
signal. Improving the antenna matching wil have little or no benefit
in dealing with external noise. Neither will a preamp.

Point taken, at least with the matching vs noise. Others have said it won't
matter much and I see why. preamp might be another issue though, especially
in light of 'amdx's post:

"http://www.kongsfjord.no/dl/Antennas/The%20Best%20Small%20Antennas%20For%20M
W,%20LW,%20And%20SW%20rev%202.pdf

The start page for that link is here;
http://www.kongsfjord.no/dl/dl.htm"

Now, using coax as a way of excluding some of the close-in interferers
isn't a bad idea at all. What you might want to consider doing, is
adding an active buffer at the longwire-to-coax feedpoint...
basically, a low-gain preamp (a few dB) designed to drive a 50-ohm
load efficiently and to overcome the (slight) signal losses in the
coax. Some people seem to favor the grounded-base or grounded-gate
broadband amplifier, as it has a modest gain, can be quite stable, and
has a good resistance to strong-signal overload. You could use a DC
inserter/block system to feed 9-12 volts up the same coax which
carries the signal down.

This is a good idea, though maybe the one I learned and reposted might do
better, it has a very attractive aspect, as it drives a simple speaker wire.
That stuff is cheap, (even the heavy stuff is cheap and durable, I use it
already for solar power..), and effectively forms a very low loss balanced
current loop that is isolated entirely from any current path anywhere else. I
wouldn't even have to bury the feed line, I could likely just run it along
the guide wire in a nearby chainlink fence or whatever is convenient because
it will very effectively reject common mode noise. It might still benefit
from a preamp current driver at the far end but as it uses a low impedance
input at the near end to take care of common mode noise it should be fine,
and a lot easier to get power to.

If you're really concerned about matching the resulting coax-carried
signal to your radio's antenna input, you could wind a fixed-ratio
broadband transformer with the correct impedance ratio.

Might try that, though more as an effort to extend learning than any need.
Right now I'm really glad I asked here because these answers are good, and I
now seem to have a better idea (the vertical long whip and balanced feed and
Norton preamp scheme) than the longwire-and-balun-and-coax I'd originally
intended. I haven't yet verified the new idea but it appears to be from
someone who has demonstrated capability among a group of radio hams, and it
certainly fits with what I know without contraindications, and it would be
much easier to arrange safely and discreetly than any other scheme I've seen.
The author even states permanently retiring his longwires and inverted L's
and so forth in favour of a phased array of two of the vertical whip. I
haven't got the 60 feet of space for the two he specifies, but I think I'll
do ok with one.