Hello Luis!
Вы писали 7 сентября 2005 г., 1:51:10:
> As this feedline is not buried but comes down through the roof, I inserted
> two common mode chokes (10 turns of RG58 through two FT140A-J)
> with the ground tap in the center (4 x 5000 mu, high permeability toroids).
The placement of this chokes, which (I hope you have few of them for
RX feedlines) and what quality of ground rods used, may vary its
properties from FB to INCREASING noise - been there, seen that ...
> the S/N ratio on the Flag is horrible.
What is more interesting is F/B ratio of your antenna!
If it is "normal" i.e. over 20 db, then you have problem with the SOURSE of
noise, If
not - then first you have to deal with Flag.
S/N and F/B is quite a different issues of course.
With FLAG antenna you REALLY need some adjastable terminating resistor
(Vactrol is the most simple form), I had a Flag on a crank up mast and
it was quite OBVIOUS that I had to readjast Vactrol every time I rised
or lowered Flag, and the optimum (on maximising F/B ratio) height was
5-7 meters by the way. The needed resistance range was from as low as
700 to 1100 ohms as I recall. All the testings was on local BC at
1134Khz (quite stable signal some 20 km away) and confirmed on 80
meters. Propely terminated flag shows about 24-26 db of F/B on a
ground signal.
Local NOISE SOURSE could be closer to the Flag then to any other of
your RX antenna and 'spread in direction' so it will actually mask
antennas real F/B and degrade its helpfulness.
We do have to carefully choose the antenna location ...
> I will let the Flag on for the winter season, hoping in any kind of miracle
> on the power line but I am afraid that this antenna is useless in this
> environment!
From my experience Falg can be used to REJECT the noise source only
and only if it is a single direction source. For getting some
directivity and more gain K9AY arrays are much better of course.
> Now this is my question:
> - but if I use a preamplifier in the switching box I must ground it, and
> also the feedline and than connect on the same ground the low impedance
> side of the transformer, is it right?
In my case transformer high impedance side ground connects to its OWN
ground rods and I use few radials too.
Transformer low impedance sides: (signal) connects to amp. input;
(earth) to amp. common;
Amp. output: (signal) to coax cenral;
(common) to coax shield;
Amp. power could be sent through the separate wire or coax`s central;
And you need to GROUND this coax`s shield some distance away from antenna
ground as cleraly shown on http://www.w8ji.com/common-mode_noise.htm.
> I have both the FT140-43 and FT114-77 cores: which do you suggest ?
FireRite binocular BN202-73 is very nice, 3 and 9 turns windings is
all you need.
--
Andrei,
RA6LBS mailto:ra6lbs@volgodonsk.ru
WWW.RA6LBS.RU
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|