[Lf] RE: [Lowfer] Big loops question for LF 135Khz to 190 Khz

[Ashlock,William]

Hi Paul,

Stewart N. did a good job of answering your three questions. Here's my two
cents from a slightly different angle:

The first step in any antenna design is to determine how much signal you
need from your antenna. I used to think 'the more the better' and this is
not true and can get you into needless problems. The signal from the antenna
does not need to be greater than 20db above the equivalent input noise of
your receiver on the quietest of quiet winter nights - or even 12 db should
be enough margin. This "signal" can be considered the actual atmospheric
noise on this quite night and will vary considerably from site to site. The
noise level from my main antenna on my quietest suburban nights is typically
2uv. Possibly some of the guys living in ideal receiving areas using the
same antenna would see 20db lower noise than this.

At my location I can read the equivalent input noise on my SV /receiver
directly off the S-meter with no antenna connected and it comes to approx
0.02uv. I can therefore read signals from the antenna down to 0.2 uv and it
would be unnecessary construct an antenna with more output. In fact I have
20db more antenna signal than I need - but at this location only, and it
could be marginal at your location.

A fairy simple way to measure equivalent input noise for a typical receiver
is to input a calibrated signal from an RF signal generator and determine
the point where it is lost in the noise. A low cost approach would be to
build a multiple stage 60db attenuator using carbon resistors calibrated at
a much higher signal level - say 1v, tweeking the final stage to 1mv using
an oscilloscope. A way of pulsing the signal from the generator to simulate
CW is useful.

Your loop antenna's the output voltage of course is based on diameter,
number of turns, and Q, and can be calculated from the math given in the
ARRL Antenna Book to allow you to determine what combination is needed based
on your receiver's equivalent input noise.

Happy Looping!

Bill WA
 
-----Original Message-----
From: Paul A. Cianciolo [mailto:paulc at snet.net]
Sent: Sunday, October 22, 2000 11:32 AM
To: lowfer at qth.net; lf-amrad; AMRAD Tacos; lofexp at egroups.com
Subject: [Lowfer] Big loops question for LF 135Khz to 190 Khz


Hello Folks,
At a recent fleamarket I found some really nice 4' fiberglass poles
that interlock with each other.  These poles are very strong.  I have
decided to replace
my old air loop, which is somewhat smaller,  with these longer poles as
supports.
Each turn on this new loop would be about
35' around. I also have 2' wide fiberglass spreaders with notches cut into
them for
wires abouts about 1" apart.   With this spacing a very high "Q" is
obtained.
This also adds a a great deal of complexity to the loop construction, and
mounting.


I have also  built multiturn loops without the spreaders that seem to work
well but do not have as
high a "Q".  Also it seems to take a lot more capacitance to turn this type
of loop.


 Here are my questions.

1) In the end as far as sensitivity is concerned is the wide spaced loop
worth the extra trouble?

2) Does the the higher "Q" increase the signal voltage extracted from the
the air?

3) I would like to hear general opinions before I start the largest loop I
have built so far.


All comments good or bad are indeed welcome


Thank you


Paulc
W1VLF