Re: LF: Big vs small RX antennas

["Dexter McIntyre W4DEX" <dmcintyre@att.net>]

The first 136 kHz US receptions of G3AQC and MØBMU made nearly two
years ago were made with my 160 meter dipole.   The dipole is
supported with a 1 meter side arm from the 95 foot level of a 100 foot,
insulated guy wire tower.  The home brew open wire feed line was fed
as a single wire with a series pot core inductor to bring the antenna to
resonance.  Later test were made comparing the TA signals received
with this antenna to a 3.3 square resonant loop.  This loop was matched
with a single turn pick up loop.  The two antennas were comparable
for 136 kHz reception most of the time.

Last year I constructed a new receive loop, octagon in shape, twenty turns, a little over 3.3 meters diameter.  This loop is center tapped and feeds a balanced preamp.  Every comparison I made showed this balanced fed loop to outperform either the old loop or the T vertical wire.  I'm a firm believer in the balanced loop design.

W4DEX
www.w4dex.com

MarkusVester@aol.com wrote:

Hi John and LF group,
 

Just curious -- since the signal to noise ratio in my receiving installation
appears to be limited by external (man-made and atmospheric) noise, rather
than noise in my preamp/receiver, how would a 100 foot tower improve that
situation?

John Andrews, W1TAG

A big vertical does help a lot against local noise-sources (neighbours' TVs and SMPSs), but with anything originating further away than a couple of 100m, it makes absolutely no difference.

However the directivity provided by a magnetic receive loop can be valuable. If all of the noise was coming in isotropically from the horizon, the figure-eight pattern would theoretically have 3dB better SNR than a vertical, and a cardioid combination would gain 4.8dB. In practice, of course one can often null out a source of QRM or a thunderstorm front and have far greater improvement.

The only problem I am having with small loops is that they seem to be much more prone to local pickup than the E-field antenna - at least in my suburban area which has underground mains wiring. The 86cm-diameter pair in the garden is often swamped by notorious 100Hz-modulated carriers which at the same time I can hardly see on the marconi.

73 de
Markus, DF6NM