Thursday, May 01, 2008

A multi-turns magnetic-loop for the top band.

Here is a description of the magnetic-loop that I am using for the 160m band. As shown on the plots published in previous posts, this is the receiving antenna (from many I have tried) wich provides the best rejection of the white band noise generated by the 380kV power line, passing some 200m from my antennas. It is made of 6 turns of RG58 cable (only the copper shield is used) on a PVC structure. The square has 0.8m by side, so 20m of cable are required. The loop is loaded by an adjustable capacitor (required value about 65pF), the coupling to the feedline is made through a small delta loop, matched with a 1/4 balun (200 ohms port to the bottom of the delta). The magnetic loop is made of 6 turns of RG58 cable on a PVC structure. You can see the power line behind ! Best power line noise rejection is obtained with the loop a 90 degrees. The measured line noise rejection is approximately 12dB, enough to make stations to emerge... You can see the loop bandwidth on the 756 spectrum display. The horizontal scale is 20kHz/div while the horizontal scale is 10dB/div. Very narrow ! For my noisy location, using the 20dB transceiver preamp, provides enough level to get the noise floor to S4 with a 2.4kHz filter, so no additional preamp is required.The MMANA-model, seems to match quite closely, the real antenna. The measured SWR is about 1.5 at resonnance, good enough for a receiving antenna... On the model the capacitor is at the top of the loop (just beacause it was easier to draw!). On the real one the capacitor is in the small electrical box at the bottom of the loop (it does not change anything !).The simulated pattern. The horizontal pattern is for the loop at low height above an average soil for 5 degrees elevation. At higher elevations, the pattern becomes an oval with low directivity, so the directivity is only useful to reject punctual sources of noise arriving at low angles, or local stations on groundwave(front to side rejection is about 30dB on the horizon). The difference between the theoretical rejection and the measured one can be explained by the fact that my noise source is not punctual...

7 comments:

Piera said...

Good post.

F6IRF said...

I must add, that though the loop provides a great improvement on local stations, it does not work very well for DX. For DX'ing the K9AY loop is much better.
Pat

F6IRF said...

A recent test I have done on the OK0EV 160m-beacon can be seen here http://tinyurl.com/OK0EV-at-SS
It shows up to 20dB improvement vs the vertical.
This antenna also works quite well sometimes on DX pathes: Last night I could decode T61AA WSPR signal (-11 dB SNR at best, 1 h before his sunrise), while I had no signal on the vertical...

F6IRF said...

Last experimentation: the loop can also be tuned to the 504Khz band. It requires a capacity around 1390pF.
This indicates an inductance value of approx. 72uH

F6IRF said...

More about my 504kHz novice experimentations at
http://wsprnet.org/drupal/node/1041

F6IRF said...

The value of the required capacitor is slightly higher than the value calculated by MMANA. With 100pF +/-2% (steafix -silver mica) the loop resonnates on 1790kHz. So The right value, is something around 80~90pF (I now use a 40pF trimmer in // with a 68pF fixed mica).
By the way, when trying to find the right value for the 600m band, I got on a Radio-France medium wave TX (on 600kHz). This was during the day, so was on "groundwave". In these conditions, the directivity is amazing with a very deep nul, when the plane of the loop is perpendicular to the transmitter direction.

Anonymous said...

Very, very interesting information. The comparation between a vertical ant. and the loop with WSPR report is a great info. Congrats for your inner experience in 500khz... and wish you enjoy it.
73 from Santander
EA1DZM