Radiation pattern of crt tv with small loop antenna

[mishima]

I was playing with an old crt TV and a small loop antenna earlier and noticed that when I hold the antenna vertically in front of the TV I get one solid spike at around 15 kHz. When I hold the antenna horizontally I get a multitude of spikes that are not there when I turn off the TV.

This suggests to me that the field lines in the space inside my antenna are oriented in such a way that the magnetic field is horizontal, and the electric field is vertical. This is since the small loop antenna is affected most by magnetic fields, and they must be passing through the loop to induce current. Is that correct?

 

[berkeman]

I was playing with an old crt TV and a small loop antenna earlier and noticed that when I hold the antenna vertically in front of the TV I get one solid spike at around 15 kHz. When I hold the antenna horizontally I get a multitude of spikes that are not there when I turn off the TV.

This suggests to me that the field lines in the space inside my antenna are oriented in such a way that the magnetic field is horizontal, and the electric field is vertical. This is since the small loop antenna is affected most by magnetic fields, and they must be passing through the loop to induce current. Is that correct?

Assuming the B-field loop probe (often called "sniffer" probes) is made correctly, then yes, you are sensing the AC magnetic field that is caused by AC currents. Here is a good intro to sniffer probe construction and use:

http://www.compliance-club.com/archive/old_archive/030718.htm

If you are not building the probe correctly, then you are sensing a mix of AC B & E fields, which is generally less useful.

 

[mishima]

Nice, thank you for the very cool link. Currently my antenna is about 50 turns of wire on a wooden frame, those small sniffer probes would be very fun to build/use as well.

 

[berkeman]

Nice, thank you for the very cool link. Currently my antenna is about 50 turns of wire on a wooden frame, those small sniffer probes would be very fun to build/use as well.

The single-turn, E-field shielded B-field probes as shown in the article generally require a preamp of 20-40dB before the signal is visible on a spectrum analyzer. You might be able to E-field shield your multi-turn coil if you follow the overall tips in the paper. The E-field shielding is necessary to keep capacitively-coupled signals out of your probe signal.

 

[alphy]

Yes, your observation is correct. The radiation pattern of a CRT TV with a small loop antenna is determined by the orientation of the magnetic and electric fields. When you hold the antenna vertically, the magnetic field lines are parallel to the antenna and induce a strong current. This results in a single spike at 15 kHz, which is the frequency of the TV signal. When you hold the antenna horizontally, the magnetic field lines are perpendicular to the antenna and do not induce a strong current. Instead, the electric field lines are parallel to the antenna and induce a weaker current, resulting in multiple spikes at various frequencies. This is due to the fact that the small loop antenna is most sensitive to magnetic fields and not electric fields. Overall, your observation is a good example of how the orientation of an antenna can affect its radiation pattern and sensitivity to different types of electromagnetic fields.