Electronics > RF, Microwave, Ham Radio

Log-period antenna for EMC measurements?

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nctnico:
New day, new EMC problem to tackle... I stumbeld onto an interesting problem where a product as a whole is radiating at several frequencies. A pre-compliance test performed outside a shielded room shows a lot of spikes which exceed the signal strength of the local FM stations and LTE signals. So I pulled out my own 80cm Bowtie antenna (broadband dipole) to get a baseline myself but none of the spikes shows up while the local radio stations and LTE bands show up just fine well beyond the noise floor. So either the problem has magically dissapeared or my measurement is wrong.

The pre-compliance scan was performed using with -what looks like- a log period antenna. Now I'm wondering: does such an antenna have a better directional sensitivity compared to my bowtie antenna? As in picking up more signal from the DUT and less noise from the environment?

ahbushnell:
Yes I would assume it would have higher gain.  How big is your bow tie and how long are the elements on the log antenna? Maybe it operates at a lower frequency? 

nctnico:

--- Quote from: ahbushnell on February 17, 2023, 07:02:33 pm ---Yes I would assume it would have higher gain.  How big is your bow tie and how long are the elements on the log antenna? Maybe it operates at a lower frequency?

--- End quote ---
The bow tie I have is about 80cm in size. Basically 2 triangular pieces of PCB material. It should be good from 80MHz and onwards (IIRC) but I don't have trouble receiving signals in the 30MHz range. My main concern is not so much about sensitivity for certain frequencies, but picking up signals coming from a particular direction.

Or put differently as a more general question: Has a log antenna, with the same frequency range specification as a bow tie antenna, a better reception of signals when pointed at a signal source a few meters away?

T3sl4co1l:
Yes, log periodic has low but useful gain, ballpark 6-10dB with common designs.  How much, depends on the... angle of the array, I think?  Basically, think of it as a Yagi with frequency range spread out by the wider range of element lengths; at any given frequency, a few adjacent elements work together (and also harmonics in longer ones) as a small Yagi embedded in the overall array.

Tim

nctnico:
I'm looking at this antenna:
https://anjo-antennen.de/Antennen/LPDA/LP1351G3S

This is available from a local dealer and not super expensive. I just don't want to waste my money  8)

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