Log-period antenna for EMC measurements?

[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]

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?

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 

[TimFox]

Remember that "antenna gain" is not actually power gain such as in an amplifier, but the difference between the "forward" response and that of a theoretical isotropic radiator.
If your directional antenna is pointing far from the direction of the radio station, it will show less signal than would a less-directional antenna with respect to other sources.

[nctnico]

Meanwhile I got a bit further. The readily made antenna I linked to earlier is a bit large and doesn't have the biconical 'antlers'. After some googling I found this website https://hamwaves.com/lpda/ which can produce a recipy to make your own log-period antenna. I have used these parameters to get to a trade-off for reasonable gain, not too many dipoles and reasonable size:

INPUT
  Lowest frequency f₁ = 200 MHz
  Highest frequency fₙ = 1500 MHz
  Diameter of the shortest element ⌀ = 6 mm
  Characteristic input impedance Zc_in = 50 Ω
  Taper τ = 0.880
  Optimal relative spacing σₒₚₜ = 0.163
  Chosen relative spacing σ = 0.085


And I ended up with this:


Still a bit larger than what I had hoped for though but I wanted to get a decent coverage from 30MHz and onwards. I also found out there is a flaw in the LPDA calculator website. It calculates the total length of the dipoles but that is only for one half of the antenna. I started with buying 8 meters of pipe for the dipoles but ended up needing nearly 13 meters. It is amazing how much length goes into an antenna like this. I used 15x15mm aluminium square tubing for the booms and 6mm round tube for the dipoles. All in all it is pretty light. I soldered the dipoles to the booms using aluminium solder I bought from Harbour Freight (I have visited all kinds of shops in the US while on holiday  ). No, this is not my best aluminium soldering job; I put this antenna together in a couple of hours.

For the antlers I used 8mm tube because all shops in the area are out of 2 meter lengths of 6mm tube 

I did some comparative testing against the 80cm bow-tie antenna I made a couple of months ago. The log period antenna is way more sensitive. I made this scan from 5MHz to 150MHz and it shows the log-period antenna is 25 dB to 30 dB more sensitive. I used a 27Mc antenna to transmit the test signal coming from the network analyser + amplifier.


The blue (noise floor) and purple (received level) traces are measured using the bow-tie antenna. The yellow (noise floor) and orange (received level) traces are measured using the log period antenna. Probably I can optimise a bit where it comes to directivity but I didn't try that yet.

I have put my new toy to good use already. I needed to measure some emissions from a device in the 900 MHz range and I could pickup the spike between the LTE bands just fine (even though the LTE signal strength is a bit higher). With the bow-tie antenna I needed to use a pre-amplifier in order to get enough signal strength into the spectrum analyser.