Designing a Loop Antenna for 25-100MHz Radiated Emissions Measurements

[likephysics]

How do I go about building a simple antenna to measure 25MHz to 100 MHz radiated emissions?
I have an oscillator in my ckt that's radiating at 75MHz. Just want to make sure it's the 3rd harmonic of fundamental frequency (25MHz).

 

[vk6kro]

You need a frequency counter or a spectrum analyser to check frequencies like this.

The antenna should not normally be connected to the output of a frequency multiplier. This is because even if the output was mostly on 75 MHz there could still be considerable output on other multiples of 25 MHz.

There are a couple of older instruments that will give a rougher indication. These are the tuned wavemeter (which is just a variable tuned circuit and a diode detector) and the gate dip oscillator which can check a tuned circuit without power connected to it.

 

[likephysics]

You need a frequency counter or a spectrum analyser to check frequencies like this.

The antenna should not normally be connected to the output of a frequency multiplier. This is because even if the output was mostly on 75 MHz there could still be considerable output on other multiples of 25 MHz.

There are a couple of older instruments that will give a rougher indication. These are the tuned wavemeter (which is just a variable tuned circuit and a diode detector) and the gate dip oscillator which can check a tuned circuit without power connected to it.

How do I make an antenna.
Why can't I look at the signals on scope. I can do an FFT and watch for peaks at 75MHz.

 

[sophiecentaur]

I found all these on another forum - worth looking at them for an antenna design.

http://www.frontiernet.net/~jadale/Loop.htm
http://www.standpipe.com/w2bri/
http://www.datacomm.ch/hb9abx/loop1-e.htm
http://www.qsl.net/mnqrp/Loop/Mag_Loops.htm
http://www.radioworks.com/nloop.html
http://www.mindspring.com/~loop_antenna/
http://www.hard-core-dx.com/nordicdx...9ay/index.html
http://www.kb6kq.com/
http://www.schmarder.com/radios/misc-stuff/loops.htm
http://www.g3ycc.karoo.net/loop.htm
http://www.alphalink.com.au/~parkerp/nodec97.htm
http://www.greertech.com/hfloop/mymagloop.html

What will your antenna be feeding? An oscilloscope? If it allows you to dump out the waveform then an fft would tell you about spectral content. But you may need to make sure that there is no aliasing and include some low pass filtering because the display may not be as fussy as you need.

 

[jim hardy]

i like the folded dipole, it's naturally 300 ohm so lends itself to inexpensive (meaning junkshop) TV antenna accessories.
and it's fairly broadband.

old jim

 

[berkeman]

How do I go about building a simple antenna to measure 25MHz to 100 MHz radiated emissions?
I have an oscillator in my ckt that's radiating at 75MHz. Just want to make sure it's the 3rd harmonic of fundamental frequency (25MHz).

How do I make an antenna.
Why can't I look at the signals on scope. I can do an FFT and watch for peaks at 75MHz.

The issue is one of signal size. The signal that you pick up with the antenna will be quite small, and you won't be able to see it on an oscilloscope without a lot of gain first. That's what using a spectrum analyzer does -- it has lots of gain, in addition to showing you what frequencies are present in the received signal.

So you need some type of preamp between your antenna and your oscilloscope. Since 75-100MHz is near the FM radio band (in the US anyway), you might be able to just use a simple TV-type antenna and preamp.

http://www.summitsource.com/images/products/AMG212.jpg

 

[berkeman]

Keep in mind, though, that you will also pick up a lot of other signals with your antenna, and it may be difficult to be sure which signals are from your device. Usually such measurements are done inside a shielded room, to eliminate the ambient RF signals that surround us.

Another option is to use a "sniffer" probe, which is a smaller probe that you hold close to your circuit to pick up the local RF. You still generally need a preamp when using a sniffer probe, though, because of the small pickup signal.

 

[vk6kro]

How do I make an antenna.
Why can't I look at the signals on scope. I can do an FFT and watch for peaks at 75MHz.

If you have an overtone crystal, the easiest way to see if it is oscillating properly is to listen with a radio receiver on 25 MHz and tune the oscillator until the 25 MHz signal disappears. There should not be any oscillation on the fundamental frequency.

In the same room, you won't need an antenna. Just a short piece of wire will be sufficient.

If you have an oscilloscope capable of operating at 75 MHz and doing FFT, then that might work, but it won't tell you if there is output at some higher frequency. There are such 'scopes available, but they are expensive.
A spectrum analyser is a device like an oscilloscope which presents a display with frequency across the horizontal axis and a logarithmic display of signal amplitude up the vertical scale.
They are very easy to use, but good ones are expensive.

A frequency counter is a good option. It is possible for an overtone oscillator to operate near the right frequency, but not be crystal controlled. So, the output is off frequency and unstable.
A 'scope will not show this, but a frequency counter will give a rapidly changing frequency display which gets worse of you move your hand near it. This will tell you the oscillator is not crystal controlled.

 

[sophiecentaur]

I think that, as is frequently the case, we don't know enough about your problem to give you any sensible answers.

If you use a short dipole (say 50cm total length) it will have a reasonable amount of 'voltage pickup but not be tuned to anywhere near to your range of frequencies so you can expect a tilt in sensitivity across the band of only a few dBs. If you are looking at low levels of signal then you really can't do without a receiver with selectivity (can you borrow a scanner with a signal strength meter on it?).

What level of signals are you aiming to measure?

 

[vk6kro]

How do I go about building a simple antenna to measure 25MHz to 100 MHz radiated emissions?
I have an oscillator in my ckt that's radiating at 75MHz. Just want to make sure it's the 3rd harmonic of fundamental frequency (25MHz).

You can't actually make an efficient loop antenna for this range of frequencies.
These antennas only operate over a small range of frequencies.

If you wanted to make a loop antenna for 75 MHz, you would take 1 wavelength of wire at 75 MHz (about 4 meters) and bend it into a square shape and feed the signal to the receiver from the center of one side.
This would have sides of 3.49 ft long

If this was used as a transmitting antenna, it would have an impedance of about 125 ohms if fed like this.

Radiation is at right angles to the plane of the loop.

 

[cmb]

How do I make an antenna.
Why can't I look at the signals on scope. I can do an FFT and watch for peaks at 75MHz.

As mentioned, depends on signal strength. I have a £200 cheapie 'scope with FFT function and it can pick up the frequency, and harmonics, of the Rad Emissions of a 10W CCFL inverter supply at 1 meter with just a 1 m rod antenna (a piece of wire will do!). If you are saying you have a scope that is good for FFT through to 75MHz, then it's going to be a fairly good 'scope to do that, so it might already have enough s/n for your app, using nothing more than a length of wire! Move the wire around 'til you get a good signal at the frequency you're expecting, then turn your kit on and off to confirm you're detecting it, rather than noise.

 

[sophiecentaur]

Your antenna will be measuring very near field and the E and H fields will not be co-phase, so there is no point in a 'high gain' design, which will only have that design gain, I think, for far field signals. Sounds to me that you need some suck it and see, with a short dipole and a loop. If you want to know the actual radiated field strength your interference is laying down then you are into another complicated ballpark.

@cmb
I agree that a sampling scope to see 75MHz reliably is likely to be costly. Most scopes that you see advertised are, essentially, 'Video' frequency. If the scope can see the 75MHz ok then, with a high signal level, an FFT could give what's needed but, in the world of dBs (unlike the linear world of scope displays) the low level products that you see may just be artefacts and so I think you may need some additional selectivity. Which is why a receiver with some level measuring meter would win - if you can borrow one.
As we still don't know what levels you need to measure, we can't yet say.