Subject: Re: Using a 'scope as a field-strength-meter
Date: Sun, 12 Jan 2003 01:43:08 -0800
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Nice thing about the scope is that it gives you
of the dynamics of the signal. Amplitude
modulation, big spurs,
general badness, that you won't see on a field
Won't help much on low level harmonics unless you
do a FFT on it.
If you put any sort of antenna on the scope input,
it will often
swamp out any signal that's coming in thru other
leakage paths...at least
on a quailty scope. Easy to check by pulling off
Roy Lewallen wrote:
> The only problem is determining what you're measuring the field strength
> of. Chances are that your rig isn't tremenously well shielded, so at six
> feet away, the scope is likely measuring leakage through the box or from
> power leads, headphone wires, or other wires leaving the box. Although
> you can tell the rig is generating a signal, the relationship between
> what you're seeing on the scope and the signal being radiated from the
> antenna might not be as direct as you'd like.
> If you're seeing a signal without the probe being attached to anything
> indicates that the signal is entering the scope via a path the scope
> isn't really intended for, such as its power cable or around its
> shielding. This means that the attenuator might not give the attenuation
> you expect, making it difficult to make even comparative measurements
> unless you stay on the same scope scale.
> These problems aren't because of the scope, which is just fine as a
> detector (although it won't have the dynamic range or shielding of a
> good FSM, if those are important). The problem is the proximity of the
> detector to the rig. To get a good idea of how much signal is getting
> radiated from the antenna, you've got to put the detector far enough
> from the rig that the signal from the antenna is much stronger than
> leakage from the rig and its re-radiation from local conductors. And, in
> the case of the scope, you'd also have to connect the input to an
> antenna that will receive a much larger signal than will leak in via
> other paths.
> Roy Lewallen, W7EL
> Paul Burridge wrote:
> > Hi,
> > I was just starting to piece together a breadboard circuit for an RF
> > field strength meter when I happened to notice that keying the
> > low-power transmitter under test (half a Watt out) resulted in a
> > substantial waveform being displayed on my 'scope about 6 feet away
> > (just by induction through the probe lead, I imagine). The question
> > then arose as to whether I needed to build a FSM at all, since the
> > 'scope is obviously sensitive enough on its own to pick up highly
> > local transmissions.
> > So I'd like to invite views on whether this is feasible or not. I
> > can't see a problem with it, but maybe someone else can. Any ideas,
> > gentlemen?
> > p.
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