It's amazing what a small movement positioning it can make, but I've noticed that with video senders, etc before at those SHF frequencies.

Not really amazing. At 2.4gHz you are looking at a wavelength 12.4cm. Imagine a sinewave in front of you with a full cycle every 12.4cm. Apart from the amplitude, there are two zero signal points in that short distance.

Picking up on the wavelength aspect in Professor973's posting-
"Not really amazing. At 2.4gHz you are looking at a wavelength 12.4cm.";
and you mentioning that the USB dongle was attached to a shelf. it may be that you had placed other items on the shelf; or in the transmission path, which caused such attenuations.

This would particularly be the case if such objects were metallic; or contained water.

Water absorbs RF emissions particularly at 2.4 GHz, hence that band, shared with WiFi, is allocated for the magnetrons in microwave cookers.

From Wikipedia

https://en.wikipedia.org/wiki/Microwave
"
Heating and power application
A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating primarily by absorption of the energy in water.
"

In reply to a post by professor973:

Imagine a sinewave in front of you with a full cycle every 12.4cm. Apart from the amplitude, there are two zero signal points in that short distance.

Ermm... no.

Only at an instant of time (ie a snapshot of the e-m radiation) or if you've got obstructions causing a standing wave pattern to occur.

The instantaneous field strength at any given point varies sinusoidally with time and the measured field strength at that point will be constant.

Edited by billford (Sun 20-Jul-14 08:46:09)

I was not intending to get into the electronic and magnetic components of RF. just trying to explain in simple terms the small wavelength and the effect of moving a small distance on signal. Possibly I am too used to drumming y(t) = A\sin(2 \pi f t + \varphi) = A\sin(\omega t + \varphi) into my RAE students!

Edited by professor973 (Sun 20-Jul-14 12:04:56)

In reply to a post by professor973:

Possibly I am too used to drumming y(t) = A\sin(2 \pi f t + \varphi) = A\sin(\omega t + \varphi) into my RAE students!

I used to find your fantasy that you somehow are a lecturer with students mildly amusing, but as the years have passed it is becoming tedious.

Please yourself you pathetic [censored] . I am equally [censored] off with you calling me a liar. How about you check with the RSGB. Why do I have to prove anything to an anorak like you, so you go off to qrz.com and check my call. You will need to register on the site. You can then look up Steve M1ACB, a FDARS club member who will confirm the students I put through the club. My late wife M0CEQ was one, but she cannot say a lot now. So off you go, take a look, then shut the [censored] up.
Go take a look sonny Jim. I have never hid from anyone. http://www.qrz.com/db/M0AUR

Edited by professor973 (Sun 20-Jul-14 23:51:53)

In reply to a post by professor973:

How about you check with the RSGB.

I checked:

In reference to rsgb.org/main/internet-guidelines/:

The following message contents fall outside the ethos of the Society and would be unacceptable:

Comments that are considered likely to disrupt, provoke, attack, impersonate or offend others. For the avoidance of doubt this includes comments that contain swear words or language that might offend, or content that is racist, sexist, sexually explicit or abusive.

Edited by deleted (Sun 20-Jul-14 23:49:52)

[censored]

Morning Bill

Regarding Standing Waves, I observed an Audio set about 1980, in a school with long corridors.

A glass had been broken on a Fire Emergency Push Switch in a major extension.

No actual fire!

The Fire Brigade made a temporary repair; and I was replacing that with a proper glass etc.


In doing so, the Alarm was started up again etc, so I had to make my way back to the Control Panel in the main building, about 300 yards all told.

The Alarms were of a constant almost sinusoidal frequency, rather than a wailing, variable frequency.

As I walked along the main corridor, I noted that there were several "dead spots" where the Alarms could hardly be heard on a quiet Sunday morning, whilst further along the volume was ear-piercing and almost overwhelming.

---------

Many years earlier, I had noted a similar effect in a factory. It was a typical, modern, large almost open-plan factory, with simple partition walled offices around the sides, no ceilings to those offices.

I was seated where I could see out of the door; and I realised that the staff in the main area, were trooping fairly fast towards the exit.

We vacated the office; but had to walk a few paces before we could hear the Alarm.

Another person working in another of the offices, confirmed that the alarm could not be heard in that one either.

-------------------------

So Standing Waves of any sort can be gye-queer things!