UWB background level - Page 1
Forum section: electronics / rf-microwave

author: jart35
author_rank: Newbie
timestamp: April 08, 2017, 07:04:16 pm
content: I just built an RF power detector using an Analog Devices 5906 evaluation board (I knew I would screw it up if I tried to make the RF circuit part myself). The detector itself is spec'd for 10Mhz to 10Ghz, and good down to about -65 dbm at the lowest frequencies, and -30 dbm at 10 Ghz. It seems to be working fine, but I'm puzzled about the readings I'm getting. There seems to be a general background in my area of around -40 to -45 dbm, which applies whether in my house with all devices turned off, or around my neighborhood, or a couple miles up in the hills in back of us with widely scattered houses only. I would expect a background < -100 dbm (effectively zero on my meter) when not anywhere near a transmitter. Is my expectation totally wrong? The nearest cell tower (that I know of) is on the top of a hill but quite far away (a couple miles at least). I've checked the circuit carefully, and if I disconnect the antenna, the reading does go to zero. (The antenna is from WA5VJB, and is spec'd for 700Mhz to 26Ghz.) Is there something about summing the signals from multiple low level sources across a wide bandwidth that adds up in a non-intuitive way? Or, maybe it's black helicopters (there is an AFB about 20 miles away).

author: hendorog
author_rank: Super Contributor
timestamp: April 08, 2017, 07:27:02 pm
content: Seems reasonable that it could be a combination of all RF including broadcast radio stations.You'll get them even though its outside the antenna spec.

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2017, 07:58:19 pm
content: Reasonable, power lines can carry RF for quite a long way, and there will be enough poorly shielded electronics around to get that amount in most areas of medium density.

author: hendorog
author_rank: Super Contributor
timestamp: April 08, 2017, 08:44:50 pm
content: I just measured the total channel power across just the FM broadcast band - which was around -34 -> -40dBmUsing a SMA connector with an approx 50cm wire soldered to the centre pin. (i.e. not a great antenna)(There is a 1dB attenuator in the path which is not corrected for in the screenshot.)The max single source in the band on max hold is about -38dBm

author: jart35
author_rank: Newbie
timestamp: April 08, 2017, 10:07:27 pm
content: Quote from: hendorog on April 08, 2017, 08:44:50 pmMy antenna is basically a dipole on a pcb strip made out of two elipses that are almost touching, not sure how well it would get 100MHz since it's lower "limit" should be 700MHz.According to this:http://www.wmo.int/pages/prog/www/TEM/SG-RFC06/Ambient-RF-noise.pdf, my readings would be very high, but then they were using a spectrum analyzer at discrete frequencies, not a simple wideband meter.Maybe I'll try driving 20 miles or so out in the desert and see what I get.

author: jart35
author_rank: Newbie
timestamp: April 08, 2017, 10:14:51 pm
content: Quote from: SeanB on April 08, 2017, 07:58:19 pmInteresting, the lines in the neighborhood are buried, but there are exposed lines less than a mile away. And then I suppose a long enough wavelength could get through a couple of feet of dirt (?). (And where I am, dirt with very low water content.)

author: whollender
author_rank: Regular Contributor
timestamp: April 10, 2017, 05:06:37 pm
content: Quote from: jart35 on April 08, 2017, 10:07:27 pmThe efficiency of your antenna will be pretty low around FM frequencies, but that won't stop it from picking up plenty of power from FM stations.To see if this is really the problem, you can try putting a high-pass filter in between the antenna and eval board. Something like this would be pretty easy:https://www.minicircuits.com/WebStore/dashboard.html?model=SHP-700%2B