RF field strength - Page 1
Forum section: electronics / beginners

author: geggi1
author_rank: Frequent Contributor
timestamp: February 28, 2015, 05:56:16 pm
content: Hi!I got a project using a AD8307 log amp chip and arduino to make a field strength meter.As s feature I would like to add distance to transmitter if the Po to the transmitter is known.After some googeling i found this page on the web.http://www.eetimes.com/document.asp?doc_id=1276321When looking at the equation furthest down I found it a bit confusing about the antenna gain.After rethinking a bit I came to that the two G for antenna gain must be times of amplification rather than dB.The reason for this as far as i understand the equation that if one use a antenna without gain over a dipole or isotropic (0 dB) the equation will become 0.But on the other hand if one uses times of multiplication 0 dB is 1 and the equation will become larger than 0.Am I going on thinking the whole thing wrong or am I on the right track?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2015, 06:36:08 pm
content: Yes, it's multiplicative, a ratio not a logarithm (dB).There are other problems. The equation only works for free space. Real environments will exhibit reflections, interference, multipath, fading and more. You could provide the calculation anyway (even if the antenna gains are known or assumed), but it won't really ever be meaningful.Tim

author: geggi1
author_rank: Frequent Contributor
timestamp: February 28, 2015, 08:07:53 pm
content: Yes I'm aware that the equation is only valide in free space,but in relative open space like on a field it will still give an good indication on the transmitted power.I'm playing with the thought to make a TETRA detector like the old days police radar detector.When the police are using the laser gun they will have to communicate with the personel at the stopping site and along the motorway and contry side roads there is not a lot of other use of TETRA ferquencys. A sensitive detector and a bandpass filter will give a relative indication of any local TETRA activicy.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2015, 10:17:16 pm
content: Hmm, so depends if it's wide open roads or urban streets or what...You'd probably have better luck with a directional antenna. A Yagi at those frequencies (300-400MHz?) isn't too bad, though it would be unsightly (if anyone happens to come up to you and ask what you're doing, you better have a good excuse..). At least that way, you have some idea which way it is -- whether you're chasing a reflection or the real thing is still hard to say, though.Tim

author: Neganur
author_rank: Unknown
timestamp: February 28, 2015, 11:39:59 pm
content: An antenna without gain compared to an isotropic has the same gain as an isotropic, it does not mean that the gain is zero, it is 1. (these are ratios and powers in Watt, not dB)The dipole has directivity compared to the isotropic i.e. 2.15 dBi, so you need to convert back to ratio G=1.64.Friis' equation works for other locations than space, you just need to take losses into account. But in this application it is rather pointless if the original poster is only interested in detecting the presence of a certain frequency.

author: jc101
author_rank: Frequent Contributor
timestamp: March 01, 2015, 03:19:16 pm
content: Quote from: geggi1 on February 28, 2015, 08:07:53 pmDownside, at least in the UK, is TERTA is used by all the Blue Light agencies and also carries data. So there is a fair bit of network chatter going on all the time, there isn't silence when no-one is talking.

author: geggi1
author_rank: Frequent Contributor
timestamp: March 01, 2015, 10:40:13 pm
content: With some filtering and a treshold it would be possible to at least have some kind of warning especialy here in Norway that is les dense populated than in the UK or other countries.