Small antenna 100MHz - 2200 MHz - Page 1
Forum section: electronics / projects

author: Sypher
author_rank: Contributor
timestamp: September 07, 2013, 09:21:42 pm
content: I was wondering if anyone could help me with an antenna question. Unfortionaly, I really don't know much about antennas.I'm in need of a small (~6inch) omni antenna capable of receiving in the 100MHz-2.2GHz range, in a short range situation (~50m). All it needs to do is be able to detect a signal, I won't be using it to capture any data or anything (so a shitty signal is okay, as long as I can measure some scaled RSS). I've search forever for an antenna in this range and haven't found anything. However, the shitty rink-dinky $2 antenna that came with my RTL-SDR will do what I want (That is i sweap a signal from 50MHz - 2200 MHz in my room and it will pick it up.).So, I want to know what gives? Why can't I find an antenna with these specs, but at the same time I grabbed 3 random (shitty pieces of crap) antennas spec'ed for small bandwidths and they will pick up the signal I'm sending out.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 07, 2013, 09:37:06 pm
content: Any piece of wire will pick up a signal, but if you want a real antenna for that range your only option is a log periodic or a GP type. Unfortunately for the low end being 100 MHz it needs to be 1.5 meters wide. If a shitty signal is ok just get a 6 inch piece of wire...

author: Sypher
author_rank: Contributor
timestamp: September 07, 2013, 09:39:10 pm
content: Thanks for the reply.All I'm trying to do is measure the time difference of arrival between two antennas (rss is important), so I don't really care about the signal. Can I use any antenna?

author: JackOfVA
author_rank: Unknown
timestamp: September 07, 2013, 10:28:12 pm
content: A simple wire antenna does not have a constant impedance with frequency. At frequencies where the wire is an odd number of quarter wavelengths long, it will have a low impedance and at frequencies where the length is an even number of quarter wavelengths long, it will have a high impedance. At in-between frequencies the impedance will vary and will exhibit capacitive reactance or inductive reactance.If a random length wire is plugged into a 50 ohm input impedance device such as a receiver or spectrum analyzer, the voltage delivered to the 50 ohm port will vary - you can think of it as a voltage divider. If at some frequency the antenna impedance is 30+j1000, it won't deliver much voltage into a 50R load. If at a different frequency the antenna impedance is 40+j10, much more of the induced voltage will be delivered to the 50 ohm port.Other things to consider is that the vertical pattern of an antenna (thinking of a vertical random length wire here) will depend on frequency as well.In many cases, you can ignore these considerations and get a usable signal from a random length of wire connected to a receiver or spectrum analyzer.If you are concerned about making accurate signal level measurements, and/or extracting every last possible bit of energy from the incident waveform, then you will have to dig much deeper into the antenna design.For decent broadband performance over 100-2.5 GHz, a discone might come close to working, usually 10:1 frequency range is possible but 25:1 will be challenging. A 100 MHz discone won't be physically small however.You need to first decide how good an antenna you need and then decide which of the available designs best meets your definition of "goodness."

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 08, 2013, 02:21:26 pm
content: A discone is a nice solution (why did I type GP?) but you are not going to do 100 MHz with a 6 inch one...