Re: Making a portable wideband VHF antenna
Cost 2000? is due that you will get detailed information about the antenna radiation pattern and a nice box to keep the antenna within.
If you are considering to purchase a biconic antenna for measurement purposes, you should also review the portfolio of a tradidtional supplier like Schwarzbeck Mess - Elektronik Homepage
Aaronia has some nice planar logper antennas at affordable prices, but I won't suggest them as a high end equipment supplier. Some of their devices, e.g. spectrum analyzers, have serious flaws.
I'm not considering to buy them. I'm just trying to figure out how do they manage to design such a wideband antenna. The purpose of my current project is to design a similar antenna, RX only, which will be used to scan the whole spectrum between 100kHz and 3GHz, searching for unauthorized wireless devices in an enviroment, like a room. I do not wish to extract any information from the source, the device must only be able to detect the presence of the signal. I know it's not possible to make it with a single antenna, but I would like to find a solution using as few antennas as possible.
Currently I've managed to design a spiral antenna with a good bandwidth (300MHz - 3000MHz). Now I'm stuck in the lower band (below 300MHz), and I'm still trying to sort things out. I've seen examples of active antennas ranging from 100kHz to 30MHz, but I still have a gap between 30MHz and 300MHz.
For finding unauthorized wireless devices in an room, is often other types of detection equipment used, if you also want to find temporarily inactive devices?
In a normal room will you be pretty much in near-field for lower frequencies so any dipole antenna or even a loop antenna can be used to find a active transmitter with enough tx range to reach outside the room. A loop is simpler to use if you also want to find the direction from were the signal radiates.
Will the loop work even if it's not tuned?
Tuned is a relative circumstance. Tuned for narrow band resonance (high Q) gives less antenna loss and suppress out of band noise but in this case I guess fine tune the antenna for each frequency is less practical.
For a loop antenna with a size<<lambda intended for use in a wider frequency range, without retuning, is a wide band transformer to prefer to better impedance match from the low antenna radiation resistance up to something 5-500 Ohm. This transformer can be a part of the antenna or winded on a separate ferrite coil. It will not be perfect impedance matching but good enough for RX.
At lower frequencies, is an alternative to use a transimpedance amplifier (OTA) instead of a matching circuit.
Whatever type of antenna, do not connect it directly to a expensive receiver as you probably are going to find strong magnetic fields from fluorescent lamp transformers, static discharges, switched power supplys.. in uncontrolled room environments which can kill a sensitive receiver. Use a easy replaceable LNA between antenna and receiver as a kind of fuse. Band pass filter is another improvement that both protects and reduces problem with saturating LNA due to strong out of band signals.