Array of antennas for 80 MHz 1

[tiger82]

Do you have any brilliant idea about an antenna array suitable to be mounted on a Helicopter for the detection of a communication handset that work on 80MHz?

(i appreciate any idea ...)

 

[MacGyverS2000]

Do you want communications, or merely detection?


Dan - Owner

http://www.Hi-TecDesigns.com

 

[tiger82]

The antenna array is for the detection and localisation of a transmitter at 80 MHz.

Thank you

 

[biff44]

I have thought about arrays of antennas that used ferrite rods as elements, possibly in magnetic shielded enclosures to reduce noise pickup. You would have to space the elements out to get any sort of spatial diversity.

 

[VEBill]

For DF-ing, you could use four antennas arranged in a "circle" (square) on a ground plane. The antennas can be blade antennas, or possibly loaded whips (+/- airworthiness issues). These are connected through an electronic switch and 'spun' (electronically) to induce an FM onto the incoming signal. The phase of the induced FM (the tangent) points towards the source. These systems are called by various names including the word Doppler. Google Doppler DF to start. Usually good for a few degrees.

For Homing, you do almost the same thing except with two antennas and when the aircraft is heading exactly to (or from) the source the switching signal nulls out. It nulls out when the two antennas are exactly the same distance from the source (in other words, aircraft facing source). You have to do the math to optimise the spacing within the limits of your airframe. Usually good for a few degrees.

There are also commercial systems from major manufactures like Rockwell Collins. Search their site for DF.

What sort of handset works at 80 MHz ??? That's in the VHF TV band. Army army army?

 

[Comcokid]

Just a suggestion. Amateur radio "Fox hunts" have developed into a fine art of locating a hidden transmitter. Frequently these are done at the 2 meter (144-148 MHz) band. Your 80 MHz need will require a different antenna size, but the frequenies are close enough that any techniques developed would apply. I'm not involved in this area, but you could do a search for articles on the web or one of the Amateur radio magazines.

 

[watchjohn]

.

It depends on your DF-equipment
usually they come with (4)antennas included
and instructions on how to adjust them to freq.

For 80 Mhz best results would give
1/4 wave whips...so they would be around 1 meter each.

Best to space antennas at minimal 1/2 wavelength
...wich is 2 meters....less will also work
but will make your indicator more "nervous"
...allthough from a heli you don´t have much refelections
(wich is the cause of that "nervousness")

If that is to long when the helicopter is on the ground
(since antennas are mounted underneath...and point down)
you could use car-antennas with a motor...once in the air
you activate them and they will extend to full length.
You could convert standard motorized car-radio-antennas for that purpose but that is a little work since these are not at the right length and have very lossy cables and connections.

But they are also available from manufacturers of so called
"covert-antennas" these are high quality antennas...usually shorter and are good enough to also transmit on.
These have HQ connecters and coax-cables and are 50 Ohms.

Google for "STI-CO" or "Hirschmann" to find those.

You could also buy 5/8 wave antennas for 150 mhz (with coils) and cut them to resonance at 80 mhz....that will give you a 25 % reduction in length (compared to 1/4 wave)
On a helicopter they might move to much (wind) so you
have to enclose them in plastic tubes.

If that is still to long...use 80 mhz "rubber-ducks"
and fill them with some compound so they get really stiff
"rubber-ducks" (helicals) will give more signal-loss.

The distance (spacing) between the antennas influences the accuracy and the stability of the directional indicator.
Also in case of a Doppler it might influence the level
of breaktrough from the antenna-scanning-frequency wich is usually an (annoying) tone between 500 and 1500 hz.

More info on what DF-er you use will help.
If its a Doppler there will be a antenna distributing
unit on a plate...the antennas are usually on that plate
usually that is 4 whips in a square.

Here´s such Doppler-DF and various antennas.

http://www.dopsys.com/ser5900.htm

80 Mhz is a little low and uncomfortable for DF-ing,
if you have a choise i would go much higher
in frequency (at least 170 mhz) that will make things
a lot smaller...but i suppose the transmitter is not
yours...so you don´t have a choise.


WatchJohn