Go Down

Topic: RF Transmitter (Read 632 times) previous topic - next topic

aweatherguy

Noah -- what sort of background do you have in RF design? It's hard to make recommendations w/o knowing that.

I think Noah is in the US -- and emission limits for a home-built intentional radiator at 70cm are likely not enough for a transmitter hunt.

I'm assuming that the ham license is required because of that, and then the power limits are much higher. There's a wider band of frequencies available (420-450MHz).


allanhurst

How does a RF foxhunr work???

Puzzled

Allan

mauried

It would help a lot if the OP could tell us what range is required.
ie are the foxhunters on foot or in cars.
Many years ago I used to do foxhunts and both cars and on foot .
The transmitters used then were from 5 to 10 watts on both 144 and 432 Mhz , but the transmissions were
AM as its much easier to make simple hand held receivers which are needed when you get close.
FM for foxhunts doesnt work all that well unless its a short range fox hunt , and very low transmitter power is used.

srnet

How's would I go about making a 70cm transmitter?
Use a RFM22B which is based on a Si4432. Its an FSK data module, but put into direct mode does a reasonable FM Morse. You can set the frequency and power level between 100mw and about 2mW. There is a 1W version to. Available in 443Mhz, 868Mhz and 915Mhz versions, but can actually be programmed from 240Mhz to 930Mhz.

I never wrote any Arduino code for it but the PICAXE version has been in orbit twice now, on one ocaision I got a accurate RDF fix with a 7 element yagi at 2000km using a RFM22B.

$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

srnet

Have you considered the legal aspects? in England it would be totally illegal -even a few mW, which won't get you very far.
For the general public, but surely a licensed Amateur can operate a remote transmitter as long as there are arrangements in place to turn it off, if requested.
$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

srnet

Semtech has lots of parts that will do that. The SX1239 SX1276 will also do 2m ham band. These are all digital modulation though -- OOK and FSK for example.
Your correct, if you put the SX1276 in direct modulation mode and put a tone (square wave) on the DIO2 pin, it transmits what an FM handheld will see as audio tones, Morse for instance. 
$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

aweatherguy

Quote
How does a RF foxhunr work???
It's a game of hide and seek with radios. One person (the "fox") goes out with a transmitter and hides, then sporadically transmits. A bunch of other guys drive or walk around and try to find the hidden transmitter by using techniques such as directional antennas.

The transmitter needs to have enough power to cover the range of the hunt -- usually at least several miles.

allanhurst

I presume then that Adcock-style antennas as used in 'huff-duff' are employed?

Allan

aarg

The easiest way is to just interface with an HT - use the mic and key inputs. It solves a lot of technical problems in one pass.
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

aweatherguy

Quote
I presume then that Adcock-style antennas as used in 'huff-duff' are employed?
Adcock antennas are mostly used at HF (i.e. below 30MHz or so). At VHF (2m or 70cm) you're more likely to see folks running around with Yagi-Uda antennas. There are other techiques which require multi-channel phase-coherent receivers; these are usually limited to folks with real money -- e.g. government agencies (regulatory enforcement) and military.

MarkT

Use a RFM22B which is based on a Si4432. Its an FSK data module, but put into direct mode does a reasonable FM Morse. You can set the frequency and power level between 100mw and about 2mW. There is a 1W version to. Available in 443Mhz, 868Mhz and 915Mhz versions, but can actually be programmed from 240Mhz to 930Mhz.
Each version has a band-specific antenna tuning circuit, you can't tune it to another band and get any sort
of output power as the VSWR will be pants.

The RFM22 is based on the RF22 chip from Hope Semiconductor, not the Si4432 from Silicon Labs.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

allanhurst

#26
Oct 13, 2017, 11:46 pm Last Edit: Oct 14, 2017, 12:14 am by allanhurst
I think the Adcock antenna would work very well at higher frequencies - and instead of using a crt to display phase and hence direction, an I-Q synchronous demod would be the approach.

I've done such things with patch antennas as well up to several GHz...

RF tricks - been around for years. And could be done pretty cheaply - you don't need government or   miilitary level funding to do this.

Allan

mauried

Usually fox hunters use small yagi antennas, usually 2 element on 144 Mhz and 6 elements on 432 Mhz.
They can easily be hand held and arnt too big to fit on the roof of a car using a pack rack pole.
Some hunts can start from 10 miles away from where the transmitter is hidden, and usually you have to walk the last part.

aweatherguy

Quote
I think the Adcock antenna would work very well at higher frequencies - and instead of using a crt to display phase and hence direction, an I-Q synchronous demod would be the approach.
This is maybe getting a bit off topic? I'm not sure what's okay here. To delve deeper I'd suggest a ham-radio discussion group -- they probably have topics on DF. You can also look at commercial DF system manufacturer web sites or internet searches. There's lots of good information on DF designs out there.

I will say that I've been doing RF engineering for a long time and that includes work with commercial DF systems. Threre are good reasons the big boys don't use Adcock/Watson-Watt type designs when they need high-qualtiy direction finding. That's not to say they don't uses such systems at VHF...but you can't get the same quality results that other more expensive designs offer.


allanhurst

Quite. It can be done.

And even hams can do sums and spend a few quid on precision RF....  You don't need AESA for this.

Allan

Go Up