Hi, I am a member of budding anarchist tech-collective in Athens, Greece. We have some experience in IT (setting up local/neighborhood networks and stuff) and now we prepare to start manufacturing small FM transmitters for squats and rural communities. Our skills with hardware are at the level of soldering kits together and using basic measurement stuff (if well documented). Our goal is to build a decent lab and manufacturing workshop and teach others how to build and run broadcast radio stations, mostly in standard FM band. We could use a lot of help from experienced builders, mostly in the area of circuit selection, building tricks, tools and equipment lists and some inevitable stupid questions. Especially in the area of antennas' black magic...
If you folks think it is a good place to ask such questions, please let me know and I will get back with more specific topics.
"Revolutions are brought about by those who think as people of action and act as people of thought." (Emma Goldman) cni-coop.net
Now, we are learning by doing, so I ask you folks here: what equipment should we put on the initial list? There is some funding available and we want to use it wisely, so your advice will be highly appreciated. As far as we already learned, the list would be:
Hand tools assortment: cutters, pliers, tweezers, hemostats, scalpels.
A set of drill bits (for coil winding).
Multimeter
SWR meter
RF Power meter
RF Oscilloscope / spectrum analyser
Thanks to a friendly makerlab we will have access to laser cutter, cnc etch[*]ing for PCB and come metalworking tools for making cases.
Now, please recommend what should we add to the list and possibly what are the best (low-budget) options to get the stuff. We are quite happy to build whatever can be sensibly built, and buy, beg or borrow the rest.
"Revolutions are brought about by those who think as people of action and act as people of thought." (Emma Goldman) cni-coop.net
If anybody cares to provide an advice or comment, here's the link to a beta version of consolidated equipment list for the stage 1 (no smd, no serial manufacturing, kit-based assembly).
Post by HighMountainRadio on Mar 18, 2018 13:00:27 GMT -6
Good Afternoon Petros !
I would be very pleased to advise you with regard to antennas for FM broadcast purposes ! I have worked with electronics and radio my entire life and I am sure that I can provide you with very useful information to get you on the right track !
As far as a very rugged, high quality VSWR / Wattmeter goes, I would highly recommend spending just a small amount more and purchasing a used Bird Model 43 Wattmeter with which you may measure forward as well as reflected RF power. It can be utilized in most any frequency range by using the correct 'element' for the intended power level and frequency range of operation. This fine, rugged instrument is THE industry standard !
Feel free to e-mail me and we can take this off-forum.
Best regards, Spooky...
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
Post by ogrevorbis on Mar 18, 2018 18:42:03 GMT -6
I second the bird meter. They work very well, and are very rugged. I think they were originally designed for military use.
In terms of the design. I think the use of a MC145170 PLL along with a Mini-circuits POS-150 is one of the best and easiest starting points because you don't really need to design the VCO yourself. You may want to once you move on to more advanced designs, but this is a good starting point. This PLL/VCO combination produces an extremely pure carrier with low spurious and phase noise compared to the all-in-one transmitter chips such as the BH series or KT series.
Post by HighMountainRadio on Mar 19, 2018 14:04:17 GMT -6
Good Afternoon Petros !
I would like to comment on at least one antenna that is certainly worth exploring for your applications. It is a 'Folded Dipole' type of antenna that is currently being sold on EBay for $14.95 plus $15.65 shipping to USA. This antenna is actually sold as a RECEIVE antenna for the FM broadcast band, 88-108 MHz. It comes with a removable 75-300 ohm Matching Transformer. This matching transformer can not handle any appreciable amount of RF power as it is strictly for receive purposes. However, splendidly, this 3 Element Folded Dipole Antenna is a terrific DIRECTIONAL antenna that can be very easily be modified for FM broadcast use in about an hour of time ! As it comes from the manufacturer it is pre-drilled and equipped with a U-Bolt intended for HORIZONTAL mounting to the support mast for use as an FM radio directional receive antenna. In order to modify this antenna for use as a TRANSMIT antenna easily capable of up to 200-250 watts RF power handling capacity, all that is required is to drill two additional holes into the support boom about an inch away from the original U-Bolt holes but opposite the existing holes so that it can be mounted VERTICALLY, the preferred plane for FM broadcast use. The 300-75 ohm matching transformer is easily removed as it will not be re-used. Since the antenna feedpoint impedance for a folded dipole antenna is 300 ohms and most users prefer to use 50-52 ohm coaxial cable for their transmitter feedline, it is necessary to easily fabricate a 'coaxial balun' made from a pre-determined length of the same coaxial cable as the feedline in order to match the 300 ohm antenna feedpoint impedance to that of the 50-52 ohm coaxial cable. It is VERY IMPORTANT to know the 'Velocity Factor' (Propagation Factor) of the coaxial cable that you will be using in order to calculate the precise length of coaxial cable needed when fabricating the balun. Simply use the Excel spreadsheet table to determine this. I have included a link to another thread on this forum that explains the procedure very thoroughly and clearly. I have also included some pictures of the antenna itself and it's gain curve plot. It will provide from 4 ~ 5.5 db of forward gain so that the signal can be directed towards a particular direction if desired. If a more omnidirectional pattern is desired you may simply remove both the director and reflector thus utilizing only the folded dipole element. If directivity is still desired but without as much gain, simply remove the director element !
If you have any questions I would be happy to assist you in any way ! Simply message me here on this forum. I hope this proves useful for you !
Keep On Rockin' In The Free World ! 73... Spooky...
Attachments:
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
As it comes from the manufacturer it is pre-drilled and equipped with a U-Bolt intended for HORIZONTAL mounting to the support mast for use as an FM radio directional receive antenna. 
