Post by HighMountainRadio on Mar 15, 2018 14:28:01 GMT -6
Greetings From High Mountain Radio !
I thought I would post this information here to assist those who may wish to utilize a 'Folded Dipole' antenna for FM Band Broadcasting. Since a folded dipole in any configuration, whether as a single driven element or as the driven element used with a reflector, with or without director elements has a feedpoint impedance of 300 Ohms, it is necessary to ensure that it is matched correctly to whatever feedline you are using. This is necessary so as to ensure that the maximum amount of transmitter power gets to the antenna where it is needed and presents a good VSWR at the transmitter (Minimum Reflected, Maximum Forward RF Power). I have included an Excel spreadsheet that I created which tabulates the values required in order to fabricate a very inexpensive 'Coaxial Balun' that serves to match the feedline impedance of 50 ohms to the folded dipole feedpoint impedance of 300 ohms. The values represented assume that you are feeding your folded dipole antenna with 50 ohm coaxial cable which is very commonly used. The figures represented DO NOT apply if you are utilizing 75 ohm coaxial cable as your feedline ! Re-calculation of the figures required will be necessary to be accurate. It is very important to know exactly what type of 50 ohm coaxial cable that you are using in order to derive the correct information from the tables included in the Excel spreadsheet ! Once you note what type of coaxial cable you have, whether it is RG-8/U, RG-58/U, RG-9913, etc., using the included table for coaxial cable specifications, note the appropriate VELOCITY FACTOR for your particular cable, e.g., 0.66, 0.73, 0.82, 0.83, 0.84, etc., then using the appropriate table in the spreadsheet, use the dimension given for your SPECIFIC OPERATING FREQUENCY to determine the correct length of coaxial cable required from which to fabricate your balun. You MUST use the same impedance cable for balun fabrication as your feedline cable impedance ! If your feedline is 50 ohms the balun section must also be 50 ohms. Follow the instructions given referencing the included pictorials as your fabrication guide. ***NOTE: Some small measure of pruning of the given length MAY be required to obtain the lowest VSWR. It is highly recommended that you utilize a good VSWR meter to determine the value during installation and setup. Whatever 'SWR Meter' that you use for this, ensure that it is capable of accurate measurement within the FM Broadcast spectrum of frequencies, 88~108 MHZ. A cheap meter will do as long as it is capable of accurate measurement in this range of frequencies. Here, I utilize a Bird, Model 43 instrument and the appropriate element for my power level and operating frequency. If you have a requirement to measure RF power levels frequently or even semi frequently, a Bird 43 is a great investment ! A good, used one can be purchased for as little as $150.00 or it may be possible to borrow one from a local CB shop or friend. Many amateur radio operators own one of these excellent, rugged instruments. I mentioned in another thread on this forum that a decent quality 3 Element Folded Dipole Yagi antenna can be purchased on EBay for as little as $14.95 and $15.65 shipping fee. It is very nicely constructed and is well worth the money in my opinion. Since I required better signal penetration in one particular direction I decided to purchase and install one of these antennas. It serves the purpose very well, yet does not significantly impact the coverage area realized when using an omnidirectional ground plane antenna. The 3 element yagi has a decent amount of forward gain shown in the included graphic representation included within the spreadsheet.
If you have any questions, feel free to message me here and I will be happy to answer any questions that you may have to the best of my ability !
"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"
on the 30-40 inch loop of cable, what do you recommend we do with it, if we coil this up, will it act as a choke? I have a large toroid, if I wrap it with the loop, will that provide extra RF shielding, or might it cause too much inductance?
I fount this... interesting antenna design, does anyone have experience with this thing it looks kinda like it would try to create a circular pattern, but who knows. I'm looking for a good cheap folded dipole or even a 1/2 wave dipole, any advice is appreciated.
I was trying to mount my home made dipole to the chimney, dropped it and it smashed to bits on the driveway, curse these buttery fingers!
Post by HighMountainRadio on Mar 29, 2018 3:20:16 GMT -6
Greetings Tester !
The best thing to do, although it may be working presently, I would get rid of the cheap 75-300 ohm matching transformer and make a '4:1 coaxial impedance matching transformer' fabricated from coaxial cable. Read this article and save the included Excel spreadsheet that is attached to that article and it should explain everything very clearly but if you still have any questions, of course, feel free to ask ! I'm always happy to help when possible !
Also, you can buy a 3 element folded dipole FM broadcast receive antenna for $14.95 on EBay that is quite nice (I have one) and it can be very easily modified using the info above for use as a great FM broadcast antenna ! I am very interested in the one you found on Amazon !! I'd like to see how well that may perform myself ! It does appear to attempt a circular pattern by it's design ! If you make and use the 4:1 impedance matching balun I have described you won't need any other balun or choke to prevent the coaxial cable feedline from radiating along it's length, all that is required is the 4:1 impedance matching transformer 'balun'... You may coil up the length required into a circle, you are only limited to just how tight of a circle by the maximum bend radius for your particular coaxial cable as per the stated manufacturer specifications.
73... 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 HighMountainRadio on Mar 29, 2018 15:05:27 GMT -6
Greetings Dave !
Thanks Dave ! I'm glad that you find my articles informative and useful. I know I seem to always get quite long-winded so I'll try to keep them shorter and more to the point without going off on extra tangents. lol ! Actually, most of the articles I write are written over the course of several hours with numerous periodic breaks along the way and when I get my 'second' wind, continue. That is why they become so lengthy ! That, and probably way too much caffeine !
73... 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"