Where is the folded dipole?
The ends of the feeder cable need tidying up (they are part of the dipole) but they should go to the ends of the folded loop.
Folding the dipole reduces its impedance to match the cable better, without it there will be very significant signal loss.
The insulation on the wires is irrelevant, a balun might make a tiny difference but folding the dipole is the real issue.
It looks nothing like the one in the web page, have you taken into account the effects of whatever it is built on, the original uses a boom and the elements are in free space.
Brian.
As shown in the picture, it's a straight 60 to 75 ohm half wave dipole, not a folded dipole. This makes sense because it's roughly matched to the 50 ohm cable, however it misses at least a ferrite core over the coax cable end or a dedicated 1:1 balun.
For such an antenna, you need to be much more precise with dimensions. I don't think you have a chance to realize it with your low mechanical precision.
Thanks for mentioning the foam board. So it's actuality a folded dipole. Unfortunately it's seriously mismatched in this case, having typically 290 ohm impedance. In addition you have the problem of asymmetrical coax connection, making the cable become part of the radiating element.
I see that the website also uses a folded dipole without an impedance matching balun. Bad idea.
Thanks for mentioning the foam board. So it's actuality a folded dipole. Unfortunately it's seriously mismatched in this case, having typically 290 ohm impedance. In addition you have the problem of asymmetrical coax connection, making the cable become part of the radiating element.
I see that the website also uses a folded dipole without an impedance matching balun. Bad idea.
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A simple solution, keeping the already designed folded dipole, would be a lambda/2 coaxial balun.Many previous threads at Edaboard discussing it. E.g. https://www.edaboard.com/showthread...h-50-Ohm-impedance-as-Balun-for-Folded-Dipole
Thank you FvM,
What kind of cable should I use to make the balun at the link you gave? The same coax cable I am already using?
Amateur
Thanks for mentioning the foam board. So it's actuality a folded dipole. Unfortunately it's seriously mismatched in this case, having typically 290 ohm impedance. In addition you have the problem of asymmetrical coax connection, making the cable become part of the radiating element.
I see that the website also uses a folded dipole without an impedance matching balun. Bad idea.
How precise do you think one has to be?
In the online design at the link I gave, the designer said to within 0.5 mm was sufficient.
foam board.
I agree that usually the folded dipole has some 4:1 balun, but it's not always true. The dipole inside a yagi isn't the same as that dipole stand-alone. The impedance is influenced by the reflectors & directors which are close to the dipole.
It think 1% tolerance on the length is acceptable. Some yagi designs are more critical than others. In general, I expect many elements to be more critical. .... a normal 3-element would be much less sensitive to tolerances.
Sounds ok. But your feed with the coax is rather large compared to the element lengt, so that fore sure will create a lot of parasitic inductance. I think you would need to go from coax to dipole feed with much less undefined extra length.
View attachment 155026
Also, your wires for the elements are rather thick compared to the length. That is possible, but wire radius will change the element length. Are the length calculated for this wire diameter? And wire insulation (=dielectric in the area of high electric field) should be removed.
Wood for the boom or foam, all that has an effect on tuning of the elements. I don't think your design with many elements will perform as expected. Go for less elements, which hopefully is more robust to tolerances.
You are right, driving element impedance depends on the reflector and director spacing. I found this table in Balanis Antenna Theory. Numbers are for regular half-wave dipole, folded dipole has 4x this impedance.
View attachment 155029
A full featured yagi design tool should be able to calculate the impedance, I didn't find respective comments in the linked antenna design.
You are right, driving element impedance depends on the reflector and director spacing. I found this table in Balanis Antenna Theory. Numbers are for regular half-wave dipole, folded dipole has 4x this impedance.
View attachment 155029
A full featured yagi design tool should be able to calculate the impedance, I didn't find respective comments in the linked antenna design.
I think the solder connections used online are "cleaner", but I think mine are structurally/mechanically stronger, due to the larger size/more solder. Frankly the solder connections of the online design looked weak. Also I just want to be sure you realize that the picture in your post that you circled in red is NOT my design, but is from the design that is online that I was using as a guide.
Yes, I understand this isn't your design, but both have "uncontrolled" wire length of several mm length between coax and dipole , which means several nH series L, which menas some dozen Ohm of series reactance. And that detunes your dipole. You would want to measure the actual input impedance (or VSWR).
I recommend to use a short length of really thin coax, so that all that wiring can be smaller. Teflon cable is nice from a practical viewpoint (tolerates heat when soldering) and low loss.
For using coax feed with a dipole, you always need some balun for doing the balaned to unbalanced conversion, even if there is no need for additional impedance transformation. A replacement might be some turns of coax cable, or other tricks as discussed here: http://dg7ybn.de/Symmetrising/Symmetrising.htm
volker,
I appreciate the replies you and others have made.
The comments section at this link are very helpful under the 3 individual commenters Saarducci, zwzserver and myjunkmailbox2013. These 3 commenters address the (unaddressed) impedance matching issue, and the connection of the coax to driven element. At least two of the three recommend a simple dipole as opposed to a folded dipole. I think I may use the simple dipole and try to follow myjunkmailbox's suggestions for making the connection between the driven element and the coax. He (she?) says to get the two inner parts of the simple dipole very close together, about 1mm apart and to use the same length of wire to attach each to the coax. I think I will also strip off the insulation for the 14 AWG copper wire that I am using to make my elements. I am already using very thin coaxial cable as it is, about 0.109 inches in outside diameter. (It might be RG-174/U.)
Any more hints and educated guesses you or others an give me, especially if based on reading the three commenters, Saarducci, zwzserver and myjunkmailbox2013, will be much appreciated.
Thanks,
Amateur
Thanks Big Boss, I will consider that type of connection.You'd better to put a simple PCB just underneath the Dipole and assembly it with proper PCB type SMA Connector otherwise you cannot be sure about antenna performance..
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