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-   -   Folded Dipole Questions (http://forum.tvfool.com/showthread.php?t=16705)

eggman531916 10-Dec-2019 1:52 PM

Folded Dipole Questions
 
I'm about to start building a folded dipole for RF channel 6 and I have some questions. First, does the spacing between the top and bottom matter? The one calculator I used has a spacing of about 4 inches while others have different spacings. I plan to use 1/2" copper tubing and four 90 degree elbows to make the bends instead of trying to bend it in a circle. I figured on making the length 66 inches long, but again, when I put through different calculators I get different lengths for each one. Coincidentally, all of them are shorter. I thought 66 inches would be the correct length. I guess what I need to know is, am I going about this correctly? Or did I miss something? Or does it not matter on the length and spacing as long as it's close? There seems to be alot of information on folded dipoles, but they never seem to match from one source to the next and I really don't want to waste 12 feet of copper through trial and error. Thank you for any guidance you all could give me.

rabbit73 10-Dec-2019 6:04 PM

You would have to use computer modeling for the precise dimensions.

Using four 90 degree elbows should be OK. I would make the spacing 3 inches, unless that creates a problem with sweating the joints together. The ends of the folded dipole are most sensitive to being touched; they are high impedance points. Use insulators for support of the tubing.

I have found that using the formula 5540 divided by the mid channel frequency in MHz gives the length in inches that is close enough for a test folded dipole.

CH 2, 54-60 MHz, mid channel 57 MHz, 5540/57 = 97 inches
CH 3, 60-66 MHz, mid channel 63 MHz, 5540/63 = 88 inches
CH 4, 66-72 MHz, mid channel 69 MHz, 5540/69 = 80 inches
CH 5, 76-82 MHz, mid channel 79 MHz, 5540/79 = 70 inches
CH 6, 82-88 MHz, mid channel 85 MHz, 5540/85 = 65 inches
CH 6 low end: 5540/82 = 68 inches
CH 6 high end: 5540/88 = 63 inches

If you are concerned about wasting expensive copper tubing, I suggest that you make a test folded dipole 65 inches long using 14 gauge solid copper THHN insulated building wire like I did for a CH 3 test antenna.

https://i.imgur.com/6qYVZsO.jpg

https://i.imgur.com/PPm31G5.jpg

https://i.imgur.com/HV6WyAs.jpg

If your CH 6 signal is still marginal with the test folded dipole, it might not be because of insufficient antenna gain; it could be that the antenna isn't in the best location or there could be interference from electrical noise which is worse on VHF-Low.

JoeAZ 10-Dec-2019 8:16 PM

FWIW, I made an antenna for Rf 6 and compared it to an inexpensive
FM antenna I purchased. Rf 6 is twenty miles away, LOS, with a massive
500 watt signal, LOL. In most every case, the FM antenna performed
better. I used a 66 inch length in hopes of avoiding FM interference from
58 Kw stations at the same location. I hope your experiment works better
than mine did!!!!!!

eggman531916 10-Dec-2019 10:38 PM

Thank you Rabbit73 and JoeAZ for the input. The 5540 divided by mid channel frequency formula is what I was looking for, basically a good rule of thumb to go by. Can't believe I didn't think to make a cheaper test version...duh. Maybe it's because I'm working the nightshift this week? Anyways, I do have a cheap directional FM antenna up and it locks in the information about what's on most days, but, can't lock into a picture. Sometimes I get an extremely pixelated picture that looks like there might be movement, but that't only once in a while. There are a decent amount of FM stations coming from the same direction as the television transmitters as well as a few somewhat local stations. Even with the FM trap on the RCA preamp, they still show up on my Hauppauge tuner listing. I bought a second FM trap on Ebay that I want to experiment with when I get a chance. Thank you again for the advice.

GroundUrMast 11-Dec-2019 9:42 PM

A three director Yagi for CH-6
 
1 Attachment(s)
Here is a design generated using software by VK5DJ.

https://forum.tvfool.com/showthread.php?t=1106

https://forum.tvfool.com/attachment....1&d=1576104088

I've built a few antennas based on this software and found them satisfactory. I have not built this particular design though.

You can use just the dipole alone or omit one or more directors if you wish. The balun design is optional, you can substitute a store bought 300/75 ohm balun.

Though the measurements are metric, I chose values that are close to imperial. The boom works out to about 8 feet of 1 inch square tube and the elements can be 3/8" to 1/2" diameter. (1 inch = 25.4 mm)

eggman531916 24-Dec-2019 3:38 AM

Thank you again for all the information. In another thread I was building an RF 6 yagi using the Changpauk (spelling?) calculator. However, someone near my area who has been trying to get approved for the forums messaged me directly as he is having the same reception problem pointed out that the folded dipole calculator gives a result that is too short. Hence the reason for all the questions. I decided to lengthen it to 66 inches in length and the reflector to a little over 69 inches. Glad he said something or I'm not sure my project would've worked. And it's working very well. Thank you all again for your help.


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