Fan Dipole tips - Page 1
Forum section: electronics / rf-microwave

author: veedub565
author_rank: Frequent Contributor
timestamp: May 08, 2020, 06:02:42 pm
content: Hi, I made a version of this fan dipolehttps://www.wolverhamptonars.co.uk/?WHAP=FDI used a commercial 1:1 balun and RG213 feedline (approx 7-8m long)So using an MFJ-259 to help with tuning, I get good low swr (1.2:1 or better) on 40m, 20m, and 1.5:1 on 10m. I'm having problems tuning the 17m and 15m elements though.I've been going back and forth between them a few times. Adjusting one affects the other. So I trim 17m to get around 1.6:1 then tune 15m. Only to find the 17m element has shifted and I have to add back on what I've just trimmed off. Which then affects the 15m. I suspect because they are quite close in frequency they are interacting, but I seem to be going in circles with this.Any tips ?Adjusting the 17m and 15m elements does not appear to be having much if any effect of the 40m, 20m, 10m elements.

author: bob91343
author_rank: Super Contributor
timestamp: May 08, 2020, 06:31:45 pm
content: I made a few experiments with fan dipoles and found that I am better off using separate transmission lines for each band. It's not that big an increment in cost and eliminates much of the grief of interaction.The shunting impedance of the off-frequency elements is not trivial. The higher frequency ones add parallel capacitance and the lower frequency ones add parallel inductance, usually, not to speak of the resistive components which radiate power (not that it's so bad, since from the outset one accepts the absence of directivity). This is partly why you get interaction.Unless you want to trim the thing over and over, my suggestion is to use separate dipoles. In addition, I don't use baluns but instead the coaxial choke seems to be a good solution. One complaint I have about baluns is that many have ferrites in them and as a result are nonlinear, causing harmonic generation and losses.Good luck with your project; if you find a simple solution let us know.

author: veedub565
author_rank: Frequent Contributor
timestamp: May 09, 2020, 05:16:02 pm
content: Thanks Bob,I did find the cause of the problem I was having. After rechecking things today, it seems that after a long hot sweaty day crawling around the loft yesterday, I had gotton a bit careless. It seems that if you cut the leg of one element, and then cut the other leg.. from a different element, then strange things happen. Couple that with trimming the wrong element entirely then you go around in circles chasing your own tail. So the interaction I was seeing i.e. trimming the 15m element interfering with the 17m element, this was most likely because I had trimmed the wrong leg or element.After getting back to a reference baseline today I was able to "succesfully" tune this antenna. Or at least do the best I could with it, I used the antenna analyser to take some measurement points, and create a rough swr graph of each band (attached).I take your point about being better off with individual dipoles, rather than multiple elements on one antenna. I did notice some physical interaction, simply folding down then ends of the 15m element shifted the lowest swr point quite dramatically. I'd say i have each element spaced apart by about 6" which is the best i can do in the space available. Who knows what other strange interactions are happening.It's all a learning experience anyway. I'll investigate coaxial chokes over baluns. I wanted to avoid RF coming back down the coax feedline which is why I went for a balun. Maybe experiment with some other antenna ideas. I wanted to work 80m but I'm short on space, and living in a large built up area my noise floor is quite high so as to make a lot of bands unusuable a lot of the time

author: bob91343
author_rank: Super Contributor
timestamp: May 09, 2020, 05:55:04 pm
content: I feel for you. When I put up decent antennas a whole world of fun opened up for me. It was no longer just getting on the air but now working serious DX and getting compliments. More than once a DX station picks me through a pileup and yells WOW! It makes it all worth it.Of course this luxury isn't for everyone. But the hobby has many benefits, including, as you say, the learning experience. Regarding an antenna analyzer, the best one (and cheapest) turned out to be the nanoVNA. I can't say enough good things about this high class piece of gear that is disguised as a toy.73, Bob

author: veedub565
author_rank: Frequent Contributor
timestamp: May 09, 2020, 06:20:37 pm
content: I only recently passed my test so it's all a steep learning curve at the moment. I was perhaps a little ambitious with this project. I've heard some good things about the nanoVNA too,for what it costs doesn't seem bad.73, Ben

author: cdev
author_rank: Super Contributor
timestamp: May 11, 2020, 01:19:27 am
content: A coaxial choke is actually a 1:1 current balun!Quote from: bob91343 on May 08, 2020, 06:31:45 pm