Bent dipole built and the trouble testing.

Thought i'd share how far i am building my own antenna. For space reasons i've decided on a bent dipole, but with the bends being the same length(58cm) of a 11m moxon. Made from 10mm and 8mm aluminum tubing, so it's telescopic. I checked on ground level with a NanoVNA what SWR i couldd get (1.14:1). Next week i'll mount it alongside my house at about 1/2 wavelength height. When i've talked with my neighbor and he doesn't object to it hanging a bit over his garden i might turn it into a moxon. I'll use it with a ferrite choke balun.Thanks for all the info you provided me with. 73

I have some trouble installing the dipole. Hung it on its place, adjusted to the correct length(some markers) and took an SWR reading (about 3:1). With the NanoVNA i determined it was resonant at 80 Mhz????? Well, after a lot of trouble (among which the fact that nanoVNAsaver doesn't work on MacOS Big sur) i got the antenna back in  and checked the connections which seem fine. Can anyone shed some light on the VNA screen, and/or the feedline connection?

Think the SWR line is a bit shaky and the Smith chart is rubbish. What could cause this? Calibrated the VNA before taking measurements.

73

The blue line is return loss or SWR, which are both the same thing, just from different points of view.

I can't read the screen in your picture, so I'm not sure what your yellow line is showing.  As I typically use said saver software I turned on my nanovna to see what yellow is set to by default, and on mine it is set to "ch0 logmag", which is return loss, which the yellow line on yours is clearly not showing...  Can you type in the yellow text at the top of the screen? or perhaps a clearer picture if possible?  I can help you more with that information.  I'm thinking perhaps something like "ch1 logmag"?  That is the only thing I can think of offhand that should have that type of pattern when measuring an antenna...

I don't see any data being shown on your smith chart, if your not going to use it, it is possible to remove it from the display.

You mentioned about being resonant at 80 MHz?  Nothing  I see on your display has anything to do with resonance, so I'm wondering how you made that assessment?


The DB

The yellow line is the smith chart. Just changed the traces 0 and 1. Blue is swr, but not a nice fluent line as i'd expect.

73

For the bumpy SWR line i suspect:
- the left and right arm of the dipole are not the same electrical length
- maybe a layer of finish(anodised/laquered?) on the outside on the inner tube adds some capacitance to the arms.

Next step to take seems to me the use of some sanding paper. This will also help in easier adjustment of  the electrical length.

73

Neither of those lines are using the smith chart layout.  Neither of your listed lines do that.  One thing to know about the smith chart is includes every possible situation for an antenna, from a dead short to an open circuit, and everything in between.

The blue line looks like a standard SWR curve, or return loss curve, both look similar, the yellow line does not.  Is it possible that one is "ch0 logmag" and the other is "ch1 logmag"?


The DB

I'll take a longer cable up to my office and run nanovna-saver on my Linux desktop (Thanks Apple ;-( ). I totally agree that the yellow line doesn't look like a smith chart graph.

73

With nanovna-saver on Linux i got a real smith chart and an SWR of 4.5:1. This seems logic as the antenna (with one new leg) lays flat on the ground. Ran the model in Cocoanec with this height and that seems consistent. I can hang it at about 1/2 wavelength and then the SWR should be about 1.05. Now i just adjusted the lengths of the legs based on the S11 quality factor of the nanoVNA to dip at 27.5Mhz. Tomorrow we'll see when it's at the correct height .......

@The DB: I think you were right about the yellow not being the SWR. Suppose the size of my fingers don't match the screen size of the nanoVNA and, given my age i'm better with a keyboard and a mouse then with a touchscreen.
PS. by the 80 Mhz resonance i meant a dip in the SWR measurement from 5 to 100Mhz, when the antenna was hanging outside, by just using the nanoVNA.

Think i'll give up for now. Perhaps it's too close to the house. i used a 1:1 ferrite choke balun and with the manual ATU after adjusting the lengths i can't get better then this. what is an acceptable SWR when hanging a dipole in the attic?

Your antenna build and testing have been interesting to read about. I have never used an atu, a nanovna or anything like it before, only basic SWR meters to help tune my experiments. I’ve made a lot of creations in the attic space previously though.

The best SWR I have achieved on a horizontal dipole hung near the top of the apex of the roof has been just under 2:1 at band centre. An inverted V in the same place 1.5:1. Vertical ¼ wave ground plane with 2 slightly sloped radials (currently using) 1.6:1. No baluns or chokes used on any of those.

Anything up there can cause havoc with turning though.. pipes, wiring, junk (I have all that). Just moving the antenna very slightly, or moving junk around, messes with it. In fact, with the ¼ wave at the moment the SWR is only at that figure with the loft ladder down. If I put it up and close the hatch it goes up by 0.3 

The top part is fairly clear of stuff, but still there are metal roof vents and joins on the wood beams...




Anodised finish on the aluminium tubing may possibly be causing you a problem. I read somewhere that some anodising processes can create a poor electrical connection. I didn’t look into it in that much depth, but because of what I had read and local cheap sources of tubing all showed as anodised, and copper tubing was actually cheaper, I just went for the latter for the ¼ wave radiator and sprayed it with a thin coating of varnish once tuned to stop it corroding fast. Maybe try sanding the metal where your tubes join?

Keep playing and moving it around slightly, I am sure you'll hit a sweet spot soon  Look forward to reading more on your trials.

Just realised after reading your post that the antenna hangs straight under the rain gutter. I have to figure out where i can hang it more freely in the air (and how to do that) or hang it perpendicular to the gutter. I don't have that much space around the house, so this will be a puzzle. To get more distance to the gutter i might hang it in the attic.

Another thing i thought about is the connection to the coax. I've soldered  some speakerwire to the center and the shield of the coax and use that to connect to the aluminium tubing. To waterproof the connection i used some vulcanizing tape. So now the 2 wires are parallel for about 5cm. They might interfere with eachother.

73

So far i've done some simulating. Hanging as an "Inverted V bent dipole" seems possible and would give no parallel elements to the gutter.
Putting it up at about 1.5m above my garage roof seems possible with 3 supporting poles(fishing pole elements). Using a surf mast of 5.5m is also possible but i think the antenna will swing too much if hanging from one point down.
Perhaps just laying it on my roof (about 40 deg slope) might also get some distance to the gutter.

73

Think i'll hang him on the pole of my vertical and hope that the 2 antennas don't influence eachother. Then i can't aim him in the direction i want, but it will do for testing.

73

Have you managed to hang on the pole of your vertical yet? Just wondered if it has reduced your SWR readings? It would be good to get it up to see how it performs as we head towards the summer months with more sporadic E.

I have taken down my ¼ wave vertical and put up an inverted V ready for the summer, this time fed via a 1:1 balun made by superyagi.com (their 250w rated one). The feed point is approximately where the top of the ¼ wave was in the photo of my previous post.

Have obtained best SWR to date for any of my attic antennas. This is a manual plot of results using Zplots Excel plugin..



The ladder in the attic does not affect the SWR of this one   The noise floor is down 1 S point compared to the ¼ wave vertical   I have also talked to one local successfully on it, so know I won’t completely miss out on the very occasional local chatter even though polarized differently

Hope you manage to reduce the SWR a little on yours (although SWR is not everything) and manage to get some good contacts on it over the summer 

We've had a bit of storm lately, didn't want to work on the bent dipole, so i decided to hang the coaxial dipole i build in the attic. It is now tuned to 25Mhz, but with my ATU i can get it between 1.3 and 1.6 from 26 to 28Mhz. I have to shorten it a bit, as it seems the velocity factor is about 0.52 instead of the expected 0.66.

73

It's up! Not on the mast of my vertical, but on a surfmast of about 5m long. SWR is 1.6 on 26 and 28 MHz and 1.3 on 27MHz. According to the calculations it should have a max gain of about 7.5 dB.

Now waiting for summer! Aimed at the West, hope to talk to you soon.

73

Fun, it behaves exactly as MMana predicted. I just did the calculations using the length of the surfpole. Now i have varied the height and the optimum seems to be 6m. I see some possibilities to raise the pole 1 m.............

73