11m coaxial dipole

I just build me a coaxial dipole for 11m according to:
I've tried modelling it in cocoaNEC and MMana, which didn't work out, so i just build it and adjusted the SWR with a nanoVNA to suit 27.55Mhz. Now i ended up with something which i would call a quarter wavelength dipole (total length about 2.6m). Not a big problem as i don't have that much space around my home. I am just worried about the gain i'll get. Can someone shed some light on this or has a model in MMana?

73

Hi Karel,

I presume the antenna you've presented is the one from here :-

https://www.hamuniverse.com/10metercoaxdipole.html


If so then the following link may help you further :-

https://owenduffy.net/antenna/PY2RML/index.htm


You'll see that if anything, you actually get a loss over a standard dipole.


I'm all for a bit of antenna experimentation and any antenna is better than no antenna

Keep up with your experiments and reading, it's the best way to learn and remember even 'setbacks' can be a valuable learning tool. 

As for antenna modelling software, (or indeed any modelling software), remember that they are just tools and reliant on their input. As with most computer based systems, garbage in equals garbage out so you need a sound understanding of what you are trying to accomplish first.

Be wary of antenna articles or actually pretty much anything published on the internet without multiple citation.
It'll save you a bit of time


Interesting stuff so thanks for posting.

All the best,
Victor

Hi Victor,

Yes, that is the antenna. I was wondering why i didn't get about 3.6m as the optimum length (0.66 x 5.5 approx.) Think i figured that out now. I used cheap RG58, so not with a foiled shield. In Antentop.org i read about leaky coax and think there is the explanation, both the center and the shield are radiating, so in fact i have an extremely thin folded dipole with a total length of a quarter wavelength.
Now for the 0.66 factor i'd have to test with a foiled coax, but i don't believe in that anymore. The outer shield is the radiating element and i expect in that case it should be about 1/2 wavelength.

As always, thanks for thinking along.
73

No worries Karel, antennas are a fascinating subject.


It does get spoiled by individuals making fantastic claims about certain setups, (including manufacturers!), because they've missed or don't fully understand the principles. I'll profess to not having enough years left in me to fully study and appreciate all the aspects.

I have no problems with compromise antennas, my own homebrew setups fit the resources available to me and although not perfect are still better than having no antenna. They may be resonant, have a good SWR, even a good impedance but that's not the whole story. Even a dummy load can be a perfect 50 ohms with a 1:1 SWR but we all know they don't make great antennas


As I said Karel, it's all interesting stuff. Look forward to you posting more with your own experiments.

All the best,
Victor

This is what i made my first aerial out of lol.
https://www.amazon.co.uk/16-25FT-Copper-Brake-Pipe/dp/B006MGAK7Y/ref=sr_1_8?crid=29X9ZRW0Q1A9B&dchild=1&keywords=copper+brake+pipe&qid=1612693047&sprefix=copper+brake%2Caps%2C228&sr=8-8

Similar story here Alan, experimented with soft copper tubing whilst playing around with magnetic loops.
Much cheaper from a plumbing supply trade outlet though

The next will probably be a regular dipole made out of coax, using the shield as radiator. I like it being lightweight, and larger diameter than regular wire. I have some 75 ohm coax laying around which is even thicker. I'll keep you posted.

73

You get a bonus if it works.
A BIG smile on your face lol.

Just connected the small coax dipole to my RTL-SDR. Fun, i got some signals, indicating it is a bit more selective than the original 6" antenna. At about 27.5 Mhz i got some music, don't know why someone would play music at this band but i gave me time to tune in on the station. Think i'll use this setup for listening and make a bent dipole to use for making contacts.

73

Nice to hear your continued antenna experiments Karel.

I'm not surprised you've heard music on the freehand area of CB as I've heard such examples many times myself. I also don't profess to understanding why?

My own experiments have always led me back to the simplicity and surprising effectiveness of the humble dipole. Amazing how well bent dipoles perform too and hence my zig-zag arrays.

Look forward to more of your work.


All the best,
Victor