element spacing

some advice if possible.
when building a fan di-pole with shared coax feed it is recommended that at least 150mm spacing be used between each element.

question. if each element was fed its own coaxial feed point can this spacing be eliminated
ie. using 7 core ribbon cable, where the spacing is only 5mm. will each element work independently or will the rest of the elements react with the element in use even when not connected.

Hi Glenn,

I'm not 100% sure in what you're asking but on my own fan dipole array all of the dipoles start from a central connector to the coax and then arc out from there, (I used 'bus-bars' and have seen everything from 'choc-blocks' to commercial dipole centres used).

The minimum space suggestion from thereafter is to keep proximity influence to a minimum, the stray capacitance/inductance from running the dipole wires too closely to each other will certainly affect each other. So as the description states, the dipoles 'fan' out from one single connection, resonance of each dipole determines everything thereafter.

My own dipoles then spread out either straight or in a zig-zag fashion but mostly as an 'Inverted V' example to fit within my loft space working within the constraints of what I have but ensuring they don't cross each other. It's not the perfect antenna system by a long shot but does give me multiple resonant dipoles from a single coax connection enabling me to successfully work everything from 160m to 10m. Definitely better than having no antenna and for me much more successful than a single 'long-wire'.

To tune I cut all of the dipoles slightly too long, (easier to cut off than add on), and started clipping/tuning from the lowest frequency or longest dipole to the shortest or higher frequency. You can bend back wire to effectively shorten their resonant length also.


I hope that helps somewhat but we can always bounce this around the forum until you achieve what you desire. I'm sure many other members here can add whatever expertise or experience they have also.


All the best,
Victor

thanks victor but the crux of my problem is only one mast
from this one mast i want to run a 17m di-pole, a 10-12m 5/8 t2lt and a 20m ground mounted1/4wave antenna. i can only use one antenna at a time
question if all three are mounted together on same mast but used independently will the sear proximity of other wires effect each antenna
question2 i was hoping to run 3 or 4 coax feeds through a length of plastic conduit to keep things tidy, will this also have an effect
i hope someone can answer these questions before i go ahead  and implement this idea.

Hi Glen. You should be fine. There probably will be some interaction as they are so close whether you feed them individually with feedlines or all fed from one line, but it will work.

My mess in the loft is fed from one feedline and currently has wires running from it on bands from 10m to 20m. The feed point..



You can see how close they all are. 10/11/12m are one dipole of 450Ω window/ladder line terminated at each end, 15/17m are the other 450Ω dipole, non-terminated as the 17m wire strings out further than the 15m wire, and the wrinkly single mains wire is resonant on 20m. The 450Ω ladder line wires are only a finger width separating them. And contrary to best practice of running the coax at a right angle aware from the dipoles mine is as short as possible and runs near the a 10m dipole leg. It all works though.

My concern would be if you were running separate feed lines to some of the antennas and wanting to operate more than one radio at the same time that your radio front ends would get overloaded when you tx.

thanks only one radio at a time

Ah, I think I see now Glen with the extra info you've supplied.

I know when I ran my 2m & 70cm double-extended-zepps to the loft not only did I have to use a duplexer to implement one coax feed I also had to keep the antennas away from each other as well as from the HF fan-array. Luckily the loft space is big enough that I could get such separation.

The willing 'volunteer' that assisted me to install them did hang them too close to my fan array feed to start with and I had interaction problems so they were moved further away from each other. It's quite the mess up there now but finally all worked.  

Thankfully the gracious donation to me of a NanoVNA helped a huge amount in resolving those difficulties but I still had to feed a further line to my VHF Horizontal Halo and luckily that antenna was in the clear.

All my coax lines run together from the shack to the loft but the use of common-mode choking has helped there too.
From there the HF array is fed from the bottom with the coax running vertically down but the VHF/UHF vertical dipoles have the feedline presented to them horizontally and the Halo also runs away from it horizontally as it is mounted in the clear.

Can be quite the challenge to muster the ability to have decent multi-band antenna systems!
Our own fault for being so greedy with band usage.
(I now have everything albeit not perfect from 160m through to 70cm - God help me if I get interested in 4 or 6m too!)


This is all going to be a problem if running everything off one mast, I can see each antenna interacting with each other especially the T2LT or coaxial shrouded dipole if the top section isn't in the clear. The coax feeds for that and the dipole for 17m will obviously interact with the 20m vertical too if that is running up the mast.

Bit of a nightmare scenario you've got there Glen!

I'm not sure off hand if this can all be resolved but I do understand why people run Cobwebbs & Hex-beams for multi-band activity being smaller footprint versions of a fan-array. It's all about impedances and proximity interaction.


I'll think on this some more and hopefully someone else may be able to provide some insight also but I think it's a difficult one.

All the best,
Victor