DIY RF EMC Biconic Antenna

[Jay_Diddy_B]

Dazz1,

Did you know that the MIL461A std has all the drawings needed to build the antenna?

The document is attached to this message:

https://www.eevblog.com/forum/testgear/diy-biconical-antenna-30-300mhz/msg2257035/#msg2257035

There are some difference between your design and the drawings in the MIL standard:



The holes are at 30 degree angle.
The holes are not flat bottomed. The flat bottom will not change the antenna  performance. Only the diameter of the holes and the diameter of the rods have tight tolerances, but I think this is for mechanical not electrical reasons.

Your design has the holes at 90 degrees to the axis:



The antenna factor is also published in the MIL std:



If you build the antenna it should be very similar to the published antenna factors.

Regards,
Jay_Diddy_B

[coppercone2]

I have been meaning to make that antenna you posted for a long time with aluminum  castings.

[wilhe_jo]

Maybe one should mention that the MIL standard is MIL-STD-461A. The newer editions miss these drawings. Btw: if someone has the original one (without the A), I'd be glad to have a look at it...

The biconical is IMHO not too good of a choice outside some chamber. You can find broad band TV Yagi antennas which work reasonably well for this purpose and allow for some tuning (front/back gain) to reduce some peaks at least for some 10-20dB.

However, if you have lots of Broadcast stations around, that's still a huge problem.

Btw, the 2 antenna calibration procedure is in mil-std as well...

73

[edit]
upps, the std-number was already mentioned... never mind
[/edit]

[dazz1]

...
The holes are at 30 degree angle.
The holes are not flat bottomed. The flat bottom will not change the antenna  performance. Only the diameter of the holes and the diameter of the rods have tight tolerances, but I think this is for mechanical not electrical reasons.

Your design has the holes at 90 degrees to the axis:
...

Regards,
Jay_Diddy_B

Hi
My design has 6 elements at 60 degree spacing.
The original Mil-Spec design used a lot of pipe fittings.  They would be expensive and plain ugly.  I believe that good engineering should look good.  It's an art.

[dazz1]

Btw, the 2 antenna calibration procedure is in mil-std as well...

I never claimed that the two antenna calibration was my idea.  I only claim that I plan to use that method.

[dazz1]

Hi
Manufacture of the bi-conic has hit a pause for a while.  I have wandered off the path.

I had a few options to configure my milling machine to make the top/bottom hats.  I chose to fit a table with angle adjustment.  When I fitted the table, I found that it clashed with the Digital Read Out encoder.   To fix that I needed to remove a gearbox cover.  When I did that I found 45 year old Soviet era grease.  Not a pretty sight.  So now I am stripping down the milling machine for deep maintenance to clean and re-lube. 

I also purchased a Philips PM3072 scope that doesn't work.  The power supply has a fault.  There may be more.

I will get back to the biconic.

[coppercone2]

lol thats how I feel about my equipment too, I found a dust bunny in a key and ended up taking a entire machine apart, washing it, and baking it. Thats electronics, but mechanical stuff too, like I end up taking crimp tools apart and shit like that because there is usually some crap with old grease, dirt, etc, in side. I hate that in my tool box. sometimes its a little risky and expensive when I screw up but its the right thing to do. 

And I mean stuff like that is totally rational too because giant machines are hard to move and stuff like that, and you might find a cracked gear or whatever. And sometimes you have energy for a project but not for the maintenance work, and having something break down because of prior inaction is a deal breaker, especially if its something crazy thats deep in the bowels of a machine, talk about change of plot.

[dazz1]

Hi
Well after a long pause and a period of completing deep maintenance on the milling machine, I have restarted work on the Biconic antenna.

I refitted the digital read-out scales and removed as much Soviet era black grease as I could.

I am currently working on the fitting that sockets the ends of the elements.  This is a conical shaped object with 12 holes drilled for the elements and sprung ball indents.  Each hole requires 4 operations to complete.  These include:
1. centre drill to guide the 1st drill bit.
2. drill a pilot hole
3. drill a full size hole to depth.
4. chamfer the hole edges.

Each fitting has 12 holes and I need 4 of these fittings for two antenna.  For a total of 96 holes that requires about 380 operations just for the holes for those fittings.
I will still have lots of things to make when the fittings are made, including the antenna elements.

[coppercone2]

just a fyi the black grease might be molybedium disulfide and the best thing might be fresh black grease, or it could be oxidized garbage, at least according to AVE.

Do I think that molybedium disulfide is the best thing? Not sure unless its like on the power  transmission for mining equipment or something with high volume titanium hogging or whatever

[coppercone2]

Also, it might not be a bad idea to alodine that stuff. You can't get the electrical one easily but the other one might still help an antenna over time.

Ultrasonic clean and dip coat them in a bucket if you want to do that, its the easiest way

at least, if they are being threaded.. if you are welding or soldering that then you should be way better

your project made me think of this, I would want to try this
https://www.sra-solder.com/al26-181-75-aluminum-solder-paste-250-gram-jar

[dazz1]

Hi
Another task completed on the path to making a matched pair of Biconic antennae.
I have finished the 4x fittings that socket the elements at the apex of the cones.

These took days to make.  All of the holes had to be drilled to close tolerances in position and depth to avoid joining up with other holes.

These fittings look remarkably similar to the UFOs from the Jerry Anderson classic U.F.O.   For those who have never heard of Jerry Anderson and the important mission of S.H.A.D.O, it is only necessary to know you led a deprived childhood from which you will never recover.

The next task is to make the elements.  You might be thinking this would be easy, but there would be no challenge or fun if it was.  I need to cut the elements to length and machine the indents for the sprung loaded balls before I start any bends.  No chance to make corrections later.   The elements are 6mm aluminium rod.   I have printed off a full scale side elevation of the elements to make sure the elements are all the same.

[dazz1]

...

your project made me think of this, I would want to try this
https://www.sra-solder.com/al26-181-75-aluminum-solder-paste-250-gram-jar

I don't have space to store fully assembled Biconics.  Mine will snap together so I can break them down to store and transport in a flat case.  They will never be exposed to the weather.   So while solder or threaded joints might be close to perfection, I think spring loaded ball detents will be good enough.

[coppercone2]

hmm i wonder if nasa has umbrella deploy types of this

[Kean]

These took days to make.  All of the holes had to be drilled to close tolerances in position and depth to avoid joining up with other holes.

Nice work!

[dazz1]

Hi
I have started making the elements. 
I have previously made the 90 degree bar bender.  I now needed a tool to form a large radius curve (535mm radius) on the short leg of the L.  There are lots of DIY slip roll machines on the internet.  The tool I made isn't like any of them.
I used a row of nails in a scrap piece of plywood to bend the element around. 

There is a lot of spring-back so I did a test bend with a arbitrary radius, compared the rod with the desired radius, then calculated the radius tool I needed to finish at 535mm.  It turns out I need to bend the rod around a 305mm radius curve to end up with the desired radius.

The large formed radius will create spherical ends to the antenna.  Will this radically improve performance ?? Probably nil observable effect, but it will look cool.   Easier to bend the rod than to drill angled holes in the top fittings.

[nctnico]

  

[coppercone2]

it will also probobly be stronger if you drop it

[dazz1]

it will also probobly be stronger if you drop it

This is the Internet.  I am looking at flat-earth theory for inspiration to invent some pretend science to justify a spherical ended antenna.

[coppercone2]

that is a very legitimate design I see it in alot of papers. But it will also be more robust if you drop it like a old BMW car.

I have had antenna tri pods tip over before. They like tipping over.

[dazz1]

that is a very legitimate design I see it in alot of papers. ...

Yes I leaned on those papers when designing my antenna, but I haven't actually seen a physical antenna with spheroid ends.
One option I have is to make another set of elements to try different shapes, including smaller/larger/conical ends.  The elements will be cheap and easy to make.

[dazz1]

Hi
Another small step for mankind with progress on the elements.
The attachments show the jig to make the 90 degree bends.

The antenna calibration process will depend on both antenna being identical.  So the jigs will go a long way to achieve that.

[dazz1]

Hi
OK, after spending way too much time making jigs for the elements, it took very little time to bend up the sets of elements.

I just plugged-in the elements into the top/lower fittings for a photo op.  I need to fit the sprung ball indents.  For that I need to make another jig to file indents into the ends of the elements. 

[dazz1]

Hi
Something I haven't yet nailed down is the choice of balun. 
I don't yet know the frequency range of the antenna, nor do I know the characteristics of the antenna.

[tautech]

Hi
Something I haven't yet nailed down is the choice of balun. 
I don't yet know the frequency range of the antenna, nor do I know the characteristics of the antenna.

You can get a fair idea with your Siglent analyzer and a RLB.

[dazz1]

Hi
Something I haven't yet nailed down is the choice of balun. 
I don't yet know the frequency range of the antenna, nor do I know the characteristics of the antenna.

You can get a fair idea with your Siglent analyzer and a RLB.

Ideally I would like to measure the balun and antenna separately.

The references indicate my antenna should be ~50ohms.    What I could do is fit one side of the bi-conic antenna to a sheet of aluminium to make it a mono-conic.  It would be something like a mono-pole antenna on a ground plane.  The aluminium sheet would act like a rf mirror.  I could then drive this directly with an unbalanced signal from the sig-gen.   The problem is I don't have a large enough sheet of aluminium.

Roberts invented his balun for the biconic antenna, so I think I will try that first.