Home brew silver rod CB 11m antenna (wire made)

[RodSilver]

I had some summer holiday time and have always wanted a go at making a 1/2 silver rod. So I copied the Sirio GPS27 1/2 wave. It has the following coil dimensions/specs. You can see photos. I made bottom of coil to earth of SO-239 and top of coil to radiating element starting at 5.6M long

26 turns, tapped at 14 from the bottom, 21 - 22mm former plastic

I do not own an Antenna analyzer and did it using SWR meter of radio.

I made the antenna and it did not tune very well, I tapped the middle turns of the coil, in fact I tried every turn from top to bottom and it will not tune below 3:1 on any of them, I made multiple taps on the best loop and it still would not go down (tap lead wired to SO-239 centre pin) and only managed VSWR 3:1 around 27.5MHz I then copied the mounting bracket of the silver rod using wire. This brought me to 2.5:1 so it seems this bracket is part of the antenna somehow which is very interesting but not altogether surprising as it is a piece of earthed metal around the coil area. (a capacitor of some kind maybe?)

Can any antenna experts see where I went wrong ?

I really would love to get VSWR below 2:1

One curious thing is I noted that by 30MHz it started to come down a little more to 2.3 :1 and looked like it might come down further (possibly) but my band edges relay kicked in and stopped low power transmit on FM so I could not very quickly check VSWR beyond 30MHz. As this possibly suggests a point of resonance at a higher frequency (30MHz +) I decided to lengthen the radiating element... but all this did was make the SWR worse at 27MHz which is very strange.

No metal objects around at all and I am testing on a fibre glass pole where other antennas tune up perfectly well. (so it is not metallic)

I continuity tested all my conections, SO-239 to coil top, tap lead, coax continuity, all are fine.

What have a done wrong ? 3 images, high resolution so you should be able to zoom clearly....





And the sirio coil...



Many thanks I would really love to get this 1/2 wave coil loaded wire emulation working.

Here is a random Sirio GPS 1/2 wave Youtube just so you can get a clear view of the the antenna....



I just had a very obvious thought. Is it because it is not mounted on a pole ? If this is the case I could add a copper wire on the earth side of say 5 meters to act as a "pretend" pole. I would have thought this would tune up without a pole though.

[Transwarp]

The mounting pole (or lack of one) affects swr on these types of antenna. Stick it on a 6 or 12ft pole and see the difference.

[Buick Mackane]

You need to tidy the loading coil up for starters so it resembles the one in Mower Junkies video. And get rid of that huge crocodile clip

[Antoni]

That loop of wire with the clip is doing you no favours, make a much more direct connection. Yes, it needs to be closer to the Sirio design. Even-out that coil turns spacing too. The wire simulation of the bracket... isn't. I'd have thought that bracket would have influenced the design a bit in a few ways.

[Buick Mackane]

The simulated bracket is too close to the loading coil. Your best bet is to chuck it in the bin and start from scratch, No offense, But you can't just carelessly wrap some wire round a plastic pipe and expected it to work, Best bet is go back to the drawing board, Paying close attention to the coil design & dimensions

[RodSilver]

You need to tidy the loading coil up for starters so it resembles the one in Mower Junkies video. And get rid of that huge crocodile clip

Thanks all I will attempt to get it better.

Have you got a video of one ?

[RodSilver]

It seems you may well be right:



I need to wind a more accurate coil. I have made numerous coil loaded antennas for lower frequencies but it may be the case that
the higher you go the more sensitive to a poor winding they are.

Thanks for the tip, I wonder how I could get the spacing better than eye sight alone. I will mull it over.

[Buick Mackane]

The coil is basically a 49:1 impedance matching transformer, Transforming the impedance from a few thousand ohms to 50 ohms, You can also ditch the ''Simulated Bracket'' Its not needed.

[RodSilver]

The bracket seems to interact as it reduced VSWR from 3:1 to 2.5:1 that is not insignificant.

I am going to rewind the coil.

How can I repeatedly tap the coil easily for fine tuning, without a croc clip ?

Sure, I can solder permanently when I have found a match.

[Buick Mackane]

The bracket seems to interact as it reduced VSWR from 3:1 to 2.5:1 that is not insignificant.

I am going to rewind the coil.

How can I repeatedly tap the coil easily for fine tuning, without a croc clip ?

Sure, I can solder permanently when I have found a match.

I think once you've properly wound the coil the simulated bracket won't be required. Use a smaller clip.A big one will interact with adjacent windings, But you really need to get the coil wound correctly

[Antoni]

Just for info, the way to get the coil dimensions exactly right or at least evenly spaced is to use a lathe in screwcutting fashion. I have two tapped homebrew inductors (from rallies) which are beautifully made in that way.

Not many folks have a lathe but you might know someone. Set a very coarse thread pitch (or power feed) and use (or grind up) a round nose cutter with abt 1mm 'diameter' and use a very low rpm. Put the plastic former in and off you go. Even then unless you've got a fancy electronic servo lathe you'd be restricted to what pitches your change wheels can give, or you could be inventive and use a variable low speed battery drill on the traverse handle's nut - I do this all the time because it gives you any power-feed you want (sometimes with a reduction drive) and it's quick.

The result should be a perfect shallow spiral groove for the coil-wire.

[NightProwler]

Im still chuckling over your user name, Rod Silver...Sounds like a porn star name

[Buick Mackane]

Im still chuckling over your user name, Rod Silver...Sounds like a porn star name

But until he tidies his coil up performance will be poor

[RodSilver]

Thanks chaps, when I have time I will try and do the best re-wind that I can. Glad the name made you chuckle.

Sadly I have no advanced machinery so I will have to attach to a door handle and do it under tension with a very watchful and patient eye.

I am going to make some solid core 1mm hook up wire tap connector points and solder them on and a tinned end of the tap wire with a crimped U shape should just about hook on hold and stay, this way I can reduce the tap wire to the shortest length possible and do away with the croc clip.
Hopefully the U will slide nicely and tightly enough onto the tap connectors.

This way I can get the loops spacing more close as well, (no croc clip to worry about) they are about 1mm on the Sirio 1/2 GPS.

With a bit of luck these improvements will see me over the finish line.

I made the former out of a plastic Denture cleaning tablet tube for £1.00

I am going to call the antenna a Dentron 99

[scanhermit]

Dentron 99

I