CB Antenna projects?

[DungButton]

Evening all, having just got my old CB rig out of temorary storage (35+ years at my Mum's house) I've not got it powered up on a bench PSU and after a bit of switch cleaning it seems to be working fine.

I'm now looking at a suitable antenna. I tried RX-ing on the HF Discone but its as deaf as a post on UK FM channels.

So, options. I'm looking at possibly making a temporary dipole after watching a couple of youtube video's. I've been experimenting this afternoon already with 2 x 2.5M lengths of wire hangin out of the 'shack' window attached to the short (2M ish) piece of spare Mini-8 Coax and a PL259. RX has improved a lot, I receive about 2-3 on the signal meter of mush but of course the SWR is appalling at >3:1

I think I need a 1:1 Balun if I'm connecting a Coax feed to a 1/4 wave dipole yes?

20230410_172915.jpg

I've tried being clever and wound the 2 x 2.5M sections of the dipole onto a piece of plumbing waste pipe but once again the SWR if still >3:1

Any advice (other than go out and buy a Sigma 4!)?

Thanks,

Dave.

[Buick Mackane]

Not surprised the SWR is high, By winding the wire you've added inductance making the wire appear electrically longer than it is, Needs to be straight

[DungButton]

Thanks Mr Buick, that makes sense, my reasoning for winding it helically was that in the 80's I had an antenna called a 'Modulator' IIRC and that had quite a long base loadin coil that was helically wound.

Meh! Back to the drawing board.

[Buick Mackane]

Thanks Mr Buick, that makes sense, my reasoning for winding it helically was that in the 80's I had an antenna called a 'Modulator' IIRC and that had quite a long base loadin coil that was helically wound.

Meh! Back to the drawing board.

Mobile antennas need loading coils to make up for their lack of height / length

[DungButton]

Okey dokey, I'm not giving up yet!

I'll try and lash up approximately 5.5m of battening to support a 2 x 2.5m lengths of wire for a 1/4 wave dipole today.

[Buick Mackane]

A total length of 5.15m should put you in the ball park

[DungButton]

A total length of 5.15m should put you in the ball park

Thanks Mr Mackane.

I was aiming for Ch20, the middle of the UK FM band (seeing as I'll be using it with my old Fidelity CB2000FM) which I found is 27.79125 MHz.

I watched a DX Commander video a while back where he explains some of the maths and allowing for end effect and cable velocity (I think the phrases were) the end result was 2.5m for each leg of 1/4 wave at that frequency I calculated.

[Buick Mackane]

Yeah, All in all a pretty easy antenna to make, Cut an inch or two off each end till you get the SWR To where you want it, Keep us posted on your progress

[DungButton]

Yeah, All in all a pretty easy antenna to make, Cut an inch or two off each end till you get the SWR To where you want it, Keep us posted on your progress

Will do Buick, I've got 'shed-time' lined up for this afternoon.

[DungButton]

Hmm, mixed results!

This afternoon I made a type of gallows (I explained to my Wife that I'm feeling OK though ) to support a centre-fed 1/4 wave dipole.

Its a beautiful thing, and just over 18ft tall....




Centrefeed, does it get any simpler?


I've insulated/supported each 'end' using non conductive washing line cord like this.


The choc-block was a simple fast way to cut a section off the radiating parts and re-attach each time, the green is the washing line cord and the black is the wire 'element'.

So I temporarily set up the rig and PSU in the shed and started checking the SWR...


Interesting, results.

1st try.
Ch1 - 2.4:1
Ch20 - 2.2:1
Ch40 - 2.0:1

Snip off about an inch.

2nd try.
Ch1 - 2:1
Ch20 - 2:1
Ch40 - 2:1

Whahey! Getting there, snip off another inch...
2.2
2.3
2.1

Meh, I've gone past the sweet spot I think. I snipped off another inch to confirm...

2.6
2.7
2.2

Erks, deffo heading in the wrong direction. By now the rain had started and the wind picked up a bit too, even though I've lashed the structure to the shed the top half moves too much for any long term use IMO but its been fun trying to do a cheap get-round.

Interestingly, laid on the ground the SWR dropped to 1.6:1 across all bands.

[Buick Mackane]

Reminds me of the clint eastwood western Hang em High

If i was to hazard a guess i'd say it's too short, The total length should be 16 feet 10 inches, It could also be that one end is too close to the ground,

Had another look at your pic and think iv'e spotted the issue, Your coax looks as if its running parallel to one of the dipole elements, It needs to run off at a right angle across your shed roof

[DungButton]

I kept singing Led Zeppelin's Gallows Pole to myself!

Yup, I started off pretty much with that in element length.This is the video I watched to help with calculations

I've got a lot more of that cable left so I'll re-wire and give it another go when the weather is better. In the long term I was planning on doing a long wire loop for my SDR experiments all aroun the garden using this wire so Its not a problem.

The wooden dipole really is too big and wobbly of course (not unlike myself) so I'm thinking about alternative soloutuions. In 1981 I started with a DV27 (I think it was called) on a metal ground plave in the loft. I'm a-wondering if I could build a 1/4 wave wire element to go vertical with (say) 4 groundplane wires spreading out at 90 degrees from the centre. I've got a handy flat roof I could put that on too.

Hmm <rubs hairy chin>.

Had another look at your pic and think iv'e spotted the issue, Your coax looks as if its running parallel to one of the dipole elements, It needs to run off at a right angle across your shed roof

Aaah, yes, you're right Buick. I didn't think of that.

[deadite66]

Callum's other video CB antenna video had ideas about attic antennas.

[Buick Mackane]

Had another look at your pic and think iv'e spotted the issue, Your coax looks as if its running parallel to one of the dipole elements, It needs to run off at a right angle across your shed roof

Aaah, yes, you're right Buick. I didn't think of that.

Would explain why the SWR Was lower when you laid it on the ground, In theory you should be able to achieve a SWR of 1.1 across the entire CB Band, Move the coax away from the bottom element and you should be set to go

[ch25]

Do it like on attached image.
Coil is 5 turns of coax on 10cm diameter.

hhhhhhh.jpg