Folded (Linear Loaded) Dipoles

Ivan wrote:Hi all,

another possibility is probably to use 5,2 m of wire (unfolded) in the dipole plus 2,2 m of dangling wire at its end for 10,1 MHz.

VBR from Ivan

Hi Ivan,
Sometimes one gets so wrapped up in a technique one cannot see the obvious.
Thank you - just a bit of common sense needed  

Best regards - Harry - sm0vpo

Hi all,

another possibility is probably to use 5,2 m of wire (unfolded) in the dipole plus 2,2 m of dangling wire at its end for 10,1 MHz.

VBR from Ivan

johan.zr1jl wrote:Good day

I am interested in your guidelines for a "Folded (linear loaded) 1/2-wave Dipole". Thank you for the instructions. I am stuck at the table for the dimensions for the "Multiband HF Antenna for Small Gardens SM0VOP". Most of the instructions and measurements make sense, but I cannot understand the last two measurements in the Length Chart (10.125 MHz = 7.310m & 7.100 MHz = 10.422m) - relative to the 1/2 dipole diagram above the three x measurements blocks. How are the lengths derived? Can you please assist? I am frustrated as I suspect there may be a simple/logical answer, but I can't figure it out. Thank you.

Hello Johan,
I thought that someone would be a bit confused, sooner or later. It is the folding technique that introduces confusion. It is the 10 MHz dipole that I skipped, mainly because I use SSB. They tend frown upon SSB on 30 metres, especially during contest weekends (which seems to be every weekend!!).

The dipoles from 29-MHz down to 14-MHz are all calculated as 145 / MHz = metres. When I get this figure I simply add the wire, BUT I always leave about 10-cm extra, "just in case I need more". The 145 is 1/2 a wavelength at 1-MHz = 150 (million) metres (minus a bit for "end effect). As you can see from the pictures, the ends of the wires are all too long, but I roll them up tightly so that I can trim them without splicing in more wire or cutting the wires. These are all pretty much straightforward.

Since 14-MHz is my lowest unmodified dipole, it means that my antenna is 5.2 metres + 5.2 metres (two 1/4-wave halves).

The 7-MHz dipole was a headache. It initially seemed to defy all logic. This is how it works (numbers rounded a bit for simplicity)
For 7MHz we need 145 / 7 = 21.8-MHz "TOTAL LENGTH" = 10.4 metres for each half. But I only have 5.2 metres on each side to play with.
So I used 5.2-metres and folded back 2.5 metres, then folded forward another 2.5 metres. This makes a total length of 10.4 metres - Great
BUT -   - folding back 2.6 plus 2.6 also cancels one of them, so inreality I have an electrical length of 7.5 metres. Then I added another 2.6 metres dangling (3-metres for tuning) to make up for the electrical length that was cancelled. When I did that the resonance dropped to 7.1-MHz. I hope that you understand this.

I didn't build the 10.1-MHz antenna, but if I was to do this then I would calculate 145 / 10.1-MHz = 14.6 metres TOTAL, that is to say 7.3 metres at either side of the diople.
My dipole is 5.4 metres each side, so my 7.3 metre wire will have a surplus of 7.3 - 5.4 = 1.9 metres that needs to be folded.
Fold back 0.95 metres, then fold again forward the last remaining 0.95 metres. Now you have the correct length BUT you have lost one metre due to cancellation. So add one metre extra dangling wire, just as I did with the 7-MHz dipole (allow extra for tuning).

7.31 metres in the table is the NON-FOLDED 1/2-wavelength for 10.1-MHz. You will in fact need about 8 metres.

Now you should be able to get the antenna running on 10.1-MHz without any problem. You should be able to trim it to almost 1:1 VSWR without an ATU.



If you are not interested in 27-MHz playground-band then you can use cable number 2 in the above drawing for 10-MHz.

- Loop 2 back at S9 to S7.
- From S7 use cable slot 3 back to S9 (spaces S7 and S8 are not used for any particular dipole length - they can be adjusted).

Your hanging wire can be fitted with a bit of plastic tube to stop it flapping to the 14-MHz dipole and the duning should be ok.

I seem to recall doing experiments for other bands when I was experimenting, inlcuding 3.5-MHz, 10-MHz, amd 50-MHz. I remember that I was happy with 10-MHz, 50-MHz was useable, but 3.5-MHz was totally impractical in my garden.

Is all this clear? I know it is confusing, but it is just the folding that coonfuses things. If you double-back TWICE then you have three length of the dipole wire in opposite directions. One of them will be cancelled out, so you have to add an extra length equal tot he electrical cancellation.

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One other suggestion is to make the 10.1-MHz dipole 5.2 + 5.2 metres, then add a coil at the feedpoint, as described in the text of the article.
If you use the 5.2 metres length, then the base loading coil will be 3.34165 uH. That is about 29 turns of 0.8mm diameter enamelled wire on a 10mm diameter plastic former, as shown in the article.



The disadvantage to this method is that it is not so easy to tune. You could use just 25 turns and leave a 10cm "dangling wire" at the ends so you can adjust it.

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So there you have two solutions to the same problem.

Please let me know if you ahve any further questions. Very best5 regards from Harry - sm0vpo

Hi Johan,

the problem is probably caused by the fact, that a folded wire apears to be (electrically) shorter than the same wire stretched. Read the text - the dipole resonated at 9,4 MHz instead of 7 MHz after being folded. Harry probably did not make a 10 MHz dipole as he does not use CW.

Let us wait Harry´s answer.

73 from Ivan OK1SIP

Good day

I am interested in your guidelines for a "Folded (linear loaded) 1/2-wave Dipole". Thank you for the instructions. I am stuck at the table for the dimensions for the "Multiband HF Antenna for Small Gardens SM0VOP". Most of the instructions and measurements make sense, but I cannot understand the last two measurements in the Length Chart (10.125 MHz = 7.310m & 7.100 MHz = 10.422m) - relative to the 1/2 dipole diagram above the three x measurements blocks. How are the lengths derived? Can you please assist? I am frustrated as I suspect there may be a simple/logical answer, but I can't figure it out. Thank you.