I’ve come across it in a couple of places but the only one I have to hand is page 19 of Backyard Antennas by Peter Dodd, G3LDO.
"The trick is to load the antenna to a lower frequency in the 21MHz region without affecting resonance on the 7MHz band. Fixing loading loops at the 21MHz high-voltage points of the antenna brings it to the correct resonance point without affecting the 7MHz resonance.
The size of the loops are determined experimentally and they should be equal in size. The shape of the loops is not important and you could use straight lengths of wire but you would need a method of tethering them. Try loops of about 300 mm to start with…
The loops can be fixed to the element temporarily using battery terminal clips and the diameter adjusted for resonance using an SWR meter. The loops can be soldered into position when the correct resonance point has been established."
Thinking about it, I think what is actually happening is that a length of wire is fixed to a dipole arm in such a way that the length of the arm inboard of the wire plus the length of the wire is resonant on 21 MHz, similar to those multi-band verticals with odd bits of rod fixed along the length to add extra bands. Anyway, it is worth trying it out, it doesn’t seem too complicated.
At least I have solved the linked dipole problem! Last Spring I made couple of linked dipoles which I could not tune correct – always SWR was close 3 or more. I was more than frustrated. In Autumn I made two 27 m long wire antennas of brand new magic wire (DX-Wire UL) with feeder line RG-174. These worked like Rock’ Roll.
During Xmas vacation I made again a new linked dipole which I could not tune in sensible SWR value. But the finding trying to tune 15 m band was that I needed to extend the wire and when I added 17 m wire, I got working 15 m antenna which SWR was 1.73.
Last Sunday I had time again thinking linked dipole. I compared my notes (which I used for measuring wires earlier) to corresponding measures which I got from Linked Antenna Designer ( http://www.sotamaps.org/extras.php ). And what was the result? Surprise, surprise the wire length for 15 m of earlier antennas was too short. No wonder that I could not tune it. Now I made new calculation with Linked Antenna Designer and build the new antenna.
What happened then? Well, outdoor I needed shorten 15 m band wire 5 cm and 17 m band wire 20 cm when tuning – nothing else. And now I am very proud owner of 5 band extra light linked dipole.
Look the SWRs, which are also good in SSB area (not presented). So, next I will make a copy of this and concentrate to work with them.
Congrats to your dipol. Of course, the SWR not always means that the so called antenna is radiating (see MM0FMF). But normally, when you measure better than 2:1 for a half wave dipol the antenna basically should be ok. But depending on the heigth over ground of both ends of the dipol you will measure a completely different swr, unless you always have the “designed” hight over ground also in the field.
As a coax line I use RG316 which has less attenuation compared to the RG174, is more rugged but is a bit less flexible.
So good luck and I hope to hear you again soon
You are fully right Andy, SWR does not tell the truth. But, but for me it tells that it is possible to operate with the antenna and those SWRs allow to operate without tuner. How this dipole radiate is fully unknown for me because I have not done any measurements and I do not have any tools for that. Next phase is go to summit and use it in praxis. BTW even I have lot of UK qsos you might be not operated - perhapse with the new antenna!
Peter, I use 10 meter telescope mast. Tuning has been done with it and the mast will be with in the summits. I suppose, based my experience so far, SWR will not change dramatically in the summits. This I will see later.
Gerald, manual jumpper I have also used. This new one will get alligator clips which I saw running well in Nick’s G4OOE antenna in Wales.
Andy is right, all these values are theory. The best possible method to clarify if the antenna is good or not and/or who does it work is keep it busy. Get power to it and listen what happens! Next possible option for me is Sunday 28th. But anyway in the close future you will hear me on the air!
I was testing new dipole last Sunday. It is five band antenna and my target was test it in all the bands. The test was 80% successful because I got qsos on 18, 14, 10 and 7 MHz but not any in 21 MHz. I do not know was the reason of badly radiating antenna or was the band closed that time. Generally it worked well but reports were two S-digits lower than typically. But time to time reports have been corresponding level with the traditional dipoles (now the gross-section of wire was 0,3 mm2 when typically it is 1,5 mm2).
RNB pointed the same what I remarked in praxis. But towards to new challenges and testing the antenna in coming summits. This one I am going to take with to the summer’s SOTA Tour.
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