Evening all , im going to build a link dipole for qrp to use from a yaesu ft818 so the max of 6w and i know we are taught to use a balun ( balanced to unbalnced ) the dipole being balanced and the coax not being and help to reduce common mode currents on the other braid … But at qrp levals does it actually make a diffrence ?
Can you get a lower vswr with a balun ?Is its quieter on the RX but on a mountain i doubt much noise anyway !
Plus i see the origional sota lonked dipole does not use a balun …
Can anyone with a bit of experience on the subject expand a bit more please ?
It makes a difference regardless of the power level. The major difference is a reduction in received noise (common mode noise). That is usually not a problem on summits, so doing without a balun/choke is fine.
When I added a 1:1 current balun to my home dipole, the noise floor dropped by 1 S-unit.
As i was thinking , made mono band dipoles and used portable in fields and the like but yet to try one at my home qth yet . Perhaps il get one up for this winter in the uk . But regarding to Sota it shouldnt make a diffrence , so i will build one to try .
At the power levels we run on summits you normally don’t need a balun to match the transmission line to the antenna - as long as the you are feeding it is near to 50 ohms impedance (normally at the centre). If you use an off centre fed dipole instead of links to get multiple bands out of one antenna, then you need a 4:1 balun.
Only my OCF antenna in my SOTA pack has a balun, all the others don’t and adding a balun adds weight, so IMHO it’s good to avoid it if you can.
My experience is that a balun helps with the adaptation, if you come from symmetric antennas to an unbalanced connection or coax cable. In addition, I had the problem with the IC 703 continues to sent CW without Balun.
A Balun with a little RG 174 over a toroid does not have much weight and hardly needs space.
What you have in your pictures is an RF-Choke. it is sometimes known as an “Ugly-Balun” and is used as you say to stop RF coming back down the screen of the co-ax. A similar option is clamp-on ferrites (of the correct frequency rating) again, to stop RF feedback from the antenna.
I believe Matt was asking about the Balun that is a transformer interfacing between balanced and unbalanced and matching impedances. If he gets RF back down the screen of the coax, the solution you show here is a good one but hopefully, if the antenna is resonant and a good impedance match he should not need even this.
Re the IC703 keyer being locked up by RF on the feedline outer, mine did that too. Apart from installing a choke balun, which will undoubtedly improve things by removing most of the RF, you can further improve the immunity to RF on the keying line by installing a wire between the key socket ground and the main PCB. I have done that on my 703 and it does make it much more tolerant of RF floating around the “shack”. Apparently the common line goes to earth via a roundabout route, shorting that out at the socket is an improvement. I think you’ll find that mod on the mods.dk website, but I think also that the description above is entirely adequate. I just put one wire about 2-3cm long from the key socket over to the PCB ground nearby.
I also had rf feedback with the speech compressor on the 817, it was going into oscillations due to RF on the earthy side of the shack, even worse when I was using a 40w amplifier. Worse on some bands than others. Inserting a choke balun into the feed connection has cured that completely according to tests done so far on summits. The antenna in that case was a zs6bkw doublet fed with ribbon.
a ugly balun or choke RF doesn´t make a lot of work and it will be better radiation, no RF return and low noise, for QRP a couple of turns of RG-174 in a toroid (Amidon FT-140-43, for example) will be enough.
A 1:1 or 4:1 balun need not be heavy or expensive:
I was taught that for a balun to be a balun, it must isolate the antenna from the coax feedline. So an rf choke IS technically a current balun, whereas, a so called 4:1 voltage balun is not actually a balun, but merely a transformer because it does not provide any isolation. I was also under the impression that a dirty balun is just a coil of coax (without any ferrite) that provides (in general) only very poor decoupling. As soon as you add the correct ferrite material, it becomes a proper balun. But I may be wrong of course.
73 de OE6FEG
For a coil of coax to work as a choke balun it needs to have enough turns and enough diameter, for assorted and varied vaules of enough. Also if you wind as a solenoid you get better performance for fewer turns or diameter.
My experience is my dipoles are better behaved with a coax choke balun. YMMV.
Yes Matt. Like all things, trying to make these things work over several octaves is not easy. I really should try a ferrite loaded choke balun but when I was making mine I had plenty of coax and no suitable ferrites.