I have used EFHW antennas with a 49:1 unun. Now I am starting to use a dipole with a connector of this type.
For 12m and 10m bands, QRP operation.
I have read that it is advisable to use a 1:1 balun. I want to build a balun with an FT82-43 toroid. But reading further, I have found out that there are two types of 1:1 baluns, one for current and one for voltage, with different wire winding diagrams.
Which of the two types of balun 1:1 should I build?
73, de Salva!
Salva - For a dipole fed with coax, typically you should prefer a CURRENT balun.
There are several designs in ARRL, RSGB etc Handbooks, and on the web.
For QRP work, âno balunâ is an option that many go for. Clearly, you can experiment with the various choices.
73 Dave
Do you have a length of coax from the radio up the mast with that coax to 4mm terminal adaptor at the top, then your dipole legs running out?
I use a choke balun made from coax. About 6 turns about 125mm diameter. The choke balun is located at the top of the coax feeder.
Hi Salva,
Not sure if this will help, but I use the SotaBeams designed kit, which has a balun at the top of the dipole. Some info here: LDK Instructions.pdf
73, Simon
A current balun does the job of preventing common mode current on the feedline, which is what you want a balun to do. A voltage balun doesnât.
And a current balun is simple: wrap enough turns of coax through your ferrite. If the core isnât large enough, you can use two parallel wires in place of the coax, using small enough wire to fit sufficient windings onto the core. âEnough turnsâ depends on the core size, type, and frequency: for small size you may need to resort to RG-178 coax (or 0.25mm wire-wrap wire).
But, to be honest, most of my portable dipoles donât use a balun.
Like my Portable Dipole Kit, that has traveled half way around the world since about 1980.
Occasionally I might run into a problem, but in most cases, even at 100 watts, it works well enough (even if not perfect) that it isnât worth the added weight and fragility of hoisting a lump of ferrite in the air.
If I do have a problem (often the SWR changes with coax length, or doesnât change when I adjust the dipole wires) then adding some extra coax may tame things a bit. (In one case, plugging in my headphones change the SWR curve.) But finding a suitable coax length to start with may avoid such issues in the field.
I have started experimenting with putting baluns on some of my cables, especially where I want to take repeatable measurements. A typical design uses two of the cores designed to slip over cables, one on each side of the center insulator, with thin coax wound through them. You can see one in this photo:
(These are larger than the ones I use for backpacking.) But the FT82-43 should also work.
A balun makes performance more predictable. But if you are always using the same wires and feedline, and you donât have problems due to lack of a problem when testing it at home, then likely you wonât when in the field, either. The balun certainly is useful for reducing noise pickup from all the electronic noisemakers in a modern home, or when using a noisy generator (or switched-mode power supply). For battery operation from a remote summit, thatâs usually less of a problem.
Donât let perfection stand in the way of âgood enoughâ.
For a QRP dipole, a 1:1 Guanella balun with 12 bifilar turns of 0.4 mm ECW wire on an FT-50-43 core are sufficient for up to 20 watts.
On an FT-82-43 core, there is space for 12 bifilar turns of 0.5 mm ECW wire.
Edit
Since the balun is primarily intended for use on the 12 and 10-meter bands, itâs worth paying special attention to the current balunâs VSWR.
This can be achieved, for example, with a 100-ohm transmission line connected in parallel.
A near-perfect solution regarding VSWR, insertion loss, common-mode impedance, and self-resonant frequency is achieved with two parallel-connected bifilar windings of nine turns each on two stacked FT-82-43 cores (as shown in the photos below).
Iâve tried many designs of bifilar baluns, none of them was satisfying in terms of vSWR across the bands. Especially above 20mhz.
Coax baluns on the other hand, you have nothing to do to keep the impedance stable. But less efficient for a given size. Coax being bulkier and harder to wind.
The problem with trying to maintain a good SWR while using bifiliar windings is maintaining the correct characteristic impedance of the windings. But in practice, the SWR error using short lengths of line (as would be the case in such baluns) is due to added reactance. So it can be corrected for when tuning your antenna.
For example, using 50cm of 125 ohm twisted pair on 10m, with a 50 ohm load, results in an SWR of about 2 : 1, with the 50 ohm coax seeing an impedance of 57 + j38 ohms. But if the dipole is shortened a bit to have an impedance of 50 - j39 ohms, the coax sees 45 + j0 ohms, for an SWR of only 1.1 : 1.
So the normal process of tuning the dipole for minimum SWR will correct for any error due to the impedance of the winding, as long as the winding length is reasonably short. (As it gets longer, the resistive components shift more.)
It does mean that your dipole wires might not have a low SWR when you use them without a balun. But wire is cheap - you can always cut a different set if needed.
That said, Iâd generally try to use coax where possible, but I happen to have small types like RG-174, RG-179 (75 ohm), and RG-178 (the next size down from RG-174) available. While the RG-178 is relatively lossy compared to RG-174, the total loss in 1m of cable is only 0.25 dB on 10m, which isnât going to make a significant difference in operation.
