I built a QRP shack in a box and want to go from RG316 coax to window line ( or maybe twisted pair or speaker wire ) and want to do the transition through a Balun. I believe I want a 1:1 Voltage Balun, but can’t seem to find good info on how to wind this. I have an FT-140-43 toroid and enameled 22 gauge wire with a max digital mode output of 10 watts.

I know wrapping the  RG316 will make a choke, but the sources I have looked at are contradictory in how to wrap a 1:1 Feedline balun. Some say use coax to wrap it others say use parallel wires to wrap it. Some say wrap 5 turns, cross the toroid and wrap another 5 turns opposite direction, others say wrap it all around in one direction.

I am confused. While I understand how a choke using coax works, I do not understand how a parallel wire wrap works.

Please point me in the right direction to figure this out.

Thanks.

I am no genius but the crossover is only or mostly to simply put the wire in the opposed position for layout purposes.  A regular wind would have your wires both exiting at 6 o’clock.  The crossover gets you in at 6 and out at 12.  Edit to add the theoretical benefit of the crossover is to put distance between your just cleaned up line away from your “dirty line”.

Here are wrapping instructions for transformers for when you get to that point in your toroid usage.  It is a great read for transformer type usage but not for choke usage like you need at the moment but do throw it in your saved radio reading files or bookmarks.

http://www.gnarc.org/wp-content/uploads/The-End-Fed-Half-Wave-Antenna.pdf


I am going to look for another site link, I think Palomar has a page on that if I can find it.

I didn’t see what I was looking for on their site, I may be remembering a youtube video they had out.  

here is a link to a picture of a kit they sell for the ubiquitous IC7300 and it shows the common mode chokes they used.  All 31 material ferrites iirc.
https://palomar-engineers.com/wp-content/uploads/Palomar-Engineers-Icom-7300-RFI-Kit-Flyer.pdf

5-7 wraps through a toroid for the coax and power supply I noted.


Here is one of their presentations and similar to what I recall watching.

https://palomar-engineers.com/wp-content/uploads/ABCs-of-RFI-for-Hams-SPARC-10282021.pdf


and since it was handy I popped the cover off my DXE max core 1:1 Balun.   Inside are 2 cores that are most probably 240-31.  Coax wrapped through center eleven times, 5, the crossover, and 5 more.  There is a blue unmarked capacitor across the coax on the post side of the car as it is built to be a center feed choke for a dipole.   I am guessing it is a high voltage 100pF rated cap. While it is a dipole box,....its main purpose is to choke common transmit power from radiating back on the coax to avoid making your dipole into a triplole.


Not loose individual wire though, sorry.

Current baluns are generally superior to voltage baluns. Explanations are here.

If you are going from 50 Ohm characteristic impedance unbalanced transmission line (RG316 coax) to 300 or 450 Ohm characteristic impedance transmission line (e.g. DX Engineering window line), then you are going to want one of two solutions:

a) non-resonant design, using an antenna tuner: 1:1 current balun

b) resonant design, no antenna tuner: current balun with ratio as close to the impedance ratio as possible, i.e. 9:1 if the ratio is 450/50.

1:1 current balun construction example:

I built one on IIRC a 240 31 mix toroid using parallel wires.  I used 14ga solid wire stripped out of some romex that still had insulation on it.  I can't remember the reason why, but something about the thick plastic insulation makes it work out to being a 1.5:1 balun instead of a perfect 1:1.

Anyhow I should dig it out and see if it's 25 or 75 ohms.  It might make it so I can use RG6.  

I need to make one of those.   I have seen arguments on that 2 pair winding and single pair winding for chokes.  Didn’t see a consensus on which was better.

I wonder if I can get the same effect on a 140 size for my QRP bag of or lower power.  Hmmm.


ETA I totally missed VOLTAGE balun being the novice trying to help out.  Sorry.

Hi Mach,
The old 1:1 voltage baluns used a tri-filer, three wire winding. An older hand book should have something on them. IIRC small zip cord is too narrow for higher frequency HF, I think it has lots of loss but don't remember if it's the insulation material or the spacing. Why a voltage balanced balun?
73,
Rob

Does this help?

The most effective choke would be a binocular winding on two longer/thinner ferrites.

Attached File


With RG-316 you could use somewhat smaller ferrites, Fair-Rite 2631625102 or 2631540002 for example. 3~4 turns is good.

Such a choke is higher choking impedance, wider bandwidth, has no issues with impedance mismatches in the choke, and is more compact in size.

Great video!

Current-mode baluns FTW!

Ok thanks for the help.

I guess I don’t want a voltage balun, I want a current balun.

I am leaning toward a non-resonant antenna dipole fed with 300 ohm 20g window line.

My primary bands will be 80 and 40 and 20 if I can get it.

This is for QRP in the back country. Where I already have QRP endfeds and a 2 x multi-band Base loaded QRP verticals, one really QRP and the other not but more efficient.

What can I say I like antennas and an efficient 80m antenna in the middle of nowhere is a bitch