quality baluns for low power UHF

[windell]

Hi, I'm building a 1/2 wavelength UHF dipole for approxmately 434MHz and I'm currently searching for a high quality balun with low loss. The balun need not be wideband since transmission will occur only at 434MHz. The output from the radio will be at most 1W. 50ohm unbalanced to 100ohm balanced is what is required.

I did some searching online, but only found high power baluns and I looked on digikey and mouser, but they seem to have baluns with a fair amount of loss.

What I am looking for is a low power balun that has low loss and is small enough to mount on a printed circuit board. Does anyone have any suggestions?

Much appreciation,
Windell

 

[VEBill]

Are you sure your dipole is 100 ohms? A dipole in free space is nominally 73 ohms.

If you can eliminate currents on the outside of your (coaxial?) feedline, then the currents inside your feedline will tend to be equal and opposite, thereby providing the basic function of a balun minus the annoying loss. This type of choke can be coils of coax, or ferrite cores.

Another perfectly reasonable and practical option is to simply not worry about it at all. So long as most of the RF power is radiated away as useful RF in the directions you want, then many of the details can ignored. Feedline radiation can provide useful coverage into the dipole nulls.

The cure may be worse than the disease. Your instincts to avoid adding a lossy balun are probably correct.

If you need to impedance match, then there are tricks with feedlines that can get the job done. For example, a quarter wavelength (accounting for the VF) of 75 ohm coax can flip 100 ohms around to land pretty close to 50 ohms.

 

[windell]

Thanks, once the dipole is built, including the balun, we will tune the dipole using a network analyzer an placing the antenna in an anechoic chamber. Unfortunately, we cannot cut any corners by not using a balun becuase we want to divide the power as equally as possible between the two antennas.

 

[VEBill]

Dipole, or two antennas?

Balun, or power divider?

 

[windell]

It is only a single dipole antenna, but it has two radiating members. Each of these members is what I am refering to as an antenna. The balun is what is dividing the power between the two radiating members.

My dipole is for a satellite therefore, I cannot depend on the environment such as angeling relative to the ground for impedance matching. I also cannot use a 75ohm cable because I would get a large power reflectance at the radio to cable connection. My transmit power is very low at 1W therefore I need to keep the impedances between components the same or at least try to keep them the same.

 

[VEBill]

A balun is BALanced to UNbalanced transformer. It doesn't necessarily ensure equal power into each element of a dipole. Depending on the type, it may help to ensure equal magnitude voltages or equal magnitude currents. Most people would lean towards the current balun if required.


With respect to the 75-ohm cable trick to match "100-ohm" (?) load to 50-ohm source, the cable used in this mode is a transformer. At the end of the quarterwave line the impedance WILL BE nominally about 50-ohms (no matter that the cable has "75 ohms" written on the jacket).

You really should follow up with some additional research of your own on this quarterwave matching technique because the description of your application ('satellite') is screaming for this sort of solution. Potentially - zero extra parts, zero extra weight, zero extra failure modes, and (as you were looking for) zero extra loss.

This quarterwave transformer line would normally be installed starting at the antenna, and then use a normal 50-ohm flat line to reach the output stage. Between trimming the antenna and the exact length of the quarterwave matching cable, you should be able to get pretty close to a perfect match.

And if you can ensure that there are zero currents on the outside of the coax assembly, then Mother Nature will help to balance the two currents at the feed point for you.

 

[Higgler]

why don't you make the two cuts on the outer sleeves of the coax.?

That's the standard simple method I've used and you don't have to buy anything. Your balun becomes integral to the antenna. You can fine tune the frequency easily by changing the length of the cuts combined with the length of your dipole arms.

I've made many dipoles this way and your frequency is high enough to do it this way too.

kch

 

[windell]

Thanks for the reply, kch. I don't know how to do that. Can you direct me to a procedure about how to do it?

 

[Higgler]

It's real simple, and once you've done one, you'll look like a genius forever.

If you take a coax. attach a 1/4 wave (in air) wire to the center conductor in the up direction (call it north), add a similar wire to the outer coax ground in the down direction (call it south) then you have a very poorly matched dipole. For initial construction, use the center conductor of the coax., strip back the outer conductor and bend the center conductor northward. You need to solder a wire to the outer conductor unless you want to strip and peel the outer conductor in the south direction (that's doable, but I've usually soldered a wire)
To tune it, take a knife/dremel tool/etc. and cut the outer conductor of the coax. in two locations while looking at VSWR.
Pictorially, on an end view of this antenna; if the center conductor wire points north and the ground outer conductor wire points south, make the cuts on the outer conductor in the east and west location. The thickness of the cuts are very friendly, they can be thin or thick. As the depth of the cut approaches 1/4 wavelength (in the dielectric, not in air) your VSWR will become excellent. If you overcut the slot length, add metal tape to reshorten it (it can help on your first one to overcut the slot and make the two wires a little long also and trim for good VSWR at desired frequency).

Now for a real rugged dipole, you can get creative.

This is a very simple and always works.

kch
PS: that'll be a dollar.
PPS: I'll also, for another dollar tell you that if you want really wide bandwidth, i.e. way wider than you ever thought a dipole would be good for, just make the wires really thick or make them flat printed circuits. And I mean way wider bandwidth.
PPPS: If you want to just print a dipole, without buying a balun, that'll be another dollar.