WA Radio Scanner Users' Group

Hello ... !
Does anyone know where to get a 1:1 Balun Current Type for VHF range locally ?
Similar to W2DU VHF-B from http://www.unadilla.com/?p=97 ... I made my homebrew 2m 4 elements quad http://www.youtube.com/watch?v=zgQiISj4cc4 and I'm trying to improve it by putting a 1:1 balun to it ...

What needs improving by using a balun? All quad antenna's I have seen (but these were HF only) did not use/need a balun.

The only vhf baluns I have ever seen advertised in Australia were from ATN Antenna's in Victoria a long time ago.

I was just reading this article about unbalanced line and balanced antenna (in this case is the driven element of the quad which happened to be a full wave loop )

If we transmit in this manner it will make the transmission line become part of the antenna and radiate RF ... apparently it beat the purpose of having a directional antenna if we do this because it will skew the radiation pattern according this article

http://www.signalengineering.com/ultima ... _quad.html

Feeding the Antenna

One of the important parts of getting a really good, high gain radiation pattern from the antenna, is to feed it correctly. Under the Coax Basics section, I discussed that coax cable is unbalanced, which means that no current flows on the shield of the coax. The quad (and yagi) are balanced feed antennas, meaning that they require currents to be balanced at their two connection points. If you would just take the coax and hook it straight up to the antenna, what do you think happens? Well, when we look at radiation patterns (of a beam feed with balanced lines), we see that they are symmetrical. If we were to take our coax and hook it straight up, our pattern will get skewed off to one side (shown in figure 5).

So, what device is used then to convert the unbalanced feed of the coax to the balanced feed requirement of the antenna? We have talked about one device already. The Gamma Match, not only does it simplify tuning, but it acts as a device to match the balanced antenna to the coax. Another device is known as the balun, which is made from the words BALanced-UNbalanced. A balun takes the input from the coax and balances it. It is possible to build your own balun, but, I am recommending that you buy a commercial one. This is one place where its just best to use a professional designed and built piece for your antenna. Figure 6 shows how you can mount your balun to the spreader arm (shown for horizontal polarization). Figure 7 shows a view of the driven element so you can see where the balun is located. On horizontal polarization, I strongly recommend that you drop the coax straight down from the antenna like I have shown. This keeps the coax out of the immediate field of the antenna and prevents the feedline from disrupting the super pattern of this antenna. There is not a whole lot you can do to avoid this if you feed the antenna for vertical polarization (as shown in figure because you can see the coax has to run down the spreader arm. This is why I recommend using horizontal polarization. If you are building this antenna your main reason must be you want the absolute highest gain, little things like dropping the coax directly away are the little things that add up. When you go to get a commercial balun, make sure you go with a current type balun (as opposed to a voltage type). The quad and yagi require balanced current for a clean pattern. Another specification that you need to get is a 1:1 ratio balun. This does not have anything to do with the SWR ratio (we know 1:1 is perfect). The ratio on the balun indicates what ratio it transforms impedance's at. Since our antenna (the 4 element quad) has a feedpoint impedance around 50 Ohms and our coax is 50 Ohms, that means we want a 1:1 ratio. If we are building a 2 element quad we would need to use a 2:1 ratio balun because the feedpoint of impedance of the 2 element quad is around 100 Ohms. Thus if we were building a 2 element we would need to match 50 Ohms (coax) to 100 Ohms (antenna), and this is a 2:1 ratio. Keep that in mind if you are building a 2 element quad (Yagi's are different). So, when you go to buy, get a "1:1 Current Type balun" for your 4 element cubical quad.

Here are the two Baluns I recommend, the first is more expensive, but it might be worth it (I have read a lot of good things about it, but I use the second one listed):

This document describes a "Bazooka Balun" which might help.

Anything with the word "bazooka" in its name must be good

Zebedee wrote:Anything with the word "bazooka" in its name must be good

Like!

Yeah definitely it also sound awesome ... !
Anyway I have been trying hard to browse the internet looking for a detailed instructions on how to actually make this bazooka ... All I can find was just diagrams that explain how everything is connected ... However I found this instruction with nice photo etc and materials but it's written in French ... At least it is something ... and better than nothing ... hahaha

http://f5zv.pagesperso-orange.fr/RADIO/ ... 25D05.html

Nosferatu wrote:Does anyone know where to get a 1:1 Balun Current Type for VHF range locally ?

A choke balun might do the trick. For VHF, 5-10 turns of RG-58U around some PVC pipe (25mm or 32mm diameter). As close to the antenna feed point as possible.

Also make sure the feed line is perpendicular to the elements (ie try not to run the feed parallel to anything on the antenna.
Also-also use low loss feed, RG-213 or better.

robbage wrote: A choke balun might do the trick. For VHF, 5-10 turns of RG-58U around some PVC pipe (25mm or 32mm diameter). As close to the antenna feed point as possible.

Also make sure the feed line is perpendicular to the elements (ie try not to run the feed parallel to anything on the antenna.
Also-also use low loss feed, RG-213 or better.

Hello Robbage,
That sounds a lot easier to make than the 'Bazooka' ...

So the choke itself made of RG-58U but the actual transmission line is made of RG-213 ?

Yes, you shouldn't bend RG-213 around a small former like a 25mm pipe. The bend radius is usually more like 75mm to 125mm depending on the manufacturer. The last yagi I made the RG-58 joins directly onto the yagi and the other end has a N-type plug into a double-ended N-type socket. The RG-213 plugs into that. Plenty of self-amalgamating tape seals it up.
Some hams don't like choke baluns but they work ok for me. Give it a go and you can always take it off if you need.

Yeah, definitely I will give it a go if I don't receive the Unadilla W2DU 1:1 VHF Balun that I ordered a few days ago ...

What I will do is to use the PVC spreader for the quad as the choke balun former ...

I don't know why I thought you were building a yagi instead of a quad. Cross out yagi anywhere I said it