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Monday, 15 April 2013

A what-un? ah, A Balun!


Whilst musing about being able to run SO2V or SO2R back here:

there are quite a few things to consider. If I am going to operate with 2 radios at the same time, the most important factor will be protecting the radio in receive from the high RF generated by the radio in transmit - this could result in some very expensive damage if not catered for correctly - more on this later.

The other thing I need to accomplish is a second antenna for the higher bands. Lets say I want to run on 20M and 15M at the same time, clearly I can use the MA5B on one radio, but I don't currently have a second antenna for either of these bands. The simplest solution (as very much a starter for 10) is to cut and hang up a simple common multiband antenna - a G5RV. I've cut one using some hard drawn copper wire I had here, some 450 ohm ladder line and a commercial balun...... hence the confusion and lack of understanding.

I've always been bothered by baluns - what do they do and how do they do it? Whats the difference between a current balun and a voltage balun - whats a 4:1 balun for? Whats a 9:1 balun for? Why don't I just go out more and not worry?

Over the coming weeks I plan to run some experiments to answer these questions, to run some tests against a commercial 1:1 balun, and to construct some baluns for myself and see what they actually do.

So, starting with the newly strung G5RV which I've included a commercial balun between the 450 ohm feedline and the coax to the shack, I see this with my newfangled antenna analyser thingamabob:

Now, I only plan to really use this antenna connected to a second radio, but as a starter for 10, the antenna looks like it's OK ish. As a quick test (clearly the SWR on my dummy load will be better!) I left the antenna and radio WSPRing on 30M last night:

So, I conclude that the antenna radiates and it looks like I can transmit a signal using it.... so it's not just a dummy load. This is a good start.

So, the next project will be some kind of band stop filters for the radios so certain radios can only be used on certain bands - this should allow me to experiment with SO2R in an RTTY contest.

Now, back to baluns.... A balun should be used where we go from a balanced arrangement (e.g. a dipole) to an unbalanced arrangement (e.g. coax cable). I would therefore expect that the output of the two balanced ports of a balun would be out of phase with each other when compared to the ground point of the input port. Here I am feeding a 10MHz signal into a commercial balun and measuring the two terminals of the balanced output:

So, the yellow and the blue (please ignore amplitude) are the two output ports and the green is the sum of the two signals.... they look to me to be about 45 degrees out of phase - this is not what I was expecting at all! I thought they would be 180 degrees out of phase such that the sum of the two was as close to zero as makes no odds.

Unfortunately (or fortunately, depending on how you look at it), I'm heading back to A71 land again today.... I'll be picking this up in about two weeks when I get back.

I'd like to hear from anyone who understands the 'scope output above though, confusing - egh?

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