Thursday, October 1, 2020

Balun musings

 

 For a while now I'm thinking about a coax wound airbalun to feed my vertical, in between the autotuner and the antenna. The design is made by PA0FRI and therefore sometimes called the PA0FRI balun. The main advantages: It's cheap and easy to make, it cannot become saturated. It is not a balun like the commercial ones you can buy. In fact it is a HF transformer, it transforms from unbalanced to balanced. I studied the website well and decided to make a switchable balun 1:1 and 1:4 ratio since I don't know the impedance of my vertical antenna system. What I overlooked is that the transformation ratio is only working for a real ohmic impedance, besides that it seems the lengths of coax on the balun have a influence on the impedance especially when they are relatively long compared to the wavelength. This is exactly what happened when I tested my prototype balun (I didn't know that before but learned from other wise men). 



I actually was surprised about the measurement results above 18 MHz. But if I had a better look on PA0FRI's page I should expect it. actually PA0FRI build a smaller balun to overcome a problem on the higher frequencies. Not shure though if this is a improvement of my antennasystem if implemented. Only testing it real time will give me a impression of how it is working. However I was not shure if I made the balun right and desoldered the relay to test the balun at 1:1 position, it was a disaster and not what I wanted to see. With 50 Ohm at the antenna side of the balun I couldn't see 50 Ohm on the other side, not on any frequency. I realize I don´t use 50 Ohm but 75 Ohm cable, could that be a problem. But my analyzer only does 50 Ohm measurements so how can I test if the balun is ok? Searching on the internet I came to the conclusion that measuring a balun with a MFJ259 is difficult. Especially if you want to construct a wideband version. The way I tested the balun is a improper way and gives a faulty impression. W8JI gives a very basic balance quality test example (find it in the MFJ259 manual as well!). However it is very basic and tells the balun is working though not what quality it is. A 1:4 balun also shows a phase difference in signal that can only be measured with a scope and signal generator. I don't have that kind of equipment as a simple hobbyist so building a proper balun will be difficult or purely experimental.

  So I'm re-thinking this whole balun concept. I probabely see it all in the wrong way, but something or someone has to open my eyes. 

I might try a compact design like the one DH1TW made. It fits in a small box and is easier to fix high in the tower between the vertical and autotuner. Besides that I still got a FT240-61 ferrite ring in my "junk" box somewhere. However I have to rely on his test results since I have no proper gear to measure it.

Yes I can buy a pre fabricated one at ON7FU. But what is the fun of that....

Another balun, or transformer project has been repaired. Remember my beverage efforts in 2019? Well, I was very enthousiastic about the results when suddenly it came to a stop. I presumed it was the rain that was terminating the fence and suddenly I didn't have good results anymore. Even when it went dry the results were not that good as before. Now I know why. One of the transformers smelled. You know that smell from burned isolation...I finally had some time to look closer and disassembled both transformers. Both were showing burns, you can see it on the wire on the photo, the light parts of the wire have no isolation on it anymore. I can't remember but I probabely transmitted on the beverage coax by accident "blowing up" both transformers. I have repaired both now and have some other experiments in mind to use them. I really have to be careful though to not transmit at all. I've been thinking, if it happens again, I will replace this "coated" transformer wire with thin wire and decent thicker isolation.

No comments:

Post a Comment

Thanks for your comment. Bedankt voor je reactie. 73, Bas