Just found a picture on a website about InnovAntennas showing a 1:1 Coax Choke Balun installed on a LFA-Q Yagi. With the text saying "remember to keep the coil separated from the boom".

Never heard before that the choke should be keept clear of the boom? All I have seen instead use the boom as a support, my own HF-beam no exception. Why this advice? what's the idéa and theory behind it? Does it apply to hf? vhf? are there anything in it?

The purpose of the choke is to introduce a high impedance into the Common-Mode path along the outside surface of the coax braid. If you tape the coax to the boom either side of the choke, the stray capacitance from braid to boom effectively "bypasses" the choke. The longer the length of coax you tape to the boom the bigger the problem.

The problem exists at HF, but is greater at higher frequencies where those stray capacitances represent a lower impedance.

Steve G3TXQ

Thnx Steve! appreciate that. I say that the lenght of coax actually touching the boom in my case might be 10cm, 4 inches, that's the "roll" and a few cm straping it to prevent it from moving in the wind.

Would you say that this little would matter on hf?

Coiled coax chokes can provide a very high choking impedance over a very narrow bandwidth - in other words they are "High-Q". But it's difficult to get them resonant at a particular operating frequency because any small change in stray capacitance (even as little as 1pF) shifts the resonant frequency significantly.

Your arrangement will almost certainly shift the resonant frequency a noticeable amount. However, the good news is that the "off frequency" choking impedance is probably still adequate providing we are talking about a 50 Ohm (nominal) antenna.

Steve G3TXQ

Wouldn't a yagi beam be considered a 50 ohm antenna? Know from litterature that a 3-elementer might be as low as 18-30 ohm, but most data sheets says 50 ohm for this type of antenna. So does as well the datasheet for this antenna Trying to learn as much as possibel from this, think this might be very little known and understood.

Yes - a Yagi beam can be considered "50 Ohm" in this context.

Steve G3TXQ

Downloaded the manual for this antenna and made a Google search and read about it a Wimo's webb as well. When reading carefully nothing actually mentioned about impedance. Nor giving any advice if it would be worth considering a balun, I guess 2:1 (25:50 ohm) would help matching. Not mentioning anything about feedpoint impedance, might it be deliberate? or am I just to suspicious

What antenna are you talking about?

Steve G3TXQ

Sorry Steve, didn't realise I didn't mention which antenna ... It's a MINI-2000, 3-Element 3-Band Mini-Beam from ZX-Yagi. Sold by Wimo, amongst others: http://www.zx-yagi.com/mini.htm

So your not susposed to tape coax to the boom now,

Guess nearly every yagi installed in the world needs to come down, :-(

I'd call that a 50 Ohm (nominal) antenna, so the requirements on the choke are not severe. Keep the amount of coax you tape to the boom either side of the choke to a minimum and it should be OK.

Steve G3TXQ

Thanks Steve!

So you see no need for a balun, perhaps 2:1, if we should think about theory and the fact that the impedance on such an antenna probably are quit a bit lower than 50 ohm?

No, I wouldn't bother with a 1:2 balun unless you've measured the impedances; I don't think you can assume the impedances will be close to 25 Ohms.

Steve G3TXQ

OK Steve, I'll rest my case until spring when I will take it down and also do a little bit of measuring on it. Based on what I have read in the antenna-handbook, a 3 element yagi should be considered being closer to 25 ohm's than 50, that's the reason for my question about the balun.

Well will be no more work on this antenna before april now, winter arrived today. Been snowing like crazy since early moorning and keeps coming and will stay until April now

Thanks for your support!