A closer look at G3TXQ on common mode chokes.

[Marconi]

The thing about all these chokes is that they are all bench tested designs with an AIM4170C vna, they are guaranteed designs built on solid principles. The big caveat with any air core choke is that small variances in design and length can quite quickly have an adverse effect and make matters worse, even promote CMC. Back to your question, I use simple home built toroid chokes on all my Yagi antennas, I don't suffer RFI issues at home, so either I didn't have any in the first place or the chokes work. The one antenna that does cause me grief is my hustler 5BTV multiband vertical, but then I'm expecting the choke to work from 10 through 80m and that just isn't going to happen, a good job all my HF DXing is currently mobile, be that in the car or on foot.

35, I'm just curious. Where is it suggested that these results were from bench testing using the AIM417C? I've read his part of impedance and this part on chokes, and he only talked about a Palstar and the 259b. Do you personally know this guy G3TXQ?

I'm not surprised that your 5BTV wasn't helped much using a choke for the very reasons you describe above.

Over time I used several chokes on some of my horizontal yagi beams, and I really could not detect any advantage...just using my radio. I thought if a choke would ever be of advantage it might be on a horizontal beam, by helping to eliminate further vertical radiation from my reciever, but I still could not sense much with receive either.

I'm still looking for reports from folks that have used this info to successfully make a choke that really worked as they expected.

 

[Lil'Yeshua]

There's no RG8X on the chart
Thanks for clarifying

 

[Marconi]

There's no RG8X on the chart
Thanks for clarifying

According to the chart the type of coax doesn't look to make much difference to me.

 

[Lil'Yeshua]

According to the chart the type of coax doesn't look to make much difference to me.

Okay. Just thought it had to be exacting

 

[northern35s]

35, I'm just curious. Where is it suggested that these results were from bench testing using the AIM417C? I've read his part of impedance and this part on chokes, and he only talked about a Palstar and the 259b. Do you personally know this guy G3TXQ?

I'm not surprised that your 5BTV wasn't helped much using a choke for the very reasons you describe above.

Over time I used several chokes on some of my horizontal yagi beams, and I really could not detect any advantage...just using my radio. I thought if a choke would ever be of advantage it might be on a horizontal beam, by helping to eliminate further vertical radiation from my reciever, but I still could not sense much with receive either.

I'm still looking for reports from folks that have used this info to successfully make a choke that really worked as they expected.

I've not spoken to Steve on air or in person, but I have many times on one particular UK based amateur radio forum, a very helpful chap and will to share his knowledge and experience with anyone that cares to ask, I asked him how he tested the chokes, this was his reply:

If your choke is wound with coax, simply connect the braid from either end of the choke across the analyser; if it's a bifilar winding, connect the two ends of the same wire across the analyser.

If you have something like an AIM4170 analyser you'll be able to "calibrate out" the connection terminals and get reasonably accurate results on the "lower-Q" broadband chokes.

However, I find the AIM4170 still adds a few pF to the measurement, and that can cause a significant error on the Type 61 ferrite and air-cored chokes. For those I now use an S21 measurement using a 2-port VNA which adds about 0.2pF across the choke.

 

[Marconi]

Okay. Just thought it had to be exacting

Well LY, I always thought that too.

I've tended to ask questions, and got little to nothing in reply, so I've been a little skeptical about such ideas. That is the case still, and why I posted this thread, although I was hoping the discussion would go more in a direction regarding feed line length and height associated with common mode currents.

Some say only the length of the coax matters, while others say the #of wraps and the former size matters...which could be the same thing said a different way. It seems everybody has something a little different that makes their idea special. 35, gives us some testing details above directly from G3TXQ, but I doubt it'll be helpful here on WWDX.

This could suggest however, that we're just missing something in understanding of the idea, and for sure maybe how and what works. This said, I hardly ever hear a story that it doesn't work out with seeming remarkable results, and I don't think I've ever heard anyone claim this idea failed, even my own trials seem to work for a while.

We also hear a lot of do's and don'ts...and then we see examples of guys violating all the do's and don'ts in images of their applications. Again I wonder how they can all be successful with such applications.

So again, how could we go wrong LY?

We solve our TVI and static problems, improve the currents in our antenna systems, and see noticeable improvements in pattern and gain in our results with a very simple modification that anybody can do. Some guys have reported increases of 1-3 Sunits in gain, no more static, and a lower takeoff angle as a result of using a coax choke, and adding radials to their end fed 1/2 wave antenna.

All I have is questions, and I wish I knew the answers.

 

[Marconi]

Another thought comes to mind in this discussion on chokes. I may have already spoken about this, but it may be worth repeating.

In my experience testing a coaxial choke idea on my old Sigma4, all that I noticed on adding a choke I made using RG8 foam, that was about 7' 7" of coax, that worked out to about 7 turns on a 4" - 5" former, was the antenna frequency changed on me by more than 35% of the CB bandwidth...about 10-20 channels of difference just adding 7' feet of line.

Here are the results I was reporting to Bob, who I was talking to at the time:

View attachment Sigma4 and feed line transformation due to adding a choke.pdf

The only other thing I reported to Bob85 was a nice improvement of the SWR bandwidth, and you will note that at the bottom of the report.

I wondered why, and later Bob gave me to understand this was due to adding length to my feed line...while the antenna did not have a perfect match at the feed point, thus showing some excessive reactance in the load.

This was further described as showing feed line transformation due to some mismatch at the feed point, and this is the same reason I say you guys using a tuned 1/2 wave line to tune your mismatched antenna...may not be getting any useful information in using this special length of line in this very likely situation, with a mismatched load. I'm implying here that there is a distinction to be made in these words.

I know all you guys out there only operate your antennas with perfect matches, but I was seeing a really nice SWR with my Sigma4 at the time and thought I was good to go with a workable but less than perfect match. This is what I learned when increasing the feed line length to my old Sigma4 on adding a choke.