A tale of two feeders

Equipment Antennas
21

Quite right Brian even if veering off topic.

of course any length of doublet can be fed with open wire line with a tuner downstream to get a multiband system. However the world of AR now has a specific antenna and feed line arrangement in mind when it speaks of the G5RV, regardless of what Varney said. Blame the RSGB if you need a scapegoat…

Something similar happened with the Windom. Windom did not invent\nt it, but he was the first adopter and the ARRL publicised it as the Windom.

73
Ron
VK3AFW

22

Nah, you can’t blame the RSGB, they printed both his articles, and the variety of feeders is covered in both! I blame the people who supply the coax fed version, often with extravagant claims!

Keeping hams on topic is like herding cats!

Brian

23

Hi Ron,

Thanks for the remarks. Please note that I said “The idea that a mismatch will INVARIABLY result …”. I did not say that the input impedance to the extension was not affected by the choice of its Zo, just that opting for a Zo different from that of the original line can EITHER degrade or improve the match between the extension and the existing line, - IF the latter carries reflection!

Mismatch will invariably occur if there is no reflection on the existing line, a highly improbable case for the systems discussed in the thread.

We do agree that the improvement may reach the heights of ELIMINATING reflection in the extension completely, resulting in a PERFECT MATCH for that section. We also agree that this is not very likely to happen without design, but the chance of improvement should not be eliminated.

I wrote the first posting because no one had pointed out the fact that the choice of Zo for the extension does not change anything for the original line; - the reflection, SWR and losses remain the same. In order to ponder the question posed in the opening posting by G8HWI, one can therefore confine himself to the investigation of what happens in the extension alone, terminated in a lumped circuit element of the same impedance as seen looking into the original line. This brings a worthwhile clarity to the problem in my opinion.

I wrote the second posting, - not because you or someone else said something wrong, but rather that I feared some readers might not see the forest for the trees. They would be left with the impression that making any change in Zo will always make whatever mismatch there is worse.

Your saying:

“HOWEVER, any change in impedance in a feed line system produces reflections ….”

may reinforce such believes. Still I fully agree with your statement!

Any ONE and single wave passing a discontinuity in Zo, be it in the forward or backward direction, will be split into two components; one that continues past the discontinuity and other that returns. But keeping track of how all such components gather to make up the steady state terminating impedance for the extension is not making things any easier.

In short, I meant what I said in either posting and still do :slight_smile:

73, Villi TF3DX

24

And the return wave will be re-reflected and encounter the discontinuity again, of course.

25

Sure, I backed out of the wording “adding all reflections and decaying re-reflections ad infinitum would not simplify …” because I felt it would draw attention from the main point.

Villi