Operation of bootstrap coax traps
 

On Sat, 18 Nov 2006 03:58:04 GMT, Owen Duffy wrote:


Picking up on a comment in a previous thread on coax traps

....

I have put some notes together on a draft page proposing a
mathematical model for these traps.

The draft is at http://www.vk1od.net/coaxtrap/index.htm .

This is a different approach to any that I have seen, principally
because it treats the coax section as a transmission line (as I
believe it is). That is not to say it is correct, so if anyone can
identify errors in the document, I welcome their feedback.

I have not constructed and measured the example trap. Construction is
easy, I just don't have equipment to perform high accuracy
measurements of RF complex impedances up to around 100k.

Comments appreciated.

Owen
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Operation of bootstrap coax traps
 

On Mon, 20 Nov 2006 15:20:30 GMT, Cecil Moore
wrote:

Owen Duffy wrote:
It seems to me that an explanation that considers that I1=I2, or that
propagation time on the coax is zero, or that the inner conductor
forms a inductance with mutual coupling to the outside of the outer
conductor is flawed.

Is the following information available?

1. Compared to a 468/f CF dipole, what is the length
of the trapped dipole wire between the traps at resonance?

2. Compared to a 468/f CF dipole, what is the
resonant feedpoint impedance of the trapped dipole?

Cecil, I don't think there is a single answer to either of the
questions, if there is, it probably depends on so many assumptions as
to be even less relevant than the answer to those questions for a
dipole without the traps.

To illustrate the point, I have seen an optimal length / diameter
ratio proposed for the traps, but none of the articles that I have
seen explains what factors were considered in the optimisation, or the
extent of the optimisation. So what does an "optimised trap" mean?

My experience with coaxtraps on low HF is that they are relatively low
L/C and do not result in much shortening of the antenna. If one of the
reasons for trapping an antenna is to reduce its length, then other
trap designs are probably better for that purpose.

Owen
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