On Fri, 09 Jun 2006 00:05:47 -0400, Buck wrote:

frequencies and their associated SWR. Knowing my internal tuner can
only handle 8:1 but the antenna shows places I am interested in using

Buck, how do you know that your tuner can handle (presumably SWR up
to) 8:1? Is it a commonly published specification for transceiver
internal tuners or external tuners?

The capability of a tuner is much more complex (pardon the pun), but
could be mapped for each band to a Smith Chart as an area of input Z
that could be transformed to 50 ohms, and then one could plot loss
contours to show the acceptable range. What would you do with the
information?

Whilst it may be appropriate to specify the tolerance of Z on a
nominal 50 ohms antenna with a maximum VSWR in a given frequency range
(eg as often done for VHF and UHF antennas), the Cobra does not
pretend to present any particular impedance on any particular
frequency (remember that user is permitted to use any length of open
wire feed operating at high VSWR, so the Z at the tx end of the balun
cannot be specified at any frequency).

The Cobra Junior, a 72' long linear loaded dipole, looks interesting
in terms of a short antenna for low HF bands with reasonably low
losses. I have run some preliminary NEC models and analysed the feed
system loss with 25m of W551 ladder line, they look promising. I will
build a complete model over HF along the lines of the ones that
underly my article on the G5RV feed system at
http://www.vk1od.net/G5RV/index.htm . More when it is done in a day or
three.

Owen
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