"Would zipline work?"
The question is open ended as it depends on what your definition of "work" is?
Yes zip line will work, but in all probability its characteristic impedance will not
be optimal. Remember you want the transmission line impedance to be the
geometric mean value of the two impedances being matched. So if you have
a 200 Ohm load and 50 Ohm source then the would want 50 * 200 = 10000
and take the square root of 10000 which will provide 100 ohms. While the
additional loss may be only an extra 0.5 dB , that 0.5 dB shows up as 10
watts of rf being converted to heat when running 100 watts.
You need to measure the zip line impedance as the conductors are
close and the formula for calculating the impedance falls apart when the conductors
are close together. Calculated values can get really weird when the "S (spacing value)
is less than two times the diameter of the conductors. Throw in the insulation is made
of who knows what, probably some PVC variant, unless you measure the velocity
factor of the zipline, you are taking a stab in the dark at what is a quarter wavelength.
You really want to use about an electrical 1/4 wavelength of line for maximum bandwidth
and not knowing the Vp (velocity factor) can easily result in a line length that is more
than 30% longer then the electrical length. So you can easily wind up with the point of
least loss in the balun being much lower in frequency than what you planned.
In my experience you get lucky about one time in a hundred to one thousand
when trying to empirically use something like zip line in an application the
vendor never envisioned you would use it for. Yeah, I have read the articles
about ops using it as transmission line for portable QRP and getting 250 miles
per milliwatt, but those articles were written when propagation was much better.
PVC is not a horrid insulation but it is not my first choice.
When you add up the likely higher losses, high chance of using the wrong length,
hoping the impedance is right, you really have a snowball's chance in Hades of
"working" being an acceptable form of expectations in most cases for a balun. Of
course you can characterize the zip cord attributes and then calculate the results
to get a decent idea of if it will meet your definition of working.
73
Chuck WD4HXG