1
$\begingroup$

I am trying to homebrew a cobweb antenna and need some help understanding the guanella balun used as described here: http://www.karinya.net/g3txq/cobweb/

This author used RG316 coax around the ferrites, but I'm having a hard time stripping the delicate wires. I then found this balun with single strand wire: http://www.m0pzt.com/blog/4to1-current-balun/

Can I use this design to match the 50 ohm coax impedance to the 12 ohm cobweb antenna impedance?

$\endgroup$
  • 1
    $\begingroup$ Teflon insulation is so slippery that conventional wire stripping tools usually don't work. I bend the coax slightly upwards and use a new single-edge razor blade to carefully slice into the outer jacket and the dielectric. Just don't knick the wire strands (or your fingers!). $\endgroup$ – Mike Waters Nov 30 at 19:27
2
$\begingroup$

At the end of section 2.4 of "Building and Using Baluns and Ununs" by Jerry Sevick, W2FMI, he uses parallel 14-gauge Thermaleze-coated wire in a transformer to match $50\Omega$ to $12.5\Omega$ as described in the article you cite. This wire is available in short lengths from Amidon. Since the relative permittivity of wire coatings varies widely, using wire with a different coating may produce different results.

$\endgroup$
  • $\begingroup$ Is there any way to actually measure the balun before I install it to make sure I have the right number of windings? $\endgroup$ – SandPiper Dec 1 at 1:20
  • $\begingroup$ To validate the impedance transformation, you can terminate the balun secondary with a $12.5\Omega$ resistor and measure the primary impedance across the frequency range of interest. It may be necessary to measure the resistor, on its own, to be sure it delivers a pure resistance at the frequency of interest. You can measure the balun loss by driving a pair of baluns, back to back, with the second balun terminated in $50\Omega$. The power loss will be the difference between the driving power and the power going into the termination. $\endgroup$ – Brian K1LI Dec 1 at 13:51
1
$\begingroup$

The balun in the article by M0PZT does not accomplish the transformation you desire. The Cobweb antenna requires that $12.5\Omega$ balanced be transformed up to $50\Omega$ unbalanced, while the M0PZT balun transforms $200\Omega$ balanced down to $50\Omega$ unbalanced.

The optimum impedance for the transmission line in such a transformer is the geometric mean of the primary and secondary impedances, i.e., the square-root of the product of the primary and secondary impedances. This is why the optimum transmission line impedance for the Cobweb transformer is $25\Omega$ - as noted in G3TXQ's article - while for the M0PZT design it is $100\Omega$.

$\endgroup$
1
$\begingroup$

Sure, common-mode chokes such as a guanella balun can be made with coax or balanced transmission lines. Two parallel (in the geometric, rather than electric sense) strands of plain wire make a balanced transmission line, so there you go.

Theoretically, there's some disadvantage to the transmission line used in the balun not being "optimal", in the sense that it won't match the characteristic impedance of the rest of the system. Consequently there will be some additional SWR loss and some additional impedance transformation besides the 4:1 intention.

However as long as the length of the transmission line is small relative to wavelength (rule of thumb, less than $\lambda / 10$) the significance of these effects is negligible. It should be pretty plain to see that a 4:1 balun can't be made with "matched" transmission line throughout anyway: should it match the high impedance end, or the low impedance end?

So use whatever you have on hand. PTFE (teflon) insulation is a good choice: it can handle high temperatures (helps with soldering), and has low dielectric loss. But if you have something else on hand and don't want to spring for some new wire, just about anything will work fine enough unless you are trying to put 2 kW through this balun.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.