On 5/3/2010 1:09 PM, Wimpie wrote:
On 3 mayo, 03:33, John wrote:
I would like to test the impedance characteristics of a rectangular
full-wave loop antenna. My test equipment is 50 ohms unbalanced.
Frequency is 450 MHz. What are my choices to achieve the conversion from
balanced to unbalanced and which might be the best (affordable) choice
for my test purposes?

Cheers,
John, KD5YI

Hello John,

There are many options for your frequency range.

Try to get some ferrite rings that tight fit around the coaxial cable
and that have high impedance around your frequency. When you have
specs (from the ferrite you use) this is the easiest one that doesn't
require any tuning.

connect the feed cable to a solid ground at 0.23 lambda from its end
(where the loop is). The low common mode Z at the short results in a
high common mode Z at the end (where the loop is). two strips of
about 0.22 lambda can also serve as a floating ground.

you can make a bazooka type choke (about quarter wave sleeve around
the cable). Best is to have lots of air between the sleeve and the
coaxial cable. This one requires some experimentation as the actual
length depends on the dielectric around the cable also.

When your loop impedance turns-out to be high, your balun must be
better as the common mode voltage component increases with high
impedance balanced antennas. I see you are working on a full wave
loop, this one is not high Z (below 200 Ohms).

Good luck with the loop measurements,


Wim
PA3DJS
www.tetech.nl
remove abc first in case of PM

Thanks, Wim. Good suggestions. I'll give them a try.

Cheers,
John, KD5YI