Ferrite bead balun for UHF?
 

I've modelled a 70 cm Moxon rectangle - with impedance of 50 +j0, and
F/B around 30dB. The cardiod radiation pattern is ideal for my
application - a repeater on one side of the service area, co-sited
with a 70 cm link repeater. The service area covers only about 25
square miles - filling in a local coverage gap. I need the rearward
null of the cardiod to minimise interference problems with a
co-channel repeater 85 miles away.

The antenna is balanced - so I need to decouple the coax feed line.

The obvious choices (at least to me) for feeding the antenna a

1. Coaxial sleeve balun: this requires a bit of measurement and
cutting, also waterproofing, and is not easy to fit in the physical
arrangement.

2. Ferrite bead balun: dead easy to install, and doesn't compromise
weatherproofness of feedline.

I have some ferrite beads which the supplier specifies as having 200
ohms impedance at 100 MHz, which snuggly fit on a short piece of
RG-400/U coax connecting to the main LDF5-50 feedline.

But who knows how what impedance they represent at 70 cm?

Can someone tell me about the effectiveness of ferrite bead balun
chokes at UHF, and how test this.

What other arrangements which would achieve the desired results?

Thanks

MikeN, ZL1BNB