Larry,
A balun is necessary and always a problem at VHF and UHF which often adds
lots of complexity to a Yagi design. The most successful and widely used
UHF baluns seem to be the half-wave type in conjunction with a "T" match.
This serves the dual purpose of matching impedance (since Yagis are often
anything but 50 Ohms) and balancing at the same time.
That being said, if you want real simplicity and your Yagi has an low
reactance and an impedance close to 50 Ohms at the center (as seems to be
implied by your request for help), the ferrite bead type of choke is very
efficient and about as simple an low cost solution as possible.
At 430 MHz, using RG58A coax, all you need is two (2) beads 1" long by 0.5"
diameter with a 0.25" hole (to pass the RG58 through). Amidon Associates
has them in their catalog as part number FB-43-1502 at $1.50 each. Just
slip them together on the coax within about 1-2 inches of the feed and
you're all set. I have shipped 1,000's of commercial and Amateur Yagis
using this technique. It works!
Finally, if you want to see if a balun is working, one simple crude test is
to monitor the VSWR while running your hand around and along the feedline
(outside the antennas immediate feed). Large changes in VSWR indicate poor
balun decoupling. The simplest way to feed small Yagis is to bring the feed
line from the driven element back behind the reflector and use rear mounting.
GL and 73,
Joe, W1JR
At 12:30 PM 7/17/2002 -0500, Larry Bush wrote:
>Am building a couple of 8 element 430 MHz Yagi on 1 inch PVC boom.
>
>The driven element is a dipole with direct feed RG-58A. I thought a
>common-mode balun wound with the RG-58 around the 1 inch OD PVC boom would
>be neat so I computed the reactance needed was .4 micro-Henry, which is five
>turns on the boom. During my research I was could not find the use of
>common-mode baluns on frequencies higher than 6 meters. I have three
>questions:
>
>
>
>1 Will a common-mode choke work on 430 MHz?
>
>2 Will the capacity coupling between turns of the balun at
>430 MHz make the choke ineffective and if so would winding the balun by
>spacing the turns the diameter of the RG-58 help?
>
>3 How would one check the common-mode currents on the yagi
>models?
>
>
>
>
>
>Larry Bush, W5NCD
>
>
>_______________________________________________
>Self Supporting Towers, Wireless Weather Stations, see web site:
>http://www.mscomputer.com
>Call 888-333-9041 to place your order, mention you saw this ad and take an
>additional 5 percent off
>any weather station price.
>_______________________________________________
>Towertalk mailing list
>Towertalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
|