Beta-Match for 6 meter yagi

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Garth Crownover:
I am in process of building a 6 meter, 3 element yagi. Mine will have the driven element split and isolated from the boom. How can I determine the dimensions for a Beta-Match? The 15th edition ARRL Antenna Handbook says about 5 to 6 inches. I will be using somewhat different tubing & spacing than the Handbook examples, how will this affect the size of my beta-match? Also the Handbook shows a 180 deg. phasing line, what is this for? I was just planning to use a balun.

With this design, do I have any other simple options for matching to 50 ohm coax?

Thank you for your time,
Garth, KW4MM

Dale Hunt:
The beta match is just a coil connected across the feedpoint.  Sometimes the coil
is in the form of a single turn loop, or a shorted feedline stub.  The required inductance
depends on the input impedance of the antenna.  Often the inductance is made
variable (for example, by using a movable short circuit across the stub) and is
adjusted along with the element length for best SWR.

If you use a "hairpin" or shorted open wire stub for the inductance, the characteristic
impedance will be close enough to 600 ohms that you can use that to calculate
the required stub length, given that you are probably going to make some find
adjustments to the length anyway. This is why the exact width of the matching
stub really isn't critical.

I don't have that particular article in front of me at the moment, but if the design
uses a half wavelength of coax it probably is to make a 4 : 1 coaxial balun.  I
suppose it is possible then to use a beta match to step the impedance of the
antenna up to 200 ohms rather than 50 ohms, but this isn't an approach that
I'd choose.

The first question about matching is, what is the input impedance of the basic
yagi.  Some 3-element designs are close enough to 50 ohms and can be fed
directly, others are around 12 ohms and can use a folded dipole.  High gain
designs like the NBS standards often have a very low feedpoint impedance -
around 8 ohms - that can make matching difficult.  A very simple method of
matching yagis around 28 ohms is to use a pair of 75 ohm cables in parallel
as a quarter wave matching stub.  This method is popularized by DK7ZB and
is explained on his excellent web site here:

http://www.mydarc.de/dk7zb/

In fact, you may want to look at his collection of 6m designs while you are
there.

Ken Merring:
The basic impedence of a diole alone is usually about 72 ohms and not present a large mismatch to a 50 ohm transmision line, so that works with most dipole antennas as is done on HF..
As soon as a reflector and directors are added, the driven element impedence usually takes a dive lower.
This is the basic reason the different matching systems are used to transform the low impedence to 50 ohm line.
The eaisest way to get a match is use a solid mounted length tuned element and apply a Gamma Match.
You will get your match without much hassle with the design/mounting/tuning of a T match system.
Admittedly there may be some small advantage to the T match but not enough to justify the extra complexity.
I have built a 6 and a 2 m beam using the Gamma match and see no peticular issue.
The 6m-5 element is on my tower but I chose to go with a large high gain 12 element T matched M2 beam rather than build one out to that length and gain size, as the price was right at the time.
With whatever system you go with, expect to do some tuning to get the match down. Mount the antenna about 10 feet up so you can get at it with an 8' step ladder for test adjustments. Put your SWR meter right at the antenna terms so you can see the match to the 50 ohm line instead of at the radio where yopu could get fooled. After the match has been attained, the reading at the radio will be nearly the same but you will now the antenna is correct where it makes the most difference.  The 'most difference' is a matter of best recieve as well as tranmit. They don't always co-inside as optiumn unless you make the tests to be sure. The transmitter will load power into the antenna within a reasonable match but you want the most recieve you can get for weak signal work if on SSB or CW.
There is a way to set for best recieve that usually results in best SWR for transmitt. This is often no spoken much about but has great value to the weak signal operator.
I put a signal out on another antenna at a low power some distance away. Using a sensitive SWR meter in it reverse reading, adjust match for the highest recieve signal and you will have a fine operating antenna. Always retest with power applied for the resulting match it offers and reset very sleightly if needed.

Dan:
Quote from: KW4MM on January 13, 2010, 04:44:29 AM

I am in process of building a 6 meter, 3 element yagi. Mine will have the driven element split and isolated from the boom. How can I determine the dimensions for a Beta-Match? The 15th edition ARRL Antenna Handbook says about 5 to 6 inches. I will be using somewhat different tubing & spacing than the Handbook examples, how will this affect the size of my beta-match? Also the Handbook shows a 180 deg. phasing line, what is this for? I was just planning to use a balun.

With this design, do I have any other simple options for matching to 50 ohm coax?

Thank you for your time,
Garth, KW4MM

You've chosen the simplest method IMO (short of designing a 50ohm feed antenna) and the method that I use. It requires you to shorten the driven element some when you use a hairpin, but that's no big deal. I build the hairpin and adjust the driven element to lowest SWR, assuming you built everything correctly and feed impedance is what it should be. I've never had a problem.

You need to know the feedpoint impedance to build the hairpin match. Some yagi's are 12.5ohm, some 22, 25, 50, etc. The 3L 6m yagi I just built was ~25ohms. The easiest method is to locate and download yagimax. Yagimax is an old DOS program that works just fine with my XP Pro computer. In it there is a program called "match" that will give you specific figures to build a match. If you have the ARRL Antenna Book, and if you don't I highly suggest you buy it as it is an invaluable resource, there are charts and formulas to help you. Yagimax is also great in that it will figure out the taper schedule for you so if you don't have exactly the amount of 1/2" tubing called for but you have lots of 3/8" tubing and 5/8" tubing, it will tell you how much of each diameter tubing you need. It really is a neat program that I use along with EZNEC.

Here's a link to a yagimax download. http://www.iw5edi.com/ham-radio/?yagimax,127

Garth Crownover:
Thank you all! This is very helpful and educational.

73,
Garth, KW4MM

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