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Author Topic: 2 meter beam  (Read 2009 times)

Posts: 26

« on: February 08, 2017, 12:37:16 PM »

I have a pair of 2 meter beams that I have not used for a few years and want to put them back into use. The feed point was originally set up for a balanced feedline and a variation of a hairpin match. The driven element is a split dipole with the matching system between the two halves. You have a shorting bar that you adjust until the match is correct. The shorting bar was aluminum against aluminum wires so it tends to corrode. I know that I can make a balun out of a piece of coax as I have done in the past but would like a more elegant solution. I am looking for a better way to handle these two problems. Can anyone suggestion either a commercial product or something that solves both of these problems?

Posts: 17484

« Reply #1 on: February 08, 2017, 02:16:11 PM »

There are many different methods for matching a yagi, but it depends to some extent on the
feedpoint impedance of the specific design you want to match.  It's important to understand
that there are two distinct steps:  matching the impedance, and a balunced-to-unbalanced

For example, if the feedpoint impedance is close to 12.5 or 28 ohms then you can use the DK7ZB match
using two parallel lengths of 50 or 75 ohm coax.  By forming the coax into a choke balun
you can perform both tasks at once.

A transformer with a 3 : 2 turns ratio will match a 22 ohm antenna to 50 ohms, but you will have
to choose the transformer core to handle your transmitter power.

You can also make a folded dipole driven element to step up the impedance to make it easier to
match.  By choosing the dimensions you can change the step-up ratio:  sometimes this is done to
get close to 200 ohms, then use a standard 4 : 1 balun to get back to 50 ohms.

The beta ("hairpin") match actually works pretty well, and might be worth using if your antennas
are already set up for it, as other matching methods may require a change to the length of the
driven element.  In practice, aluminum-to-aluminum joints tend to have fewer corrosion problems
than copper-to-aluminum joints, especially with a bit of NoAlOx or OxGard applied to them.
Otherwise, the introduction of a stainless steel washer or clamp between the two parts may help.

The matching coil doesn't have to be in the form of a shorted stub, though that makes it easy to
adjust and allows the center point to be grounded to the boom.  You can also wind a discrete coil
or use a length of transmission line to form the matching element.

Or you can redesign the yagis to give a 50 ohm impedance using the OWA (Optimized Wideband
Antenna) approach popularized by the late W4RNL.  I converted a 6m yagi this way by adding
an extra element to allow a direct 50 ohm feed (though it still needed a 1 : 1 balun.)

So, yes, there are other options, but you need to know the impedance of your antenna without the
added matching to know what the best approach might be.  That probably can be calculated from
the length and width of the matching stub and the diameter of the conductors.

Posts: 26

« Reply #2 on: February 10, 2017, 08:39:10 AM »

When I originally purchased the beams they came with a stacking kit. While that is long gone they were parallel lengths of aluminum tubing that met halfway between the beams on the mast. As I recall there was a coax connector on that meeting point for the feedline to the shack. No balun was required. I never used that method. Instead I made a balun for each beam mounted at the driven element and a coax phasing harness. It seemed to work quite well until I burned through the jacket on the coax running high power. This time I only want to use one of them mounted vertically instead of horizontally. I think they are 10 element Hygain yagi's. I know I still have the instructions around here somewhere. Thank you for your detailed response. Bill - K4NAE
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