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Author Topic: 50 mhz yagi  (Read 8450 times)
WB6BYU
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Posts: 16879




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« Reply #15 on: October 23, 2016, 06:32:14 PM »

Quote from: KP2BH

...the problem here is the matching section. i was looking for a already made gamma match
but no lock .we keep checking and see if i find something not too complicated.




There are a number of simple matching methods.  For example, the OWA designs
(like the 6m beam I redesigned) use direct 50 ohm feed (with a current balun,
of course.)  That actually can be pretty simple:  I found a fiberglass rod (electric
fence post from the farm supply store) that slipped inside the middle of my
driven element and passed it through the boom.  The elements then attach on
the outside of that so they don't contact the metal boom, and the coax has
lugs that attach under the screws that hold on the elements.

When the antenna has a lower feedpoint impedance, just use a "beta match".
That consists of a hairpin of wire, or a short stub of feedline, connected across
the same terminals with the feedline.  I find this approach much easier than
a gamma match.
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KB1GMX
Member

Posts: 1485




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« Reply #16 on: October 24, 2016, 06:08:32 PM »

KP2BH,

Why not buy the replacement parts from Cushcraft for the A505S?
Or look up DX engineering to see if they have parts you can use.

all a gamma match is mechanically is the center of a peice of coax (rg213 sans jacket and braid)
a tube that fits over it and a socket for the coax to the antenna plus a bracket made of bent aluminum
to hook the tube to the element and space it out.  All of the dimensions are variable as the antenna
you are making is not exactly like something so fully adjustable is the way to go till the SWR is good
at the desired frequency.  This is all covered in antenna books and the Radio Amateurs Handbook
(any year from about 1953 to current).


Allison

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K7RBW
Member

Posts: 476




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« Reply #17 on: October 26, 2016, 04:36:48 AM »

A Yagi antenna consists of a driven element surrounded by a collection of parasitic elements (the reflector and directors). The more parasitic elements you add, the better the gain (or, more precisely, the percentage of your signal that goes one direction instead of the other) and the lower the antenna's impedance at the feed-point.

If you feed a Yagi with a simple dipole (maybe the most common configuration), then you're starting with a feed point of about 50 ohms, which will only decrease as you add parasitic elements. That type of antenna will require some sort of matching arrangement in order to feed it with a 50-ohm coax. On my 50 MHz, 3-element, M2 antenna, a gamma matching network is used.

Another option is to feed the antenna with a folded dipole, which starts off with a feed-point impedance around 300 ohms. With that, it's possible to add enough parasitic elements such that the feed point of the antenna drops to something near 50 ohms where no other matching system is required. However, you'll need the modeling software to figure out what that antenna looks like.
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WB6BYU
Member

Posts: 16879




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« Reply #18 on: October 26, 2016, 08:58:07 AM »

Quote from: K7RBW

...The more parasitic elements you add, ... the lower the antenna's impedance at the feed-point...



Well, that is often true with some simple older designs, but there are plenty of exceptions.  It is
quite possible to make a long yagi with good gain that has a 50 ohm feedpoint impedance.

One of the older approaches to do this are the DL6WU equations for yagis between 2 and 39
wavelengths.  Here is a calculator for them.  These have a 50 ohm feedpoint impedance,
though they are often fed with a folded dipole and a 4 : 1 coax balun.

A newer approach is the OWA yagi from WA3FET and NW3Z.  This not only has a direct
50 ohm feedpoint impedance, but achieves a wide SWR bandwidth with only a minor loss in forward
gain.  The late W4RNL did a lot to popularize these designs and featured designs for many bands
on his website.  This is the approach I used for my 6m element, by adding an extra element close
to the driven element that controls the feedpoint impedance.  G0KSC has several designs of this type,
along with his special "Loop Fed Yagis" (LFA) that also feature a 50 ohm feedpoint impedance.

Another approach is used for the WA5VJB "Cheap Yagis", where a "half-folded" driven element
provides a 4 : 1 step-up, though in this case the antenna itself is designed for a feedoint
impedance of 12.5 ohms (for 50 ohm feed) to 20 ohms (for 75 ohm feed.)  This has become
one of my favorite methods of feeding VHF yagis, though it isn't as convenient at 6m because
the elements aren't self-supporting when made with stiff wire.

DK7ZB has some 6m yagis with direct 50 ohm feed, and others that use a quarter wave
matching section made with 2 lengths of coax.  In that case you can see that simply adding
more elements on a longer boom doesn't necessarily lower the feedpoint impedance.  Rather,
it depends on the exact tuning and spacing of the elements:  a 50 ohm feedpoint impedance
is achievable with a slight reduction in maximum gain, even for long yagis with high gain.

All of these designs are well enough defined that they can be assembled and used with only
a minimum amount of adjustment:  much easier than trying to fuss with a gamma match.

(I used to use a twisted wire gamma match to get a low SWR, then I discovered that
it rotated the polarization on my quads by about 50 degrees from what was expected.
Now I use other options, and even at 440 MHz I can throw a design together and use
it without any adjustment for SWR.  If I do need a solid, unbroken driven element I use
the WA5VJB approach, or a balanced delta or tee match with the twisted wire approach.)
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K7RBW
Member

Posts: 476




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« Reply #19 on: October 27, 2016, 04:22:45 AM »

Another approach is used for the WA5VJB "Cheap Yagis", where a "half-folded" driven element...
Interesting...
Thanks for the links!
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VK3MEG
Member

Posts: 793




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« Reply #20 on: November 12, 2016, 01:51:35 AM »

i build a yu7ef 5 element yagi it perform well i started with a 4 elements to get the construction and matching sorted.
nto hard to make they need the elements isolated from the boom i use connectors from hydrolic companies stauff for the stand offs .
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WA4UF
Member

Posts: 56


WWW

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« Reply #21 on: December 16, 2016, 08:06:15 AM »

I'd second the OWA recommendations. I recently took down my 6m beam (4el on an 8' boom - adequate but not great) and rebuilt it as a W4RNL (RIP Larry Cebik, dean of antenna designers!) "Route 66" OWA beam (6el on a 14' boom, direct 50ohm feed) and it works very well indeed.

The G0KSC LFAs are also intriguing. If you're in a noisy environment (and these days who isn't, what with cheap Chinese switching power supplies in everything from TVs to PCs to cell phone chargers) they'll likely be quite beneficial.

I'm not sure I'd go with a WA5VJB "Cheap Yagi" on 6m; the build technique and the driven element design might get a trifle unwieldy for 6m, and I don't think Kent's published a Cheap Yagi design for 6.  For 2m or 70cm? They're a great way to get yourself on-air for very little money, and since they're so inexpensive to build they're great for experimenting with stacking bunches of 'em for weak signal or EME.
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N8EKT
Member

Posts: 580




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« Reply #22 on: December 16, 2016, 05:25:03 PM »

Build a quagi

Quad reflector and driven element with yagi directors

Direct fed and produces better gain, bandwidth, and f/b ratio
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KB1GMX
Member

Posts: 1485




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« Reply #23 on: December 17, 2016, 09:03:39 AM »

Kent did not do a 6M cheap-yagi.  However if you scale the 2M version to 6 and use 3/8ths rod
it would be doable.   I'd suggest a more robust boom like a good fir 2x4 painted.  For 3 elements
that should fly. for more elements its going to be a structural challenge for a boom longer than 6ft
though.

The easy way out is the ARROW 52-4S 4 elements and tough as nails plus inexpensive.

Allison
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KP2BH
Member

Posts: 171




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« Reply #24 on: January 20, 2017, 09:59:35 AM »

Thanks to all for the response to my message. After reading and searching the net i found a very simple
design for a 50 mhz yagi. is a 5 elements on a 12 ft boom..i build the antenna and match it with a hairpin
to 50 ohms put it up at 35 ft and i been making contacts with good reports.
thanks to all
jimmy  KP2BH
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