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Author Topic: 50 mhz yagi  (Read 9964 times)

Posts: 171

« on: October 16, 2016, 09:01:29 AM »

Hi all
I am planning to build a 5 element Yagi for 50 mhz on a 20 ft boom. i would like to know if i could feet this antenna direct
and if i could do that, do the elements need to be insulated from the boom?
Thanks for the help: KP2BH /JIMMY

Posts: 225

« Reply #1 on: October 16, 2016, 12:03:40 PM »

Here is how I would do a 5 element 50 MHz yagi, the directional and reflector element mount directly to the boom with no insulation between the boom and elements. The only element that would be insulated from the boom is the drive element consisting of two element, each one half of a dipole configuration.  The drive element could be directly connected to the 50 ohm coax.

There are other methods of coupling the coax to the drive element and these can also be found on the internet.

Have you done a web search home brew/made 50 MHz yagi?  I would suggest you do some additional home work of searching the internet for plans and discussions on making such and antenna.

Good Luck,


Posts: 171

« Reply #2 on: October 16, 2016, 04:52:12 PM »

Thanks David will be checking on the net for plans. 73's

Posts: 635

« Reply #3 on: October 16, 2016, 05:24:54 PM »

If you use a quad driven element its direct fed with coax

Posts: 6748

« Reply #4 on: October 16, 2016, 06:36:29 PM »

BH:  Other sources of information on antenna building is the ARRL Antenna Book and of course the old standby, the ARRL Handbook.  I feel that too many new hams (a) don't belong or support the ARRL who are the only ones between us and 'them.' and (b) Taking advantage of the many publications the ARRL produces. 

While the Internet is a great source of any kind of information you want...... IMHO there's nothing like setting down with a book that can be studied....notes made...and picked up in a heartbeat whenever a question comes to mind.

A Pessimist is Never Disappointed!

Posts: 17477

« Reply #5 on: October 16, 2016, 06:45:24 PM »

I recently went through the process of updating an old 5-element 6m yagi (on a 12'? boom) and
found that by adding an extra element to turn it into a W4RNL "OWA" (Optimized Wideband
Antenna) I could get good gain and bandwidth with a direct 50 ohm feed.

The exact feedpoint impedance of a yagi depends on the exact lengths and spacings of all the
elements:  it can be as high as 50 ohms or as low as 8 ohms.  Generally, 50 ohm yagis will have
less gain, unless you add an extra element (as I did to the 5-element I was fixing up.)

Note that there isn't a single design for 5 elements on a 20' boom:  there are thousands of
possibilities by varying the element lengths and spacings.  The element diameters also make
a difference, as does the taper if you use telescoping sections rather than a single diameter
for the whole element.

In most yagi designs, the elements can be either insulated or grounded to the boom, but
the element lengths are different
for the two cases by the boom correction factor.

Then we come to the question of what exactly you mean by "direct feed".  For example, the
DK7ZB designs are fed with coax, but include a quarter wave  matching section made from
two parallel lengths of coax to match the antenna impedance:

Other good yagi designers to search for include YU7EF, G0KSC, and VE7BQH, and some
others that I can't remember at the moment.  But that should keep you busy for a while
(and check each site for links to other sources as well.)

And there is a lot of good general information in GM3SEK's Long Yagi Workshop page:

Posts: 764

« Reply #6 on: October 16, 2016, 08:50:46 PM »

1964 Handbook has details for open wire fed 6m yagi.

Posts: 815

« Reply #7 on: October 17, 2016, 03:36:47 PM »

I am using a 3 element built several years ago and uses a direct feed through a 6 turn coax choke.
The dimensions came off of a 1976 ARRL Antenna book graph.
It exhibits a sharp and deep null off the sides.
For the driven element, use a flat plate from the builders deck hardware selection.
Large enough and strong enough to stand up the some loading on the elements.
Uses 2 each  Nylon spacers about 1" long to mount the driven elements on the flat plate.
Use Antenna analyzer to tune in the lengths and match or just trial and error..
Should end up pretty wide banded for match and work pretty good for an easy to build and simple arrangement.
Good luck.

Posts: 755

« Reply #8 on: October 17, 2016, 04:02:43 PM »

Hi. You might try looking at "NBS Antenna" on the web.
I think you'll find that gain is pretty much a function of boom length rather than the number of elements......
Lots of options. Have fun...


Posts: 1006

« Reply #9 on: October 18, 2016, 09:30:41 AM »

 I've built a few 6 meter yagis and always used a gamma match. Although it works very well, it's technically not the ultimate arrangement since an unbalanced feed line (coax) is being coupled to a balanced feedpoint.  The result can be pattern distortion with common mode currents radiating from the feedline.

My 5 element yagi has been down for a while and I've been considering reinstalling it - but not before redesigning the feed arrangement.

I like M2's feed design which uses a split driven element with a hairpin match fed by a 1:1 current balun.

Posts: 171

« Reply #10 on: October 18, 2016, 07:10:37 PM »

Thanks to all for the links. this weekend i be working on a design that i found on the net.
Is a 6 elements Yagi with split driven element and direct feed but i will be installing only 5 elements..
will be adjusting the Yagi with an analyser.
Thanks kp2bh

Posts: 17477

« Reply #11 on: October 19, 2016, 11:33:05 AM »

The presence / absence of the last director can make a significant change in the
performance of the antenna, especially if you are trying to optimize it.

Posts: 455

« Reply #12 on: October 20, 2016, 09:26:41 PM »

You can't remove an element from an existing design and then use a antenna analyzer to match to 50 ohms and expect to have your antenna perform, you may very well end up with an antenna with a gain loss of a couple of db or more.

Especially at 6 meters and above the following recommendations are a must.

Gain is king on 6meters, even 1 db is noticeable on this band.

Errors of 1/2 inch in element lengths and element spacing placement errors along the boom of an inch or two can add up to a couple of db or more loss in gain with even pattern direction reversal at 6 meters.

To accurately build and model your antenna you really need to use the same element mounting method for all elements.

 Thru boom mounting on some elements with insulated mounting on others is a nightmare to model and without modeling and optimizing your element lengths and placement along your boom, your 5 element design  very well can result in a 5 el yagi with performance no better than a two or three element antenna.

After 15 years of building 6 meter antennas that didn't work as expected and then changing my methodology to more modern methods of element mounting and element taper, I firmly recommend that you use insulated elements of the same diameter with elements mounted to a square boom using a mounting scheme such as using stauff insulated blocks simply because it dramatically simplifies the modelling of the antenna and results in a valid design.

Why not tell us what diameter tubing you have to start with?

Are you planning to use one tube diameter for the entire length of all elements or do you plan to use tapered tubing elements?

What is your boom material, round or square tube?

With some of this information we can help you come up with a working antenna.

Additionally, Before starting your design I suggest obtaining a copy of the 4NEC antenna modelling program (free) and/or  eznec, paid but well worth it.

check these videos out.
« Last Edit: October 20, 2016, 09:49:31 PM by WB8VLC » Logged

Posts: 1560

« Reply #13 on: October 23, 2016, 06:12:36 AM »

For five elements the optimum length is around 18ft plus or minus.
You cannot use an arbitrary feed without modeling it first.

Like others have said if properly executed a 50 ohm feeds tend to be wider
band but lower gain.

Look at the LFA and OWB designs for hints.  But don't just do mixy-matchy
of several designs without modeling.

Grounded elements are the norm and work well.   Make sure you include
element length adjustment for the boom.

Most attempts at yagis work.  Yagis unless butchered want to work but the
gain can be much less than expected.   The whole point of a long boom yagi
is gain.  When you go to more than three elements the process become more
critical to doing it right.  There are plenty of known good designs to follow and
the tools [4nec2, QY4, YO] are there.

FYI I run a 5 element on about 17.5ft, its a Cushcraft A505SS with W5WVO
mods for assembly and boom per his article.  That changes the boom from
12 to 17.5ft and then alters every element length. Allowable error is less than
a half inch and changing the diameter of elements breaks it.  After much modeling
and range testing of before and after it was a real improvement.


Posts: 171

« Reply #14 on: October 23, 2016, 05:23:21 PM »

Hi Allison  KB1GMX es thanks for the response.
I will be downloading one of the antenna software and play with it.
probably i will choose a 4 element to start, i have a 20 foot Boom
available and about 5 elements from 3/8 to 1/2.
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.
Thanks es 73
KP2BH / jimmy
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