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Author Topic: How to tell if 2m Yagi needs a balun?  (Read 251 times)

2E0ILY

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How to tell if 2m Yagi needs a balun?
« on: May 14, 2020, 09:07:21 AM »

I am putting up a secondhand 2 metre  13 element Yagi I bought at a rally years ago. The dipole is brand new and never used, the other elements are undamaged and I have just replaced all the plated steel hardware with stainless steel fasteners. But how do I decide if it should have a balun, and if it should, what type please? I intend to feed it with something better than 213, but lesser than Andrew LD*.... ;) It will be topmost on a 40 foot lattice work wind up / tilt over mast. Thanks. Very new to VHF and UHF, I have been working mainly down on 136 kHz for years! 2E0ILY
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Best regards, Chris Wilson.

G4AON

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Re: How to tell if 2m Yagi needs a balun?
« Reply #1 on: May 15, 2020, 11:49:13 PM »

Pretty much any balanced antenna needs a balun, or at least a feed line choke.

You don’t mention if the yagi is 50 Ohm feed or lower... for a folded dipole driven element you can use a coil of mini coax, such as RG316 or at a push RG58. The neat solution is a ferrite sleeve choke as sold by Innov Antennas.
https://innovantennas.com/en/shop-page/357/1/vhf-uhf-ham-radio-antennas/144mhz-low-noise-lfa-yagis/2m-innovant-balun-1kwInnovAntennas.html

Regarding coax, Hyperflex 10 and associated matching plugs is hard to improve on. You can buy it direct from M&P or Martin Lynch: https://messi.it/en/home.htm

At 1/3rd of the cost, some hams are quite happy to use HDF400 coax, however it has a solid centre core and that may eventually give rise to problems when used with a rotator. The coax can be obtained from:
https://www.wifi-antennas.co.uk/hdf400-coaxial-cable-per-metre

Both the above types of coax need a different plug to the types that fit RG213, it is not a problem but just be aware. Also foam dielectric cable is easily damaged by crushing, either from over tightening cable ties or by standing on it.

73 Dave
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WB6BYU

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Re: How to tell if 2m Yagi needs a balun?
« Reply #2 on: May 16, 2020, 11:48:27 AM »

Quote from: 2E0ILY

I am putting up a secondhand 2 metre  13 element Yagi I bought at a rally years ago. The dipole is brand new and never used, the other elements are undamaged and I have just replaced all the plated steel hardware with stainless steel fasteners. But how do I decide if it should have a balun, and if it should, what type please? ...



I'd agree that most beams need some sort  of balun for good
performance.  The question of what type, however, depends
on the design of the driven element and the antenna itself.

There are three basic types of driven elements:  those where
the center is open to connect the feedline (like a common HF
dipole), those that use a continuous element with no gap, often
fed using a gamma match (sometimes called "plumbers delight"
construction because it can be made from copper pipe with
no insulators), and those that use a folded dipole or other
other multi-element design.  (There are others as well - these
are the main ones.)

You also need to know the design impedance for the antenna
feedpoint:  yagis can be designed anywhere in the range from
about 5 ohms to 75 ohms.  Some can be used without any
impedance matching (though a balun is still a good idea) and
some are designed for 12.5 ohms to use with a 4 : 1 impedance
step-up in the balun (or from a folded dipole element).
DK7ZB likes to design his for 28 ohms, as that permits matching
with a quarter wave section made by paralleling two pieces of
75 ohm coax.  Many designs come out in the 20 - 25 ohm range,
which can be matched to 50 ohms with a shunt coil (often a "hairpin").
Some commercial designs use the hairpin to transform the antenna
impedance up to 200 ohms, then use a 4 : 1 balun or folded dipole
to step it back down to 50 ohms.

So you really need to know how the original antenna was designed
to be fed.  That will give us a better idea of what sort of balun to
recommend.


To the other question of "how do I know if I need a balun", one
answer is to put up the antenna, tune in a constant signal, and plot
the radiation pattern as you turn the antenna.  Radiation from the
coax (which is what a balun is designed to prevent) will often
distort the pattern, though you might not notice it if the signal source
and antenna are both horizontal and the coax runs vertically.

(Installing the antenna for vertical polarization requires attention to
other details, such as keeping the coax and metal mast/tower out
of the radiation pattern.)

2E0ILY

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Re: How to tell if 2m Yagi needs a balun?
« Reply #3 on: May 17, 2020, 01:37:36 PM »

Thanks very much for the two detailed replies. I had a look at the commercial ferrite tube over co-ax baluns and my second hand, make unknown Yagi has a moulded in N type socket, not two posts for the ends of the co-ax. If I knew what ferrite tubes to use I could make my own, slipped over the end of the feeder co-ax before fitting the N type plug.

The Yagi has a one piece driven element, the ends moulded into the same plastic as the N type socket, so I have no idea as to its impedance, but if I had to guess would say it is 50 Ohm


As an aside the elements are mounted uninsulated atop the square alloy section boom. More modern Yagis seem to have the elements above the boom on plastic insulators. I have read that the two methods demand different element lengths, but I have not seen if one has better performance than the other, although noise floor was mentioned as worse with uninsulated elements. Is there a marked difference please? Thanks
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Best regards, Chris Wilson.

G4AON

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Re: How to tell if 2m Yagi needs a balun?
« Reply #4 on: May 18, 2020, 04:09:25 AM »

Chris, my old Tonna 2m yagi has a sleeve balun that comprises an aluminium tube 390mm long x 12mm internal diameter that has the coax routed through it.

It is clipped to the boom with a stainless steel clip at the end furthest from the dipole and is positioned to clear the rear of the antenna plug by approx 12mm. While there might be more effective methods of isolating the driven element from the outer of the coax, it's low cost and easy to add.

Whether insulating the elements from the boom is of benefit... it makes for easier building from software antenna models as much as anything. You will find a lot of amateur antenna models do not include the boom, yet a metal one does make a difference. Many modern designs use insulated elements, whereas older designs generally do not. In my days of dealing with commercial VHF and UHF radio, the parasitic elements of a yagi were all directly "metal to metal"... probably based on the need to produce as rugged an antenna as possible.

73 Dave

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W9IQ

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Re: How to tell if 2m Yagi needs a balun?
« Reply #5 on: May 18, 2020, 04:36:40 AM »

Ferrite based baluns on 2 meters are difficult to make effective. Dave's picture is an example of a 1/4 wave sleeve balun. These are quite effective on a single band VHF or UHF antenna. They are not practical for HF antennas due to their size.

When designing such a balun, keep in mind that the open end of the sleeve must point towards the feed point of the antenna. This is sometimes incorrectly depicted. The length of the sleeve must be an electrical odd multiple of 1/4 wavelength. The velocity factor of the outer coax jacket, not of the coax itself, must be utilized to compute the electrical length.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

2E0ILY

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Re: How to tell if 2m Yagi needs a balun?
« Reply #6 on: May 18, 2020, 07:46:31 AM »

Dave, I am so glad you posted that photo, I had never known the make of this Yagi, but the driven element looked very familiar, so a Google search brought this link up:

http://www.zen70432.zen.co.uk/EME/Tonna.htm

and it shows a knackered one like my (brand new) element in close up. I see old UK Radioworld adverts show a replacement element and it seems to come with a new sleeve balun.

https://www.radioworld.co.uk/tonna_accessories/210131_spare_144_146mhz_element_for_220809_220818_220817_and_220

So now I know this thing is a Tonna and I have a brand new driven element, but what I do not have is the sleeve balun that you show and describe, and is in the old radioworld ad'.

Can I make one? I am a race car engineer so have decent workshop facilities (milling machine, lathe, band saw, surface grinder, TIG, MIG blah blah). What's inside the plastic "funnel" part? I don't want to to continue work to get it up aloft and find it's useless without this particular balun... I am also wondering if it's possible to better waterproof this new driven element without de-tuning it, to stop what is obviously a known issue?

Thanks again everyone :)
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G4AON

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Re: How to tell if 2m Yagi needs a balun?
« Reply #7 on: May 18, 2020, 08:05:51 AM »

Chris, the “plastic funnel” is just a rain shield to keep the connections dry. The aluminium tube is just that. Self amalgamating tape with a wrap of black PVC tape is far better than relying on a slide over sleeve. Commercial antenna riggers often cut off those sleeves as they serve no useful purpose... except for temporary use when /P, etc.

My antenna is the 9 ele portable Tonna, model 20089, it is around 15 years old and is very lightweight. Sadly Tonna ceased trading some years ago.

Good luck with your installation.

73 Dave
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W9WQA

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Re: How to tell if 2m Yagi needs a balun?
« Reply #8 on: May 18, 2020, 10:03:56 AM »

just yesterday i was leafing thru an old "rsgb" book and stopped at a 2 mtr yagi section. very interesting stuff in there, fyi...
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2E0ILY

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Re: How to tell if 2m Yagi needs a balun?
« Reply #9 on: May 18, 2020, 02:08:28 PM »

Thanks, I have few older RSGB handbooks so will have look, much appreciated.
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Best regards, Chris Wilson.
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